Computer Computer A Level(Form Six) Computer Study Notes Form Six (A Level)

COMPUTER A LEVEL(FORM SIX) NOTES – C PROGRAMMING

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EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

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C PROGRAMMING

AN INTRODUCTION TO C PROGRAMMING

It is one of the most used programing language in software development area. At the beginning, C was designed and developed for the development of UNIX operating system. Today C has been widely used to develop many types of application software such as basic calculator word processor games, operating system etc.

Editing in C language

Editing is a process of writing the C source code. programmers write C source code using editor program. widely used editing programs are Linux, platform via and Emacs.
On windows platform, we can use Notepad as editor program but most use software package for C such as C++ builder, DOS C++. This software provides complete programming environment forwriting, managing, compiling, debugging and testing C source code. This is known as Integrated Development Environment (IDE)
Compiling in C language
The C source code has to be translated into machine language code. Machine language is a language which are in the form of binary numbers that a computer understand. The process of code translation is called compiling. During compilation process, the compiler can detect any syntax error.
Execution
The execution stage is where you can run the program to check whether it produce the desired output.This stage can be a testing stage where you can check if you get the desired output.
C program layout
Basically, a C program consists of two sections
1. Pre- processor directives
2. Main function

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

A simple C program
The following program is written in the C programming language which can display the word “program in C is easy”

#include

main()

{

printf(“Programming in C is easy.n”);

}

Sample Program Output

Programming in C is easy.

_

A NOTE ABOUT C PROGRAM
In C, lowercase and uppercase characters are very important! All commands in C must be lowercase. The C programs starting point is identified by the word

main()

This informs the computer as to where the program actually starts. The brackets that follow the keyword main indicate that there are no arguments supplied to this program The two braces, { and }, signify the begin and end segments of the program. The purpose of the statement

is to allow the use of the printf statement to provide program output. Text to be displayed by printf() must be enclosed in double quotes. The program has only one statement

printf(“Programming in C is easy.n”);

printf() is actually a function (procedure) in C that is used for printing variables and text. Where text appears in double quotes “”, it is printed without modification. There are some exceptions however. This has to do with the and % characters. These characters are modifiers, and for the present the followed by the n character represents a newline character. Thus the program prints

Programming in C is easy.

and the cursor is set to the beginning of the next line. As we shall see later on, what follows the character will determine what is printed, ie, a tab, clear screen, clear line etc. Another important thing to remember is that all C statements are terminated by a semi-colon

Summary of major points so far

DATA TYPES AND CONSTANTS
The four basic data types are

INTEGER
These are whole numbers, both positive and negative. Unsigned integers (positive values only) are supported. In addition, there are short and long integers.

The keyword used to define integers is,

An example of an integer value is 32. An example of
declaring an integer variable called sum is,

FLOATING POINT
These are numbers which contain fractional parts, both positive and negative. The keyword used to define float variables is,

An example of a float value is 34.12. An example of declaring a float variable called money is,

DOUBLE
These are exponetional numbers, both positive and negative. The keyword used to define double variables is,

An example of a double value is 3.0E2. An example of declaring a double variable called big is,

CHARACTER

These are single characters. The keyword used to define character variables is,

An example of a character value is the letter A. An example of declaring a character variable called letter is,

letter = ‘A’;

Note the assignment of the character A to the variable letter is done by enclosing the value in single quotes. Remember the golden rule: Single character – Use single quotes.

Sample program illustrating each data type

#include < stdio.h >

sum = 10; /* assign integer value */

money = 2.21; /* assign float value */

letter = ‘A’; /* assign character value */

pi = 2.01E6; /* assign a double value */

printf(“value of sum = %dn”, sum );

printf(“value of money = %fn”, money );

printf(“value of letter = %cn”, letter );

printf(“value of pi = %en”, pi );

Sample program output

value of sum = 10

value of money = 2.210000

value of letter = A

value of pi = 2.010000e+06

PREPROCESSOR STATEMENTS
The define statement is used to make programs more readable. Consider the following examples,

#define TRUE 1 /* Don’t use a semi-colon , # must be first character on line */

#define FALSE 0

#define EQUALS ==

game_over = TRUE;

while( list_pointer != NULL )

…………….

Note that preprocessor statements begin with a # symbol, and are NOT terminated by a semi-colon. Traditionally, preprocessor statements are listed at the beginning of the source file.

Preprocessor statements are handled by the compiler (or preprocessor) before the program is actually compiled. All # statements are processed first, and the symbols (like TRUE) which occur in the C program are replaced by their value (like 1). Once this substitution has taken place by the preprocessor, the program is then compiled.

In general, preprocessor constants are written in UPPERCASE.

Class Exercise C4
Use pre-processor statements to replace the following constants

#define smallvalue 0.312

#define letter ‘W’

#define smallint 37

LITERAL SUBSTITUTION OF SYMBOLIC CONSTANTS USING #define
Lets now examine a few examples of using these symbolic constants in our programs. Consider the following program which defines a constant called TAX_RATE.

#define TAX_RATE 0.10

balance = 72.10;

tax = balance * TAX_RATE;

printf(“The tax on %.2f is %.2fn”, balance, tax );

The pre-processor first replaces all symbolic constants before the program is compiled, so after preprocessing the file (and before its compiled), it now looks like,

#define TAX_RATE 0.10

balance = 72.10;

tax = balance * 0.10;

printf(“The tax on %.2f is %.2fn”, balance, tax );

YOU CANNOT ASSIGN VALUES TO THE SYMBOLIC CONSTANTS
Considering the above program as an example, look at the changes we have made below. We have added a statement which tries to change the TAX_RATE to a new value.

#define TAX_RATE 0.10

balance = 72.10;

TAX_RATE = 0.15;

tax = balance * TAX_RATE;

printf(“The tax on %.2f is %.2fn”, balance, tax );

This is illegal. You cannot re-assign a new value to a symbolic constant.

ITS LITERAL SUBSTITUTION, SO BEWARE OF ERRORS

As shown above, the preprocessor performs literal substitution of symbolic constants. Lets modify the previous program slightly, and introduce an error to highlight a problem.

#define TAX_RATE 0.10;

balance = 72.10;

tax = (balance * TAX_RATE )+ 10.02;

printf(“The tax on %.2f is %.2fn”, balance, tax );

In this case, the error that has been introduced is that the #define is terminated with a semi-colon. The preprocessor performs the substitution and the offending line (which is flagged as an error by the compiler) looks like

tax = (balance * 0.10; )+ 10.02;

However, you do not see the output of the preprocessor. If you are using TURBO C, you will only see

tax = (balance * TAX_RATE )+ 10.02;

flagged as an error, and this actually looks okay (but its not! after substitution takes place).

MAKING PROGRAMS EASY TO MAINTAIN BY USING #define
The whole point of using #define in your programs is to make them easier to read and modify. Considering the above programs as examples, what changes would you need to make if the TAX_RATE was changed to 20%.

Obviously, the answer is once, where the #define statement which declares the symbolic constant and its value occurs. You would change it to read

#define TAX_RATE = 0.20

Without the use of symbolic constants, you would hard code the value 0.20 in your program, and this might occur several times (or tens of times).

This would make changes difficult, because you would need to search and replace every occurrence in the program. However, as the programs get larger, what would happen if you actually used the value 0.20 in a calculation that had nothing to do with the TAX_RATE!

SUMMARY OF #define

PREPROCESSOR STATEMENTS
The define statement is used to make programs more readable. Consider the following examples,

#define TRUE 1 /* Don’t use a semi-colon , # must be first character on line */

#define FALSE 0

#define EQUALS ==

game_over = TRUE;

while( list_pointer != NULL )

…………….

Note that preprocessor statements begin with a # symbol, and are NOT terminated by a semi-colon. Traditionally, preprocessor statements are listed at the beginning of the source file.

Preprocessor statements are handled by the compiler (or preprocessor) before the program is actually compiled. All # statements are processed first, and the symbols (like TRUE) which occur in the C program are replaced by their value (like 1). Once this substitution has taken place by the preprocessor, the program is then compiled.

In general, preprocessor constants are written in UPPERCASE.

Class Exercise C4
Use pre-processor statements to replace the following constants

CLASS EXERCISE C4
Use pre-processor statements to replace the following constants

#define smallvalue 0.312

#define letter ‘W’

#define smallint 37

LITERAL SUBSTITUTION OF SYMBOLIC CONSTANTS USING #define
Lets now examine a few examples of using these symbolic constants in our programs. Consider the following program which defines a constant called TAX_RATE.

#define TAX_RATE 0.10

balance = 72.10;

tax = balance * TAX_RATE;

printf(“The tax on %.2f is %.2fn”, balance, tax );

The pre-processor first replaces all symbolic constants before the program is compiled, so after preprocessing the file (and before its compiled), it now looks like,

#define TAX_RATE 0.10

balance = 72.10;

tax = balance * 0.10;

printf(“The tax on %.2f is %.2fn”, balance, tax );

YOU CANNOT ASSIGN VALUES TO THE SYMBOLIC CONSTANTS
Considering the above program as an example, look at the changes we have made below. We have added a statement which tries to change the TAX_RATE to a new value.

#define TAX_RATE 0.10

balance = 72.10;

TAX_RATE = 0.15;

tax = balance * TAX_RATE;

printf(“The tax on %.2f is %.2fn”, balance, tax );

This is illegal. You cannot re-assign a new value to a symbolic constant.

ITS LITERAL SUBSTITUTION, SO BEWARE OF ERRORS
As shown above, the preprocessor performs literal substitution of symbolic constants. Lets modify the previous program slightly, and introduce an error to highlight a problem.

#define TAX_RATE 0.10;

balance = 72.10;

tax = (balance * TAX_RATE )+ 10.02;

printf(“The tax on %.2f is %.2fn”, balance, tax );

In this case, the error that has been introduced is that the #define is terminated with a semi-colon. The preprocessor performs the substitution and the offending line (which is flagged as an error by the compiler) looks like

tax = (balance * 0.10; )+ 10.02;

However, you do not see the output of the preprocessor. If you are using TURBO C, you will only see

tax = (balance * TAX_RATE )+ 10.02;

flagged as an error, and this actually looks okay (but its not! after substitution takes place).

MAKING PROGRAMS EASY TO MAINTAIN BY USING #define
The whole point of using #define in your programs is to make them easier to read and modify. Considering the above programs as examples, what changes would you need to make if the TAX_RATE was changed to 20%.

Obviously, the answer is once, where the #define statement which declares the symbolic constant and its value occurs. You would change it to read

#define TAX_RATE = 0.20

Without the use of symbolic constants, you would hard code the value 0.20 in your program, and this might occur several times (or tens of times).

This would make changes difficult, because you would need to search and replace every occurrence in the program. However, as the programs get larger, what would happen if you actually used the value 0.20 in a calculation that had nothing to do with the TAX_RATE!

SUMMARY OF #define

HEADER FILES
Header files contain definitions of functions and variables which can be incorporated into any C program by using the pre-processor #include statement. Standard header files are provided with each compiler, and cover a range of areas, string handling, mathematical, data conversion, printing and reading of variables.

To use any of the standard functions, the appropriate header file should be included. This is done at the beginning of the C source file. For example, to use the function printf() in a program, the line

should be at the beginning of the source file, because the definition for printf() is found in the file stdio.h All header files have the extension .h and generally reside in the /include subdirectory.

#include “mydecls.h”

The use of angle brackets <> informs the compiler to search the compilers include directory for the specified file. The use of the double quotes “” around the filename inform the compiler to search in the current directory for the specified file.

Practice Exercise 1: Defining Variables

1. Declare an integer called sum

2. Declare a character called letter

3. Define a constant called TRUE which has a value of 1

4. Declare a variable called money which can be used to hold currency

5. Declare a variable called arctan which will hold scientific notation values (+e)

6. Declare an integer variable called total and initialise it to zero.

7. Declare a variable called loop, which can hold an integer value.

8. Define a constant called GST with a value of .125

Answers to Practice Exercise 1: Defining Variables

1. Declare an integer called sum

2. Declare a character called letter

3. Define a constant called TRUE which has a value of 1

4. Declare a variable called money which can be used to hold currency

5. Declare a variable called arctan which will hold scientific notation values (+e)

6. Declare an integer variable called total and initialise it to zero.

7. Declare a variable called loop, which can hold an integer value.

8. Define a constant called GST with a value of .125

#define GST 0.125

ARITHMETIC OPERATORS
The symbols of the arithmetic operators are:-

Operation	Operator	Comment	Value of Sum before	Value of sum after
Multiply	*	sum = sum * 2;	4	8
Divide	/	sum = sum / 2;	4	2
Addition	+	sum = sum + 2;	4	6
Subtraction	–	sum = sum -2;	4	2
Increment	++	++sum;	4	5
Decrement	—	–sum;	4	3
Modulus	%	sum = sum % 3;	4	1

The following code fragment adds the variables loop and count together, leaving the result in the variable sum

sum = loop + count;

Note: If the modulus % sign is needed to be displayed as part of a text string, use two, ie %%

modulus = sum % 10;

printf(“The %% of %d by 10 is %fn”, sum, modulus);

Sample Program Output

The % of 50 by 10 is 0.000000

CLASS EXERCISE C5
What does the following change do to the printed output of the previous program?

printf(“The %% of %d by 10 is %.2fn”, sum, modulus);

ANSWERS: CLASS EXERCISE C5

modulus = sum % 10;

printf(“The %% of %d by 10 is %.2fn”, sum, modulus);

The % of 50 by 10 is 0.00

Practice Exercise 2: Assignments

1. Assign the value of the variable number1 to the variable total

2. Assign the sum of the two variables loop_count and petrol_cost to the variable sum

3. Divide the variable total by the value 10 and leave the result in the variable discount

4. Assign the character W to the char variable letter

5. Assign the result of dividing the integer variable sum by 3 into the float variable costing. Use type casting to ensure that the remainder is also held by the float variable.

Answers: Practise Exercise 2: Assignments

1. Assign the value of the variable number1 to the variable total

total = number1;

2. Assign the sum of the two variables loop_count and petrol_cost to the variable sum

sum = loop_count + petrol_cost;

3. Divide the variable total by the value 10 and leave the result in the variable discount

discount = total / 10;

4. Assign the character W to the char variable letter

letter = ‘W’;

5. Assign the result of dividing the integer variable sum by 3 into the float variable costing. Use type casting to ensure that the remainder is also held by the float variable.

costing = (float) sum / 3;

PRE/POST INCREMENT/DECREMENT OPERATORS
PRE means do the operation first followed by any assignment operation , POST means do the operation after any assignment operation. Consider the following statements

++count; /* PRE Increment, means add one to count */

count++; /* POST Increment, means add one to count */

In the above example, because the value of count is not assigned to any variable, the effects of the PRE/POST operation are not clearly visible.

Lets examine what happens when we use the operator along with an assignment operation. Consider the following program,

int count = 0, loop;

loop = ++count; /* same as count = count + 1; loop = count; */

printf(“loop = %d, count = %dn”, loop, count);

loop = count++; /* same as loop = count; count = count + 1; */

printf(“loop = %d, count = %dn”, loop, count);

Sample Program Output

loop = 1, count = 1

loop = 1; count = 2

If the operator precedes (is on the left hand side) of the variable, the operation is performed first, so the statement

loop = ++count;

really means increment count first, then assign the new value of count to loop.

Which way do you write it?
Where the increment/decrement operation is used to adjust the value of a variable, and is not involved in an assignment operation, which should you use,

The answer is, it really does not matter. It does seem that there is a preference amongst C programmers to use the post form.

Something to watch out for
Whilst we are on the subject, do not get into the habit of using a space(s) between the variable name and the pre/post operator.

Try to be explicit in binding the operator tightly by leaving no gap.

GOOD FORM
Perhaps we should say programming style or readability. The most common complaints we would have about beginning C programmers can be summarized as,

Your programs will be quicker to write and easier to debug if you get into the habit of actually formatting the layout correctly as you write it.

For instance, look at the program below

int sum,loop,kettle,job;

whoKnows=’A’;

printf(“Whoknows=%c,kettle=%dn”,whoknows,kettle);

It is our contention that the program is hard to read, and because of this, will be difficult to debug for errors by an inexperienced programmer. It also contains a few deliberate mistakes!

Okay then, lets rewrite
the program using good form.

int sum, loop, kettle = 0, job;

whoknows = ‘A’;

printf( “Whoknows = %c, kettle = %dn”, whoknows, kettle );

We have also corrected the mistakes. The major differences are

the { and } braces directly line up underneath each other

This allows us to check indent levels and ensure that statements belong to the correct block of code. This becomes vital as programs become more complex

Practise Exercise 3: printf() and scanf()

1. Use a printf statement to print out the value of the integer variable sum

2. Use a printf statement to print out the text string “Welcome”, followed by a newline.

3. Use a printf statement to print out the character variable letter

4. Use a printf statement to print out the float variable discount

5. Use a printf statement to print out the float variable dump using two decimal places

6. Use a scanf statement to read a decimal value from the keyboard, into the integer variable sum

7. Use a scanf statement to read a float variable into the variable discount_rate

8. Use a scanf statement to read a single character from the keyboard into the variable operator. Skip leading blanks, tabs and newline characters.

Answers: Practise Exercise 3: printf() and scanf()

1. Use a printf statement to print out the value of the integer variable sum

printf(“%d”, sum);

2. Use a printf statement to print out the text string “Welcome”, followed by a newline.

printf(“Welcomen”);

3. Use a printf statement to print out the character variable letter

printf(”
%c”, letter);

4. Use a printf statement to print out the float variable discount

printf(“%f”, discount);

5. Use a printf statement to print out the float variable dump using two decimal places

printf(“%.2f”, dump);

6. Use a scanf statement to read a decimal value from the keyboard, into the integer variable sum

scanf(“%d”, &sum);

7. Use a scanf statement to read a float variable into the variable discount_rate

scanf(“%f”, &discount_rate);

8. Use a scanf statement to read a single character from the keyboard into the variable operator. Skip leading blanks, tabs and newline characters.

scanf(” %c”, &operator);

THE RELATIONAL OPERATORS
These allow the comparision of two or more variables.

Operator	Meaning
==	equal to
!=	not equal
<	less than
<=	less than or equal to
>	greater than
>=	greater than or equal to

In the next few screens, these will be used in for loops and if statements.

may be legal in Pascal, but is illegal in C.

ITERATION, FOR LOOPS
The basic format of the for statement is,

for( start condition; continue condition; re-evaulation )

program statement;

/* sample program using a for statement */

main() /* Program introduces the for statement, counts to ten */

for( count = 1; count <= 10; count = count + 1 )

printf(“%d “, count );

printf(“n”);

Sample Program Output

1 2 3 4 5 6 7 8 9 10

The program declares an integer variable count. The first part of the for statement

for( count = 1;

initialises the value of count to 1. The for loop continues whilst the condition

evaluates as TRUE. As the variable count has just been initialised to 1, this condition is TRUE and so the program statement

printf(“%d “, count );

is executed, which prints the value of count to the screen, followed by a space character.

Next, the remaining statement of the for is executed

count = count + 1 );

which adds one to the current value of count. Control now passes back to the conditional test,

which evaluates as true, so the program statement

printf(“%d “, count );

is executed. Count is incremented again, the condition re-evaluated etc, until count reaches a value of 11.

When this occurs, the conditional test

evaluates as FALSE, and the for loop terminates, and program control passes to the statement

printf(“n”);

which prints a newline, and then the program terminates, as there are no more statements left to execute.

/* sample program using a for statement */

int n, t_number;

for( n = 1; n <= 200; n = n + 1 )

t_number = t_number + n;

printf(“The 200th triangular_number is %dn”, t_number);

Sample Program Output

The 200th triangular_number is 20100

The above program uses a for loop to calculate the s
um of the numbers from 1 to 200 inclusive (said to be the triangular number).

The following diagram shows the order of processing each part of a for

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

An example of using a for loop to print out characters

for( letter = ‘A’; letter <= 'E'; letter = letter + 1 ) {

printf(“%c “, letter);

Sample Program Output

An example of using a for loop to count numbers, using two initialisations

int total, loop;

for( total = 0, loop = 1; loop <= 10; loop = loop + 1 ){

total = total + loop;

printf(“Total = %dn”, total );

Sample Program Output

In the above example, the variable total is initialised to 0 as the first part of the for loop. The two statements,

for( total = 0, loop = 1;

are part of the initialisation. This illustrates that more than one statement is allowed, as long as they are separated by commas.

Graphical Animation of for loop
To demonstrate the operation of the for statement, lets consider a series of animations.

The code we will be using is

for( x = 1; x <= 6; x = x + 1 ) {

printf(“%d”, y );

printf(“n%d”, z );

Sample Program Output

The following diagram shows the initial state of the program, after the initialization of the variables x, y, and z.

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

On entry to the for statement, the first expression is executed, which in our example assigns the value 1 to x. This can be seen in the graphic shown below (Note: see the Variable Values: section)

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

The next part of the for is executed, which tests the value of the loop variable x against the constant 6.

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

It can be seen from the variable window that x has a current value of 1, so the test is successful, and program flow branches to execute the statements of the for body, which prints out the value of y, then adds 1 to y. You can see the program output and the state of the variables shown in the graphic below.

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

After executing the statements of the for body, execution returns to the last part of the for statement. Here, the value of x is incremented by 1. This is seen by the value of x changing to 2.

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

Next, the condition of the for variable is tested again. It continues because the value of it (2) is less than 6, so the body of the loop is executed again.

Execution continues till the value of x reaches 7. Lets now jump ahead in the animation to see this. Here, the condition test will fail, and the for statement finishes, passing control to the statement which follows.

EcoleBooks | COMPUTER A LEVEL(FORM SIX) NOTES - C PROGRAMMING

EXERCISE C6
Rewrite the previous program by calculating the 200th triangular number, and make the program shorter (if possible).

CLASS EXERCISE C7
What is the difference between the two statements,

CLASS EXERCISE C8
Change the printf line of the a
bove program to the following,

printf(” %2d %2dn”,n,t_number);

What does the inclusion of the 2 in the %d statements achieve?

EXERCISE C9
Create a C program which calculates the triangular number of the users request, read from the keyboard using scanf(). A triangular number is the sum of the preceding numbers, so the triangular number 7 has a value of

7 + 6 + 5 + 4 + 3 + 2 + 1

Answer: EXERCISE C6

int n = 1, t_number = 0;

for( n <= 200; n++ )

t_number = t_number + n;

printf(“The 200th triangular_number is %dn”, t_number);

Answer: CLASS EXERCISE C7

a == 2 equality test

a = 2 assignment

Answer: CLASS EXERCISE C8
The inclusion of the 2 in the %d statements achieves a field width of two places, and prints a leading 0 where the value is less than 10.

Answer: EXERCISE C9

int n = 1, t_number = 0, input;

printf(“Enter a numbern”);

scanf(“%d”, &input);

for( n <= input; n++ )

t_number = t_number + n;

printf(“The triangular_number of %d is %dn”, input, t_number);

Practice Exercise 4: for loops

1. Write a for loop to print out the values 1 to 10 on separate lines.

2. Write a for loop which will produce the following output (hint: use two nested for loops)

3. Write a for loop which sums all values between 10 and 100 into a variable called total. Assume that total has NOT been initialized to zero.

4. Write a for loop to print out the character set from A-Z.

PRACTISE EXERCISE 4

for loops

1. Write a for loop to print out the values 1 to 10 on separate lines.

for( loop = 1; loop <= 10; loop = loop + 1 )

printf(“%dn”, loop)

2. Write a for loop which will produce the following output (hint: use two nested for loops)

for( loop = 1; loop <= 5; loop = loop + 1 )

for( count = 1; count <= loop; count = count + 1 )

printf(“%d”, loop );

printf(“n”);

3. Write a for loop which sums all values between 10 and 100 into a variable called total. Assume that total has NOT been initialized to zero.

for( loop = 10, total = 0; loop <= 100; loop = loop + 1 )

total = total + loop;

4. Write a for loop to print out the character set from A-Z.

for( ch = ‘A’; ch <= 'Z'; ch = ch + 1 )

printf(“%c”, ch );

printf(“n”);

THE WHILE STATEMENT
The while provides a mechanism for repeating C statements whilst a condition is true. Its format is,

while( condition )

program statement;

Somewhere within the body of the while loop a statement must alter the value of the condition to allow the loop to finish.

/* Sample program including while */

while( loop <= 10 ) {

printf(“%dn”, loop);

Sample Program Output

The above program uses a while loop to repeat the statements

printf(“%dn”, loop);

whilst the value of the variable loop is less than or equal to 10.

Note how the variable upon which the while is dependent is initialised prior to the while statement (in this case the previous line), and also that the value of the variable is altered within the loop, so that eventually the conditional test will succeed and the while loop will terminate.

This program is functionally equivalent to the earlier for program which counted to ten.

THE DO WHILE STATEMENT
The do { } while statement allows a loop to continue whilst a condition evaluates as TRUE (non-zero). The loop is executed as least once.

/* Demonstration of DO…WHILE */

int value, r_digit;

printf(“Enter the number to be reversed.n”);

scanf(“%d”, &value);

r_digit = value % 10;

printf(“%d”, r_digit);

value = value / 10;

} while( value != 0 );

printf(“n”);

The above program reverses a number that is entered by the user. It does this by using the modulus % operator to extract the right most digit into the variable r_digit. The original number is then divided by 10, and the operation repeated whilst the number is not equal to 0.

It is our contention that this programming construct is improper and should be avoided. It has potential problems, and you should be aware of these.

One such problem is deemed to be lack of control. Considering the above program code portion,

r_digit = value % 10;

printf(“%d”, r_digit);

value = value / 10;

} while( value != 0 );

there is NO choice whether to execute the loop. Entry to the loop is automatic, as you only get a choice to continue.

Another problem is that the loop is always executed at least once. This is a by-product of the lack of control. This means its possible to enter a do { } while loop with invalid data.

Beginner programmers can easily get into a whole heap of trouble, so our advice is to avoid its use. This is the only time that you will encounter it in this course. Its easy to avoid the use of this construct by replacing it with the following algorithms,

initialise loop control variable

while( loop control variable is valid ) {

adjust control variable if necessary

Okay, lets now rewrite the above example to remove the do { } while construct.

/* rewritten code to remove construct */

int value, r_digit;

while( value <= 0 ) {

printf(“Enter the number to be reversed.n”);

scanf(“%d”, &value);

if( value <= 0 )

printf(“The number must be positiven”);

while( value != 0 )

r_digit = value % 10;

printf(“%d”, r_digit);

value = value / 10;

printf(“n”);

Sample Program Output

Enter the number to be reversed.

The number must be positive

Enter the number to be reversed.

SELECTION (IF STATEMENTS)

The if statements allows branching (decision making) depending upon the value or state of variables. This allows statements to be executed or skipped, depending upon decisions. The basic format is,

if( expression )

program statement;

if( students < 65 )

++student_count;

In the above example, the variable student_count is incremented by one only if the value of the integer variable students is less than 65.

The following program uses an if statement to validate the users input to be in the range 1-10.

while( valid == 0 ) {

printf(“Enter a number between 1 and 10 –>”);

scanf(“%d”, &number);

/* assume number is valid */

if( number < 1 ) {

printf(“Number is below 1. Please re-entern”);

if( number > 10 ) {

printf(“Number is above 10. Please re-entern”);

printf(“The number is %dn”, number );

Sample Program Output

Enter a number between 1 and 10 –> -78

Number is below 1. Please re-enter

Enter a number between 1 and 10 –> 4

The number is 4

EXERCISE C10
Write a C program that allows the user to enter in 5 grades, ie, marks between 0 – 100. The program must calculate the average mark, and state the number of marks less than 65.

Consider the following program which determines whether a character entered from the keyboard is within the range A to Z.

printf(“Enter a character –>”);

scanf(” %c”, &letter );

if( letter >= ‘A’ ) {

if( letter <= 'Z' )

printf(“The character is within A to Zn”);

Sample Program Output

Enter a character –> C

The character is within A to Z

The program does not print any output if the character entered is not within the range A to Z. This can be addressed on the following pages with the if else construct.

Please note use of the leading space in the statement (before %c)

scanf(” %c”, &letter );

This enables the skipping of leading TABS, Spaces, (collectively called whitespaces) and the ENTER KEY. If the leading space was not used, then the first entered character would be used, and scanf would not ignore the whitespace characters.

COMPARING float types FOR EQUALITY
Because of the way in which float types are stored, it makes it very difficult to compare float types for equality. Avoid trying to compare float variables for equality, or you may encounter unpredictable results.

if else
The general format for these are,

if( condition 1 )

else if( condition 2 )

else if( condition 3 )

The else clause allows action to be taken where the condition evaluates as false (zero).

The following program uses an if else statement to validate the users input to be in the range 1-10.

while( valid == 0 ) {

printf(“Enter a number between 1 and 10 –>”);

scanf(“%d”, &number);

if( number < 1 ) {

printf(“Number is below 1. Please re-entern”);

else if( number > 10 ) {

printf(“Number is above 10. Please re-entern”);

printf(“The number is %dn”, number );

Sample Program Output

Enter a number between 1 and 10 –> 12

Number is above 10. Please re-enter

Enter a number between 1 and 10 –> 5

The number is 5

This program is slightly different from the previous example in that an else clause is used to set the variable valid to 1. In this program, the logic should be easier to follow.

/* Illustates nested if else and multiple arguments to the scanf function. */

int invalid_operator = 0;

float number1, number2, result

printf(“Enter two numbers and an operator in the formatn”);

printf(” number1 operator number2n”);

scanf(“%f %c %f”, &number1, &operator, &number2);

if(operator == ‘*’)

result = number1 * number2;

else if(operator == ‘/’)

result = number1 / number2;

else if(operator == ‘+’)

result = number1 + number2;

else if(operator == ‘-‘)

result = number1 – number2;

invalid_operator = 1;

if( invalid_operator != 1 )

printf(“%f %c %f is %fn”, number1, operator, number2, result );

printf(“Invalid operator.n”);

Sample Program Output

Enter two numbers and an operator in the format

number1 operator number2

23.2 + 12 is 35.2

The above program acts as a simple calculator.

Practice Exercise 5: while loops and if else

1. Use a while loop to print the integer values 1 to 10 on the screen

2. Use a nested while loop to reproduce the following output

3. Use an if statement to compare the value of an integer called sum against the value 65, and if it is less, print the text string “Sorry, try again”.

4. If total is equal to the variable good_guess, print the value of total, else print the value of good_guess.

Answers: Practice Exercise 5: while loops and if else

1. Use a while loop to print the integer values 1 to 10 on the screen

while( loop <= 10 ) {

printf(“%d”, loop);

printf(“n”);

2. Use a nested while loop to reproduce the following output

while( loop <= 5 ) {

while( count <= loop ) {

printf(“%d”, count);

printf(“n”);

3. Use an if statement to compare the value of an integer called sum against the value 65, and if it is less, print the text string “Sorry, try again”.

printf(“Sorry, try again.n”);

4. If total is equal to the variable good_guess, print the value of total, else print the value of good_guess.

if( total == good_guess )

printf(“%dn”, total );

printf(“%dn”, good_guess );

COMPOUND RELATIONALS ( AND, NOT, OR, EOR )

Combining more than one condition
These allow the testing of more than one condition as part of selection statements. The symbols are

Logical and requires all conditions to evaluate as TRUE (non-zero).

Logical or will be executed if any ONE of the conditions is TRUE (non-zero).

logical not negates (changes from TRUE to FALSE, vsvs) a condition.

Logical eor will be excuted if either condition is TRUE, but NOT if they are all true.

The following program uses an if statement with logical OR to validate the users input to be in the range 1-10.

while( valid == 0 ) {

printf(“Enter a number between 1 and 10 –>”);

scanf(“%d”, &number);

if( (number < 1 ) || (number > 10) ){

printf(“Number is outside range 1-10. Please re-entern”);

printf(“The number is %dn”, number );

Sample Program Output

Enter a number between 1 and 10 –> 56

Number is outside range 1-10. Please re-enter

Enter a number between 1 and 10 –> 6

The number is 6

This program is slightly different from the previous example in that a LOGICAL OR eliminates one of the else clauses.

COMPOUND RELATIONALS ( AND, NOT, OR, EOR )

printf(“The flag is not set.n”);

printf(“The value of flag is %dn”, flag);

Sample Program Output

The flag is not set.

The value of flag is 1

The program tests to see if flag is not (!) set; equal to zero. It then prints the appropriate message, changes the state of flag flag becomes equal to not flag equal to 1. Finally the value of flag is printed.

COMPOUND RELATIONALS ( AND, NOT, OR, EOR )

Range checking using Compound Relational
Consider where a value is to be inputted from the user, and checked for validity to be within a certain range, lets say between the integer values 1 and 100.

while( valid == 0 ) {

printf(“Enter a number between 1 and 100”);

scanf(“%d”, &number );

if( (number < 1) || (number > 100) )

printf(“Number is outside legal rangen”);

printf(“Number is %dn”, number );

Sample Program Output

Enter a number between 1 and 100

Number is outside legal range

Enter a number between 1 and 100

Number is outside legal range

Enter a number between 1 and 100

The program uses valid, as a flag to indicate whether the inputted data is within the required range of allowable values. The while loop continues whilst valid is 0.

if( (number < 1) || (number > 100) )

checks to see if the number entered by the user is within the valid range, and if so, will set the value of valid to 1, allowing the while loop to exit.

Now consider writing a program which validates a character to be within the range A-Z, in other words alphabetic.

while( valid == 0 ) {

printf(“Enter a character A-Z”);

scanf(” %c”, &ch );

if( (ch >= ‘A’) && (ch <= 'Z') )

printf(“Character is outside legal rangen”);

printf(“Character is %cn”, ch );

Sample Program Output

Enter a character A-Z

Character is outside legal range

Enter a character A-Z

Character is outside legal range

Enter a character A-Z

In this instance, the AND is used because we want validity between a range, that is all values between a low and high limit. In the previous case, we used an OR statement to test to see if it was outside or below the lower limit or above the higher limit.

witch() case:
The switch case statement is a better way of writing a program when a series of if elses occurs. The general format for this is,

switch ( expression ) {

program statement;

program statement;

program statement;

The keyword break must be included at the end of each case statement. The default clause is optional, and is executed if the cases are not met. The right brace at the end signifies the end of the case selections.

Rules for switch statements

values for ‘case’ must be integer or character constants

the order of the ‘case’ statements is unimportant

the default clause may occur first (convention places it last)

you cannot use expressions or ranges

int menu, numb1, numb2, total;

printf(“enter in two numbers –>”);

scanf(“%d %d”, &numb1, &numb2 );

printf(“enter in choicen”);

printf(“1=additionn”);

printf(“2=subtractionn”);

scanf(“%d”, &menu );

switch( menu ) {

case 1: total = numb1 + numb2; break;

case 2: total = numb1 – numb2; break;

default: printf(“Invalid option selectedn”);

if( menu == 1 )

printf(“%d plus %d is %dn”, numb1, numb2, total );

else if( menu == 2 )

printf(“%d minus %d is %dn”, numb1, numb2, total );

Sample Program Output

enter in two numbers –> 37 23

enter in choice

37 minus 23 is 14

The above program uses a switch statement to validate and select upon the users input choice, simulating a simple menu of choices.

EXERCISE C11
Rewrite the previous program, which accepted two numbers and an operator, using the switch case statement

THE switch case STATEMENT

EXERCISE C11
Rewrite the previous program, which accepted two numbers and an operator, using the switch case statement.

/* Illustates nested if else and multiple arguments to the scanf function. */

int invalid_operator = 0;

float number1, number2, result

printf(“Enter two numbers and an operator in the formatn”);

printf(” number1 operator number2n”);

scanf(“%f %c %f”, &number1, &operator, &number2);

if(operator == ‘*’)

result = number1 * number2;

else if(operator == ‘/’)

result = number1 / number2;

else if(operator == ‘+’)

result = number1 + number2;

else if(operator == ‘-‘)

result = number1 – number2;

invalid_operator = 1;

if( invalid_operator != 1 )

printf(“%f %c %f is %fn”, number1, operator, number2, result );

printf(“Invalid operator.n”);

/* Illustates switch */

int invalid_operator = 0;

float number1, number2, result

printf(“Enter two numbers and an operator in the formatn”);

printf(” number1 operator number2n”);

scanf(“%f %c %f”, &number1, &operator, &number2);

switch( operator ) {

case ‘*’ : result = number1 * number2; break;

case ‘/’ : result = number1 / number2; break;

case ‘+’ : result = number1 + number2; break;

case ‘-‘ : result = number1 – number2; break;

default : invalid_operator = 1;

switch( invalid_operator ) {

case 1 : printf(“Invalid operator.n”); break;

default : printf(“%f %c %f is %fn”, number1, operator, number2, result );

Practice Exercise 6

Compound Relational and switch

1. if sum is equal to 10 and total is less than 20, print the text string “incorrect.”.

2. if flag is 1 or letter is not an ‘X’, then assign the value 0 to exit_flag, else set exit_flag to 1.

3. rewrite the following statements using a switch statement

if( letter == ‘X’ )

else if ( letter == ‘Z’ )

valid_flag = 1;

else if( letter == ‘A’ )

printf(“Unknown letter –>%cn”, letter );

Answers: Practice Exercise 6

Compound Relational and switch

1. if sum is equal to 10 and total is less than 20, print the text string “incorrect.”.

if( (sum == 10) && (total < 20) )

printf(“incorrect.n”);

2. if flag is 1 or letter is not an ‘X’, then assign the value 0 to exit_flag, else set exit_flag to 1.

if( (flag == 1) || (letter != ‘X’) )

3. rewrite the following statements using a switch statement

if( letter == ‘X’ )

else if ( letter == ‘Z’ )

valid_flag = 1;

else if( letter == ‘A’ )

printf(“Unknown letter –>%cn”, letter );

switch( letter ) {

case ‘X’ : sum = 0; break;

case ‘Z’ : valid_flag = 1; break;

case ‘A’ : sum = 1; break;

default : printf(“Unknown letter –>%cn”, letter );

ACCEPTING SINGLE CHARACTERS FROM THE KEYBOARD

getchar
The following program illustrates this,

for( i = 1; i<= 5; ++i ) {

ch = getchar();

Sample Program Output

The program reads five characters (one for each iteration of the for loop) from the keyboard. Note that getchar() gets a single character from the keyboard, and putchar() writes a single character (in this case, ch) to the console screen.

The file ctype.h provides routines for manipulating characters.

BUILT IN FUNCTIONS FOR STRING HANDLING

string.h
You may want to look at the section on arrays first!. The following macros are built into the file string.h

strcat Appends a string

strchr Finds first occurrence of a given character

strcmp Compares two strings

strcmpi Compares two strings, non-case sensitive

strcpy Copies one string to another

strlen Finds length of a string

strlwr Converts a string to lowercase

strncat Appends n characters of string

strncmp Compares n characters of two strings

strncpy Copies n characters of one string to another

strnset Sets n characters of string to a given character

strrchr Finds last occurrence of given character in string

strrev Reverses string

strset Sets all characters of string to a given character

strspn Finds first substring from given character set in string

strupr Converts string to uppercase

To convert a string to uppercase

char name[80]; /* declare an array of characters 0-79 */

printf(“Enter in a name in lowercasen”);

scanf( “%s”, name );

strupr( name );

printf(“The name is uppercase is %s”, name );

Sample Program Output

Enter in a name in lowercase

The name in uppercase is SAMUEL

BUILT IN FUNCTIONS FOR CHARACTER HANDLING
The following character handling functions are defined in ctype.h

isalnum Tests for alphanumeric character

isalpha Tests for alphabetic character

isascii Tests for ASCII character

iscntrl Tests for control character

isdigit Tests for 0 to 9

isgraph Tests for printable character

islower Tests for lowercase

isprint Tests for printable character

ispunct Tests for punctuation character

isspace Tests for space character

isupper Tests for uppercase character

isxdigit Tests for hexadecimal

toascii Converts character to ascii code

tolower Converts character to lowercase

toupper Converts character to uppercase

To convert a string array to uppercase a character at a time using toupper()

printf(“Enter in a name in lowercasen”);

scanf( “%s”, name );

for( loop = 0; name[loop] != 0; loop++ )

name[loop] = toupper( name[loop] );

printf(“The name is uppercase is %s”, name );

Sample Program Output

Enter in a name in lowercase

The name in uppercase is SAMUEL

Validation Of User Input In C

Basic Rules

/* example one, a simple continue statement */

int valid_input; /* when 1, data is valid and loop is exited */

char user_input; /* handles user input, single character menu choice */

valid_input = 0;

while( valid_input == 0 ) {

printf(“Continue (Y/N)?n”);

scanf(” %c”, &user_input );

user_input = toupper( user_input );

if((user_input == ‘Y’) || (user_input == ‘N’) ) valid_input = 1;

else printf(“07Error: Invalid choicen”);

Sample Program Output

Continue (Y/N)?

Error: Invalid Choice

Continue (Y/N)?

/* example two, getting and validating choices */

int exit_flag = 0, valid_choice;

char menu_choice;

while( exit_flag == 0 ) {

valid_choice = 0;

while( valid_choice == 0 ) {

printf(“nC = Copy FilenE = ExitnM = Move Filen”);

printf(“Enter choice:n”);

scanf(” %c”, &menu_choice );

if((menu_choice==’C’) || (menu_choice==’E’) || (menu_choice==’M’))

valid_choice = 1;

printf(“07Error. Invalid menu choice selected.n”);

switch( menu_choice ) {

case ‘C’ : ………………..(); break;

case ‘E’ : exit_flag = 1; break;

case ‘M’ : ………………..(); break;

default : printf(“Error— Should not occur.n”); break;

Sample Program Output

Error. Invalid menu choice selected.

Other validation examples

THE CONDITIONAL EXPRESSION OPERATOR
This conditional expression operator takes THREE operators. The two symbols used to denote this operator are the ? and the :. The first operand is placed before the ?, the second operand between the ? and the :, and the third after the :. The general format is,

condition ? expression1 : expression2

If the result of condition is TRUE ( non-zero ), expression1 is evaluated and the result of the evaluation becomes the result of the operation. If the condition is FALSE (zero), then expression2 is evaluated and its result becomes the result of the operation. An example will help,

s = ( x < 0 ) ? -1 : x * x;

If x is less than zero then s = -1

If x is greater than zero then s = x * x

Example program illustrating conditional expression operator

printf(“I will tell you if the number is positive, negative or zero!”n”);

printf(“please enter your number now—>”);

scanf(“%d”, &input );

(input < 0) ? printf("negativen") : ((input > 0) ? printf(“positiven”) : printf(“zeron”));

Sample Program Output

I will tell you if the number is positive, negative or zero!

please enter your number now—> 32

CLASS EXERCISE C12
Evaluate the following expression, where a=4, b=5

least_value = ( a < b ) ? a : b;

Answers: CLASS EXERCISE C12
Evaluate the following expression, where a=4, b=5

least_value = ( a < b ) ? a : b;

least_value = 4

Little Boxes on the hillside
Arrays are a data structure which hold multiple variables of the same data type. Consider the case where a programmer needs to keep track of a number of people within an organisation. So far, our initial attempt will be to create a specific variable for each user. This might look like,

int name1 = 101;

int name2 = 232;

int name3 = 231;

It becomes increasingly more difficult to keep track of this as the number of variables increase. Arrays offer a solution to this problem.

An array is a multi-element box, a bit like a filing cabinet, and uses an indexing system to find each variable stored within it. In C, indexing starts at zero.

Arrays, like other variables in C, must be declared before they can be used.

The replacement of the above example using arrays looks like,

names[0] = 101;

names[1] = 232;

names[2] = 231;

We created an array called names, which has space for four integer variables. You may also see that we stored 0 in the last space of the array. This is a common technique used by C programmers to signify the end of an array.

Arrays have the following syntax, using square brackets to access each indexed value (called an element).

so that x[5] refers to the sixth element in an array called x. In C, array elements start with 0. Assigning values to array elements is done by,

and assigning array elements to a variable is done by,

In the following example, a character based array named word is declared, and each element is assigned a character. The last element is filled with a zero value, to signify the end of the character string (in C, there is no string type, so character based arrays are used to hold strings). A printf statement is then used to print out all elements of the array.

/* Introducing array’s, 2 */

word[0] = ‘H’;

word[1] = ‘e’;

word[2] = ‘l’;

word[3] = ‘l’;

word[4] = ‘o’;

printf(“The contents of word[] is –>%sn”, word );

Sample Program Output

The contents of word[] is Hello

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