12 lec 12 loop

Goals
By the end of this unit, you should
understand …
• … basic loop concepts, including pretest &
posttest loops, loop initialization ,condition
and increment.
• … how to program a while loop.
• … how to program a do-while loop.
• … how to program a for loops.

What is a loop?
• A loop is a programming structure that
allows an action to repeat until the
program meets a given condition.
• After each iteration of a loop, the loop
checks against a loop control expression
to see if the program met the given
condition. If it did, the loop stops. If not,
the loop moves on to the next iteration.

Types of Loops
• C supports two categories of loops,
based on where the program tests the
condition:
– Pretest Loops
– Post-test Loops

Pretest Loops
• With each iteration, the program tests the
condition first before executing the loop’s
block.
• If the condition results to true, the loop
continues and executes the block; if the
condition results to false, the loop
terminates.
• With a pretest loop, there is a chance the
loop may never execute once in the program.

Post-Test Loops
• With each iteration, the program
executes the loop’s block first and tests
against a condition.
• If the condition tests to true, the loop
continues and executes another
iteration; if the condition tests to false,
the loop terminates.
• With a post-test loop, the loop will
always execute at least once!

Pretest vs. Post-Test Loops

from Figure 6-2 in Forouzan & Gilberg, p. 305

Variable Initialization
• int i=5;
• I have initialized the variable i with a value i=5;

Updating a Loop
• A loop update is what happens inside a
loop’s block that eventually causes the
loop to satisfy the condition, thus ending
the loop.
• Updating happens during each loop
iteration.
• Without a loop update, the loop would be
an infinite loop.

Loop Comparison

from Table 6-1 in Forouzan & Gilberg, p. 308

C Implementation of Loops
• Pretest Loops
– while Loop
– for Loop
• Post-test Loop
– do … while loop
• All loops in C execute so long as a
condition evaluates to true and terminate
when the condition evaluates to false.

The while Loop

from Figure 6-11 in Forouzan & Gilberg, p. 311

Example of while loop
#include<stdio.h>
#include<conio.h>
If we input value
int main()
of end as 10 then
{ output will be 1 to
int start, end; 10
scanf("%d",&end);
start = 0;
while ( start < end)
{
start++;
printf("%dn", start);
}
return 0;

Example
#include<stdio.h> The output of the postfix and
prefix increment example will
int main() look like this:
{ int i; 1
2
i = 0; 3
4
while(i++ < 5)
5
{ printf("%dn", i);
} 1
printf("n"); 2
3
i = 0; 4
while(++i < 5)
{ printf("%dn", i); }
return 0; }

The for Loop
• A for loop is a pretest loop that includes three
expressions in its header:
– Loop initialization statement
– Limit test expression(condition)
– Loop update statement

• The for loop is often used as a start-controlled
loop since we can accurately predict the
maximum number of iterations.

Comparing while with for
i = 1; sum = 0;
sum =0; for (i = 1; i <= 20; i++)
while (i <= 20) {
{ scanf(“%d”, &a);
scanf(“%d”, &a); sum = sum+a;
sum = sum+a; }//end for
i++
}//end while
The while Loop The for Loop

Example of for loop
main ( )
{
int p, n, count ;
float r, si ;
for ( count = 1 ; count <= 3 ; count = count + 1 )
{
printf ( "Enter values of p, n, and r " ) ;
scanf ( "%d %d %f", &p, &n, &r ) ;
si = (p * n * r) / 100 ;
printf ( "Simple Interest = Rs.%fn", si ) ;
}
}

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int i = 0, j = 8;
printf("Times 8 Tablen");
for(i = 0; i <= 12; i = i + 1)
{
printf("%d x %d = %dn", i, j, j*i);
}
printf("n");
}

Nested for Loops
• We can nest any statement, even
another for loop, inside the body of
a parent for loop.
• When we nest a child for loop, it
iterates all of it’s cycles for each
iteration of the parent.

Example of nested for loop
• /* Demonstration of nested loops */
When you run this
main( ) program you will get the
{ following output:
r = 1 c = 1 sum = 2
int r, c, sum ; r = 1 c = 2 sum = 3
r = 2 c = 1 sum = 3
for ( r = 1 ; r <= 3 ; r++ ) /* outer loop */ r = 2 c = 2 sum = 4
{ r = 3 c = 1 sum = 4
r = 3 c = 2 sum = 5
for ( c = 1 ; c <= 2 ; c++ ) /* inner loop */
{
sum = r + c ;
printf ( "r = %d c = %d sum = %dn", r, c, sum ) ;
}
}
}

The do … while Loop
• C implements a post-test loop using a
structure called a do … while loop.
• In the do … while, the loop begins with
the keyword do, followed by the body,
followed by the keyword while and the
loop expression.
• A semi-colon (;) follows the loop
expression.

Example of do-while
/* Execution of a loop an unknown number of time*/
main( )
{
Output:
char another ; Enter a number 5
int num ; square of 5 is 25
Want to enter another number y/n y
do Enter a number 7
square of 7 is 49
{ Want to enter another number y/n n
printf ( "Enter a number " ) ;
scanf ( "%d", &num ) ;
printf ( "square of %d is %d", num, num * num ) ;
printf ( "nWant to enter another number y/n " ) ;
scanf ( " %c", &another ) ;
} while ( another == 'y' ) ;}

Same using for loop
/* odd loop using a for loop */
main( )
{
char another = 'y' ;
int num ;
for ( ; another == 'y' ; )
{

Same using while loop
/* odd loop using a while loop */
main( )
{
char another = 'y' ;
int num ;
while ( another == 'y' )
{
}}

Comparing
do … while with while

The break Statement in Loops
• We often come across situations where we want
to jump out of a loop instantly, without waiting to
get back to the conditional test.
• The keyword break allows us to do this.
• When break is encountered inside any loop,
control automatically passes to the first
statement after the loop.

Example of break
• Write a program to determine whether a number is
prime or not. A prime number is one, which is
divisible only by 1 or itself.
• All we have to do to test whether a number is prime or
not, is to divide it successively by all numbers from 2 to
one less than itself.
• If remainder of any of these divisions is zero, the number
is not a prime. If no division yields a zero then the
number is a prime number.

Example
main( )
{int num, i ;
i=2;
while ( i <= num - 1 )
{if ( num % i == 0 )
{
printf ( "Not a prime number" ) ;
break ;
}
i++ ;
} if ( i == num )
printf ( "Prime number" ) ;
}

Example
main( )
The keyword break
breaks the {
control only int i = 1 , j = 1 ;
from the while in while ( i++ <= 100 )
which it is placed. {
while ( j++ <= 200 )
{
if ( j == 150 )
break ;
else
printf ( "%d %dn", i, j ) ;
}
}
}

The continue Statement
• In some programming situations we want to take
the control to the beginning of the loop,
bypassing the statements inside the loop,
which have not yet been executed. The keyword
continue allows us to do this.
• When continue is encountered inside any
loop, control automatically passes to the
beginning of the loop.

Example of continue statement
main( )
{
The output of the
int i, j ;
above program would
for ( i = 1 ; i <= 2 ; i++ ) be...
{ 12
for ( j = 1 ; j <= 2 ; j++ ) 21
{
if ( i == j )
continue ;
printf ( "n%d %dn", i, j ) ;
}
}

12 lec 12 loop

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