BRANCHING AND LOOPING in C

6. Write short notes on the following: (JAN 2009)
for loop
while loop
dowhile loop
Switch case  (MAY 2009/FEB 2009/FEB 2010)

1. 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 */
#include <stdio.h>
main() /* Program introduces the for statement, counts to ten */
{
int count;
for( count = 1; count <= 10; count = count + 1 )
printf(“%d “, count );

printf(“\n”);
}
The program declares an integer variable count. The first part of the for statement for (count = 1; initialized the value of count to 1.
The for loop continues with the condition count <= 10; evaluates as TRUE. As
the variable count has just been initialized 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, count <= 10; 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 count <= 10; 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

2. 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.
Example:
/* Sample program including while */
#include <stdio.h>
main()
{
int loop = 0;
while( loop <= 10 ) {
printf(“%d\n”, loop);
++loop;
}
}
The above program uses a while loop to repeat the statements
printf(“%d\n”,loop); ++loop; the value of the variable loop is less than or
equal to 10.
3. 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.
Example:
/* Demonstration of DO...WHILE */
#include <stdio.h>
main()
{
int value, r_digit;
printf(“Enter the number to be reversed.\n”);
scanf(“%d”, &value);
do {
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.

4. SWITCH 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 ) { case value1:
program statement;
program statement;
...... break;
case valuen:
program statement;
....... break; default:
.......
....... break;
}
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.
Example:
#include <stdio.h>
main()
{
int menu, numb1, numb2, total;
printf(“enter in two numbers à”); scanf(“%d %d”, &numb1, &numb2 ); printf(“enter in choice\n”); printf(“1=addition\n”); printf(“2=subtraction\n”);
scanf(“%d”, &menu );
switch( menu ) {
case 1: total = numb1 + numb2; break;
case 2: total = numb1 – numb2; break;
default: printf(“Invalid option selected\n”);
}
if( menu == 1 )
printf(“%d plus %d is %d\n”, numb1, numb2, total );
else if( menu == 2 )
printf(“%d minus %d is %d\n”, numb1, numb2, total );
}
The above program uses a switch statement to validate and select upon the users input choice, simulating a simple menu of choices.