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Lectu re 6

Construct 3 Repetition / looping

By Prof. Elijah OmwengaOctober 2012

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In t roduct ion to Loop ing

Loops cause a section of your program to berepeated a certain number of times. Therepetition continues while a condition is true.

When a condition becomes false, the loopends and control is passed to the statementsfollowing the loop. If a loop continuesforever, it is called an infinite loop.

In looping, a sequence of statements isexecuted until some condition for thetermination of the loop are satisfied.

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A program loop therefore consists of twosegments:

1.Control statement.2.Body of the loop

The control statement sets certain conditionsand then directs the repeated execution ofthe statements contained in the body of theloop.

There are three kinds of loops in C: the forloop, the while loop, and the do loop.

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The for loopThe forloop is an entry controlled loop that provides aconcise loop control. The general form of the loop is:

for (initialisation; test condition; increment){

statement;statement;

} // No semicolon here

The execution of the forstatementis done as, by firstly

initialising the control variables using assignmentstatements such as i=0. the variables i is known as loopcontrol variable.

The value of the control variable is tested using the testcondition. See flowchart below:

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The for loop

The body of the loop can contain one ormultiple statements. The brace {} areused to group the multiple statements.

However it is a good programming practiceto use braces even if there is only onestatement in the body of the loop.

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The for loop

Test

Body of loop

Exit

True

False

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The test condition is a relational expression,such as i

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#include void main()

{ int count, number, sum;double average;sum =0;for (count=0;count

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The while loop

The test condition is evaluated first and if thecondition is true, the body of the loop is executedzero or more times.

After execution of the body, the test condition is

once again evaluated and if it is true, the body isexecuted once again.

This process of repeated execution of the bodycontinues until the test condition finally become

false. On exit, the program continues with thestatement immediately after the body of the loop.

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whi le loop

True

Test

Body of loop.Increment count

Exit

True

False

initialize count

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The general form of the while loop is:

while (test-condition){

statement;statement;

} // No semicolon here

NB:The difference between while loop and the for loop

is that the forloop does something a fixed numberof times while the while loop can be used where thenumber of loops is not known.

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#include void main(){

int count, number, sum;double average;sum =0;count=0;

while (count

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The do .. wh i le loop

Unlike the while loop in which a condition istested before the loop is executed.

In do.. while (condition), the testing is doneafter the body of the loop is executed at leastonce.

This means that the execution is one ormore times.

See the flowchart on the next slide...

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Do whi le loop

Test

do{Body of loop.

}

Exit

True

False

initialize count

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The general form of the do loop is:do{

statement;statement;

} // No semicolon here

while (test-condition); // semi colons here

On reaching the do statement, the programproceeds to evaluate the body of the loop first. Atthe end of the loop, the test condition in the while

statement is evaluated. If the condition is true, theprogram continues to evaluate the body of the looponce again.

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This process continues as long as thecondition is true.

When the condition becomes false, the loopwill be terminated and the control goes to thestatement that appears immediately after thewhile statement.

Since the test condition is evaluated at thebottom of the loop, the do. Whileconstruct provides an exit controlled loopand therefore the body of the loop is always

executed at least once.

do while loop

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#include

void main()

{

int count, number, sum;double average;

sum =0;

count=0;

do

{

printf ("Enter a number");

scanf("%d", &number);

sum=sum+number;

count++;

}

while (count

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Sentinel controlled loop

In a Sentinel-Controlled loop, a special value called asentinel value is used to change the loop control

expression from true to false.

For example, when reading data we may indicate the "end

of data" by a special value, like -1 and 999. The controlvariable is called sentinel variable.

A sentinel-Controlled loop is often called indefinite

repetition loop because the number of repetitions is not

known before the loop begins executing.

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Sent inel contro l led loop

#include

void main()

{

int number, count,sum;

double average;

sum =0;count=0;

printf ("Enter a number ");

scanf("%d", &number);

while (number!=-1)

{

sum=sum+number;

printf ("Enter a number ");

scanf("%d", &number);

count++;

}

if (count>0)

{

average=(double)sum/count;

printf("The Average is %7.2lf\n",

average);

}

else

printf("Sum is %d\n", sum);

}

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Break and continue

Break and continue are used to alter the flow of control. Break is used to exit from a loop (for, while, or do) or a

switch, passing control to the first statement outside the loop

or switch. A break within a loop should always be protected

within an if statement which provides the test to control the

exit condition. Continue is similar to break but only works within loops

where its effect is to force an immediate jump to the loop

Boolean test.

In a while loop, jump to the test statement. In a do while

loop, jump to the test statement. In a for loop, jump to thetest, and perform the iteration. Like a break, continue should

be protected by an if statement.

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Break example: Prime number testing

Write your own program that tests if the given number is

prime number.#include #include void main() {

int i, n, prime; prime=1; // prime is trueprintf("Input natural number :");scanf("%d", &n);for( i=2; i

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Prime numbers

To test if a numbern is prime, we could loop through 2 ton - 1 and test whether each number divides exactly into n((n % test) == 0). If any of them do, the number is notprime.

However, since by definition any number which is notprime can be divided by at least one other prime number,a more efficient way to do it is to test only the primenumbers less than n.

Therefore, our program maintains a list of prime numbersalready found, and uses that to test n.

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Use of break and continue

Write a program that reads integers. If the given number issmaller than zero, program should print error messageand stop reading numbers.

If the given number is bigger than 100, it should beskipped and program should read another number. Allother numbers should be red and printed. Program must

stop reading numbers when 0 or error shows up.

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Break and con t inue

#include void main() {int x;

do {printf ("Input number :\n");scanf("%d", &x );if (x < 0) {

printf("Not allowed value\n");break; /* escapes the loop */

}

if (x > 100) {printf("Skipping the value\n");

continue;/* jumps to second iteration */}printf ("Given number is : %d", x);

} while (x != 0);}

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Mathematical OperatorsOperator Name /Function Description

+ Addition

The result of addition of two integers is an

integer. The result of addition of a float and aninteger is a float. A float plus a float results in a

float

- Subtraction

The result of subtraction of two integers is an

integer. The result of subtraction of a float and an

integer is a float. A float minus a float results in a

float

* Multiplication

Multiplication of two integers results in an

integer. The result of multiplication of a float and

an integer is a float. A float times a float results in

a float

/ DivisionDivision of two integers results in an integer. Anyremainder is lost. The result of division of a float

and an integer is a float. A float divided by a float

results in a float

% modulusActs on integers. Returns the remainder of an

integer division