6/16/24, 11:56 AM C - Loops
C - Loops
Loops are a programming construct that denote a block of one or more statements
that are repeatedly executed a specified number of times, or till a certain condition is
reached.
Repetitive tasks are common in programming, and loops are essential to save time
and minimize errors. In C programming, the keywords while, do–while and for are
provided to implement loops.
Looping constructs are an important part of any processing logic, as they help in
performing the same process again and again. A C programmer should be well
acquainted with implementing and controlling the looping construct.
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.
Flowchart of C Loop Statement
Given below is the general flowchart of a loop statement which is applicable to any
programming language −
The statements in a C program are always executed in a top-to-bottom manner. If
we ask the compiler to go back to any of the earlier steps, it constitutes a loop.
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Example: Loops in C
To understand the need of loops in a program, consider the following snippet −
#include <stdio.h>
int main (){
// local variable definition
int a = 1;
printf("a: %d\n", a);
a++;
printf("a: %d\n", a);
a++;
printf("a: %d\n", a);
a++;
printf("a: %d\n", a);
a++;
printf("a: %d\n", a);
return 0;
}
Output
On running this code, you will get the following output −
a: 1
a: 2
a: 3
a: 4
a: 5
The program prints the value of "a", and increments its value. These two steps are
repeated a number of times. If you need to print the value of "a" from 1 to 100, it is
not desirable to manually repeat these steps in the code. Instead, we can ask the
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6/16/24, 11:56 AM C - Loops
compiler to repeatedly execute these two steps of printing and incrementing till it
reaches 100.
Example: Using While Loop in C
You can use for, while or do-while constructs to repeat a loop. The following
program shows how you can print 100 values of "a" using the "while" loop in C −
#include <stdio.h>
int main () {
// local variable definition
int a = 1;
while (a <= 100){
printf("a: %d\n", a);
a++;
}
return 0;
}
Output
Run this code and check the output −
a: 1
a: 2
a: 3
a: 4
.....
.....
a: 98
a: 99
a: 100
If a step redirects the program flow to any of the earlier steps, based on any
condition, the loop is a conditional loop. The repetitions will stop as soon as the
controlling condition turns false. If the redirection is done without any condition, it is
an infinite loop, as the code block repeats forever.
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6/16/24, 11:56 AM C - Loops
Parts of C Loops
To constitute a loop, the following elements are necessary −
Looping statement (while, do–while or for)
Looping block
Looping condition
Loops are generally of two types −
Counted Loops in C
If the loop is designed to repeat for a certain number of times, it is a counted loop.
In C, the for loop is an example of counted loop.
Conditional Loops in C
If the loop is designed to repeat till a condition is true, it is a conditional loop. The
while and do–while constructs help you to form conditional loops.
Looping Statements in C
C programming provides the following types of loops to handle looping requirements
−
Sr.No. Loop Type & Description
while loop
1 Repeats a statement or group of statements while a given condition is
true. It tests the condition before executing the loop body.
for loop
2 Executes a sequence of statements multiple times and abbreviates the
code that manages the loop variable.
do-while loop
3 It is more like a while statement, except that it tests the condition at the
end of the loop body.
4 nested loops
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You can use one or more loops inside any other while, for or do-while
loop.
Each of the above loop types have to be employed depending upon which one is
right for the given situation. We shall learn about these loop types in detail in the
subsequent chapters.
Loop Control Statements in C
Loop control statements change the execution from its normal sequence. When
execution leaves a scope, all automatic objects that were created in that scope are
destroyed.
C supports the following control statements −
Sr.No. Control Statement & Description
break statement
1 Terminates the loop or switch statement and transfers execution to the
statement immediately following the loop or switch.
continue statement
2 Causes the loop to skip the remainder of its body and immediately retest
its condition prior to reiterating.
goto statement
3
Transfers the control to the labeled statement.
The break and continue statements have contrasting purposes. The goto
statement acts as a jump statement if it causes the program to go to a later
statement. If the goto statement redirects the program to an earlier statement,
then it forms a loop.
The Infinite Loop in C
A loop becomes an infinite loop if a condition never becomes false. An infinite loop
is a loop that repeats indefinitely because it has no terminating condition, or the
termination condition is never met or the loop is instructed to start over from the
beginning.
Although it is possible for a programmer to intentionally use an infinite loop, they are
often mistakes made by new programmers.
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Example: Infinite Loop in C
The for loop is traditionally used for creating an infinite loop. Since none of the three
expressions that form the "for" loop are required, you can make an endless loop by
leaving the conditional expression empty.
#include <stdio.h>
int main (){
for( ; ; ){
printf("This loop will run forever. \n");
}
return 0;
}
Output
By running this code, you will get an endless loop that will keep printing the same
line forever.
This loop will run forever.
This loop will run forever.
........
........
This loop will run forever.
When the conditional expression is absent, it is assumed to be true. You may have
an initialization and increment expression, but C programmers more commonly use
the for(;;) construct to signify an infinite loop.
Note − You can terminate an infinite loop by pressing the "Ctrl + C" keys.
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