Loops

LOOPS

You may encounter situations when a block of code needs to be executed
several number of times. In general, statements are executed sequentially: The
first statement in a function is executed first, followed by the second, and so on.
Programming languages provide various control structures that allow for more
complicated execution paths.
A loop statement allows us to execute a statement or group of statements
multiple times. Given below is the general form of a loop statement in most of
the programming languages:
C programming language provides the following types of loops to handle looping
requirements.

Loop Type Description

while loop Repeats a statement or group of statements while a
given condition is true. It tests the condition before
executing the loop body.
for loop Executes a sequence of statements multiple times and
abbreviates the code that manages the loop variable.
11. LOOPS
do...while loop It is more like a while statement, except that it tests
the condition at the end of the loop body.
nested loops You can use one or more loops inside any other while,
for, or do..while loop.

While Loop

A while loop in C programming repeatedly executes a target statement as long
as a given condition is true.

Syntax
The syntax of a while loop in C programming language is:

while(condition) { statement(s); }

Here, statement(s) may be a single statement or a block of statements.
The condition may be any expression, and true is any nonzero value. The loop
iterates while the condition is true.
When the condition becomes false, the program control passes to the line
immediately following the loop.

Flow Diagram

Here, the key point to note is that a while loop might not execute at all. When
the condition is tested and the result is false, the loop body will be skipped and
the first statement after the while loop will be executed.
Example

#include <stdio.h> int main () { /* local variable definition */ int a = 10; /* while loop execution */ while( a < 20 ) { printf("value of a: %d\n", a); a++; } return 0; }

When the above code is compiled and executed, it produces the following result:

value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19

for Loop

A for loop is a repetition control structure that allows you to efficiently write a
loop that needs to execute a specific number of times.
Syntax
The syntax of a for loop in C programming language is:

for ( init; condition; increment ) { statement(s); }

Here is the flow of control in a ‘for’ loop:
1. The init step is executed first, and only once. This step allows you to
declare and initialize any loop control variables. You are not required to
put a statement here, as long as a semicolon appears.
2. Next, the condition is evaluated. If it is true, the body of the loop is
executed. If it is false, the body of the loop does not execute and the flow
of control jumps to the next statement just after the ‘for’ loop.
3. After the body of the ‘for’ loop executes, the flow of control jumps back up
to the increment statement. This statement allows you to update any
loop control variables. This statement can be left blank, as long as a
semicolon appears after the condition.
4. The condition is now evaluated again. If it is true, the loop executes and
the process repeats itself (body of loop, then increment step, and then
again condition). After the condition becomes false, the ‘for’ loop
terminates.

Flow Diagram
Example

#include <stdio.h> int main () { /* for loop execution */ for( int a = 10; a < 20; a = a + 1 ) { printf("value of a: %d\n", a); C Programming 63 } return 0; }

When the above code is compiled and executed, it produces the following result:

value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19

do…while Loop

Unlike for and while loops, which test the loop condition at the top of the loop,
the do...while loop in C programming checks its condition at the bottom of the
loop.
A do...while loop is similar to a while loop, except the fact that it is guaranteed
to execute at least one time.
Syntax
The syntax of a do...while loop in C programming language is:

do { statement(s); }while( condition );

Notice that the conditional expression appears at the end of the loop, so the
statement(s) in the loop executes once before the condition is tested.
If the condition is true, the flow of control jumps back up to do, and the
statement(s) in the loop executes again. This process repeats until the given
condition becomes false.

Flow Diagram
Example

#include <stdio.h> int main () { /* local variable definition */ int a = 10; /* do loop execution */ do { printf("value of a: %d\n", a); a = a + 1; }while( a < 20 ); return 0; }

When the above code is compiled and executed, it produces the following result:

value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19

Nested Loops

C programming allows to use one loop inside another loop. The following section
shows a few examples to illustrate the concept.
Syntax
The syntax for a nested for loop statement in C is as follows:

for ( init; condition; increment ) { for ( init; condition; increment ) { statement(s); } statement(s); }

The syntax for a nested while loop statement in C programming language is as
follows:

while(condition) { while(condition) { statement(s); } statement(s); }

The syntax for a nested do...while loop statement in C programming language
is as follows:

do { statement(s); do { statement(s); }while( condition ); }while( condition );

A final note on loop nesting is that you can put any type of loop inside any other
type of loop. For example, a ‘for’ loop can be inside a ‘while’ loop or vice versa.

Example

The following program uses a nested for loop to find the prime numbers from 2
to 100:

#include <stdio.h> int main () { /* local variable definition */ int i, j; for(i=2; i<100; i++) { for(j=2; j <= (i/j); j++) if(!(i%j)) break; // if factor found, not prime if(j > (i/j)) printf("%d is prime\n", i); }return 0; }

When the above code is compiled and executed, it produces the following result:

2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 53 is prime 59 is prime 61 is prime 67 is prime 71 is prime 73 is prime 79 is prime 83 is prime 89 is prime 97 is prime