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Loops in programming come into use when we need to repeatedly execute a block of statements. For
example: Suppose we want to print “Hello World” 10 times. This can be done in two ways as shown
below:
Iterative Method
#include <stdio.h>
Int main()
Return 0;
Output:
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Using Loops
In Loop, the statement needs to be written only once and the loop will be executed 10 times as shown
below.
In computer programming, a loop is a sequence of instructions that is repeated until a certain condition
is reached.
An operation is done, such as getting an item of data and changing it, and then some condition is
checked such as whether a counter has reached a prescribed number.
Counter not Reached: If the counter has not reached the desired number, the next instruction in the
sequence returns to the first instruction in the sequence and repeat it.
Counter reached: If the condition has been reached, the next instruction “falls through” to the next
sequential instruction or branches outside the loop.
Entry Controlled loops: In this type of loops the test condition is tested before entering the loop body.
For Loop and While Loop are entry controlled loops.
Exit Controlled Loops: In this type of loops the test condition is tested or evaluated at the end of loop
body. Therefore, the loop body will execute atleast once, irrespective of whether the test condition is
true or false. Do – while loop is exit controlled loop.
For Loop
A for loop is a repetition control structure which allows us to write a loop that is executed a specific
number of times. The loop enables us to perform n number of steps together in one line.
Syntax:
In for loop, a loop variable is used to control the loop. First initialize this loop variable to some value,
then check whether this variable is less than or greater than counter value. If statement is true, then
loop body is executed and loop variable gets updated . Steps are repeated till exit condition comes.
Initialization Expression: In this expression we have to initialize the loop counter to some value. For
example: int i=1;
Test Expression: In this expression we have to test the condition. If the condition evaluates to true then
we will execute the body of loop and go to update expression otherwise we will exit from the for loop.
For example: I <= 10;
Update Expression: After executing loop body this expression increments/decrements the loop variable
by some value. For example: i++;
Example:
#include <stdio.h>
Int main()
Int i=0;
For (I = 1; I <= 10; i++)
Return 0;
Output:
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
While Loop
While studying for loop we have seen that the number of iterations is known beforehand, i.e. the
number of times the loop body is needed to be executed is known to us. While loops are used in
situations where we do not know the exact number of iterations of loop beforehand. The loop execution
is terminated on the basis of test condition.
Syntax:
We have already stated that a loop is mainly consisted of three statements – initialization expression,
test expression, update expression. The syntax of the three loops – For, while and do while mainly
differs on the placement of these three statements.
Initialization expression;
While (test_expression)
// statements
Update_expression;
Flow Diagram:
Example:
#include <stdio.h>
Int main()
{
// initialization expression
Int I = 1;
// test expression
While (I < 6)
// update expression
I++;
Return 0;
Output:
Hello World
Hello World
Hello World
Hello World
Hello World
Do while loop
In do while loops also the loop execution is terminated on the basis of test condition. The main
difference between do while loop and while loop is in do while loop the condition is tested at the end of
loop body, i.e do while loop is exit controlled whereas the other two loops are entry controlled loops.
Note: In do while loop the loop body will execute at least once irrespective of test condition.
Syntax:
Initialization expression;
Do
// statements
Update_expression;
} while (test_expression);
Flow Diagram:
Example:
Int main()
Do
// loop body
// update expression
I++;
Output:
Hello World
In the above program the test condition (i<1) evaluates to false. But still as the loop is exit – controlled
the loop body will execute once.
An infinite loop (sometimes called an endless loop ) is a piece of coding that lacks a functional exit so
that it repeats indefinitely. An infinite loop occurs when a condition always evaluates to true. Usually,
this is an error.
#include <stdio.h>
Int main ()
Int I;
// This is an infinite for loop as the condition
// expression is blank
For ( ; ; )
/*
While (I != 0)
i-- ;
*/
/*
While (true)
*/
Output:
……………….
Important Points:
Use for loop when number of iterations is known beforehand, i.e. the number of times the loop body is
needed to be executed is known.
Use while loops where exact number of iterations is not known but the loop termination condition is
known.
Use do while loop if the code needs to be executed at least once like in Menu driven programs
Related Articles:
Quiz on Loops
This article is contributed by Harsh Agarwal. If you like GeeksforGeeks and would like to contribute, you
can also write an article using contribute.geeksforgeeks.org or mail your article to
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