Author: saqibkhan

The break statement in C is used in two different contexts. In switch-case, break is placed as the last statement of each case block. The break statement may also be employed in the body of any of the loop constructs (while, dowhile as well as for loops).

When used inside a loop, break causes the loop to be terminated. In the switch-case statement, break takes the control out of the switch scope after executing the corresponding case block.

Flowchart of Break Statement in C

The flowchart of break in loop is as follows −

The following flowchart shows how to use break in switch-case −

In both the scenarios, break causes the control to be taken out of the current scope.

Break Statements in While Loops

The break statement is never used unconditionally. It always appears in the True part of an if statement. Otherwise, the loop will terminate in the middle of the first iteration itself.

while(condition1){......if(condition2)break;......}

Example of break Statement with while Loop

The following program checks if a given number is prime or not. A prime number is not divisible by any other number except itself and 1.

The while loop increments the divisor by 1 and tries to check if it is divisible. If found divisible, the while loop is terminated.

#include <stdio.h>/*break in while loop*/intmain(){int i =2;int x =121;printf("x: %d\n", x);while(i < x/2){if(x % i ==0)break;
  i++;}if(i &gt;= x/2)printf("%d is prime", x);elseprintf("%d is not prime", x);return0;}</code></pre>


Output



On executing this code, you will get the following output −



x: 121
121 is not prime




Now, change the value of "x" to 25 and run the code again. It will produce the following output −



x: 25
25 is not prime




Break Statements in For Loops



You can use a break statement inside a for loop as well. Usually, a for loop is designed to perform a certain number of iterations. However, sometimes it may be required to abandon the loop if a certain condition is reached.



The usage of break in for loop is as follows −



for(init; condition; increment){...if(condition)break;...}



Example of break Statement with for Loop



The following program prints the characters from a given string before a vowel (a, e, I, or u) is detected.



#include <stdio.h>#include <string.h>intmain(){char string[]="Rhythmic";int len =strlen(string);int i;for(i =0; i < len; i++){if(string[i]=='a'|| string[i]=='e'|| string[i]=='i'|| string[i]=='o'|| string[i]=='u')break;printf("%c\n", string[i]);}return0;}



Output



Run the code and check its output −



R
h
y
t
h
m




If break appears in an inner loop of a nested loop construct, it abandons the inner loop and continues the iteration of the outer loop body. For the next iteration, it enters the inner loop again, which may be broken again if the condition is found to be true.



Example of break Statement with Nested for Loops



In the following program, two nested loops are employed to obtain a list of all the prime numbers between 1 to 30. The inner loop breaks out when a number is found to be divisible, setting the flag to 1. After the inner loop, the value of flag is checked. If it is "0", the number is a prime number.



#include <stdio.h>intmain(){int i, num, n, flag;printf("The prime numbers in between the range 1 to 30:\n");for(num =2; num <=30; num++){
  flag =0;for(i =2; i &lt;= num/2; i++){if(num % i ==0){
        flag++;break;}}if(flag ==0)printf("%d is prime\n",num);}return0;}</code></pre>


Output



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




Break Statement in an Infinite Loop



An infinite loop is rarely created intentionally. However, in some cases, you may start an infinite loop and break from it when a certain condition is reached.



Example of break Statement with Infinite Loop



In the following program, an infinite for loop is used. On each iteration, a random number between 1 to 100 is generated till a number that is divisible by 5 is obtained.



#include <stdio.h>#include <stdlib.h>#include <time.h>intmain(){int i, num;printf("Program to get the random number from 1 to 100: \n");srand(time(NULL));for(;;){
  num =rand()%100+1;// random number between 1 to 100printf(" %d\n", num);if(num%5==0)break;}}</code></pre>


Output



On running this code, you will get an output like the one shown here −



Program to get the random number from 1 to 100:
6
56
42
90




Break Statements in Switch Case



To transfer the control out of the switch scope, every case block ends with a break statement. If not, the program falls through all the case blocks, which is not desired.



Example of break Statement with switch



In the following code, a series of if-else statements print three different greeting messages based on the value of a "ch" variable ("m", "a" or "e" for morning, afternoon or evening).



#include <stdio.h>intmain(){/* local variable definition */char ch ='m';printf("Time code: %c\n\n", ch);switch(ch){case'm':printf("Good Morning \n");break;case'a':printf("Good Afternoon \n");break;case'e':printf("Good Evening \n");break;default:printf("Hello");}}



Output



Here, the break statement breaks the program execution after checking the first case.



Time code: m

Good Morning




Now, comment the break statements and run the code again. You will now get the following output −



Time code: m

Good Morning 
Good Afternoon 
Good Evening 
Hello

In C language, an infinite loop (or, an endless loop) is a never-ending looping construct that executes a set of statements forever without terminating the loop. It has a true condition that enables a program to run continuously.

Flowchart of an Infinite Loop

If the flow of the program is unconditionally directed to any previous step, an infinite loop is created, as shown in the following flowchart −

An infinite loop is very rarely created intentionally. In case of embedded headless systems and server applications, the application runs in an infinite loop to listen to the client requests. In other circumstances, infinite loops are mostly created due to inadvertent programming errors.

How to Create an Infinite Loop in C?

To create an infinite loop, you need to use one of the loop constructs (while, do while, or for) with a non-zero value as a test condition. Generally, 1 is used as the test condition, you can use any non-zero value. A non-zero value is considered as true.

Example

#include <stdio.h>intmain(){while(1){printf("Hello World");}return0;}

Hello WorldHello WorldHello WorldHello WorldHello WorldHello World
Hello WorldHello WorldHello WorldHello WorldHello WorldHello World
Hello WorldHello WorldHello ...

Types of Infinite Loops in C

In C language, infinite while, infinite do while, and infinite for are the three infinite loops. These loops execute the code statement continuously. Let us understand the implementation of infinite loops using all loop constructs.

Infinite While Loop

The while keyword is used to form a counted loop. The loop is scheduled to repeat till the value of some variable successively increments to a predefined value. However, if the programmer forgets to put the increment statement within the loop body, the test condition doesnt arrive at all, hence it becomes endless or infinite.

Example 1

Take a look at the following example −

#include <stdio.h>// infinite while loopintmain(){int i =0;while(i <=10){// i++;printf("i: %d\n", i);}return0;}

Output

Since the increment statement is commented out here, the value of “i” continues to remain “0”, hence the output shows “i: 0” continuously until you forcibly stop the execution.

i: 0
i: 0
i: 0
...
...

Example 2

The parenthesis of while keyword has a Boolean expression that initially evaluates to True, and is eventually expected to become False. Note that any non-zero number is treated as True in C. Hence, the following while loop is an infinite loop:

#include <stdio.h>// infinite while loopintmain(){while(1){printf("Hello World \n");}return0;}

Output

It will keep printing “Hello World” endlessly.

Hello World 
Hello World 
Hello World 
...
...

The syntax of while loop is as follows −

while(condition){......}

Note that the there is no semicolon symbol in front of while, indicating that the following code block (within the curly brackets) is the body of the loop. If we place a semicolon, the compiler treats this as a loop without body, and hence the while condition is never met.

Example 3

In the following code, an increment statement is put inside the loop block, but because of the semicolon in front of while, the loop becomes infinite.

#include <stdio.h>// infinite while loopintmain(){int i =0;while(i <10);{
  i++;printf("Hello World \n");}return0;}</code></pre>


Output



When the program is run, it won't print the message "Hello World". There is no output because the while loop becomes an infinite loop with no body.




Infinite For Loop



The for loop in C is used for performing iteration of the code block for each value of a variable from its initial value to the final value, incrementing it on each iteration.



Take a look at its syntax −



for(initial val; final val; increment){......}



Example 1



Note that all the three clauses of the for statement are optional. Hence, if the middle clause that specifies the final value to be tested is omitted, the loop turns infinite.



#include <stdio.h>// infinite for loopintmain(){int i;for(i=1;; i++){
  i++;printf("Hello World \n");}return0;}</code></pre>


Output



The program keeps printing Hello World endlessly until you stop it forcibly, because it has no effect of incrementing "i" on each turn.



Hello World 
Hello World 
Hello World 
...
...




You can also construct a for loop for decrementing the values of a looping variable. In that case, the initial value should be greater than the final test value, and the third clause in for must be a decrement statement (using the "--" operator).



If the initial value is less than the final value and the third statement is decrement, the loop becomes infinite. The loop still becomes infinite if the initial value is larger but you mistakenly used an increment statement.



Hence, both the for loops result in an infinite loop.



Example 2



Take a look at the following example −



#include <stdio.h>intmain(){// infinite for loopfor(int i =10; i >=1; i++){
i++;printf("Hello World \n");}}</code></pre>


Output



The program will print a series of "Hello World" in an infinite loop −



Hello World 
Hello World 
Hello World 
...
...




Example 3



Take a look at the following example −



#include <stdio.h>intmain(){// infinite for loopfor(int i =1; i <=10; i--){
i++;printf("Hello World \n");}}</code></pre>


Output



The program keeps printing "Hello World" in a loop −



Hello World 
Hello World 
Hello World 
...
...




Example 4



If all the three statements in the parenthesis are blank, the loop obviously is an infinite loop, as there is no condition to test.



#include <stdio.h>intmain(){int i;// infinite for loopfor(;;){
  i++;printf("Hello World \n");}}</code></pre>


Output



The program prints "Hello World" in an endless loop −



Hello World 
Hello World 
Hello World 
...
...




Infinite Do While Loop



An infinite loop can also be implemented using the do-while loop construct. You have to use 1 as the test condition with the while.



Example



The following example demonstrates an infinite loop using do while:



#include <stdio.h>intmain(){do{printf("Hello World\n");}while(1);return0;}



Hello World
Hello World
Hello World
Hello World
...
...




How to Break an Infinite Loop in C?



There may be certain situations in programming where you need to start with an unconditional while or for statement, but then you need to provide a way to terminate the loop by placing a conditional break statement.



Example



In the following program, there is no test statement in for, but we used a break statement to make it a finite loop.



#include <stdio.h>intmain(){// infinite while loopfor(int i =1;;){
i++;printf("Hello World \n");if(i ==5)break;}return0;}</code></pre>


Output



The program prints "Hello World" till the counter variable reaches 5.



Hello World
Hello World
Hello World
Hello World




How to Stop an Infinite Loop Forcefully in C?



When the program enters an infinite loop, it doesnt stop on its own. It has to be forcibly stopped. This is done by pressing "Crtrl + C" or "Ctrl + Break" or any other key combination depending on the operating system.



Example



The following C program enters in an infinite loop −



#include <stdio.h>intmain(){// local variable definitionint a =0;// do loop execution
   LOOP:
  a++;printf("a: %d\n", a);goto LOOP;return0;}</code></pre>


Output



When executed, the above program prints incrementing values of "a" from 1 onwards, but it doesnt stop. It will have to be forcibly stopped by pressing "Ctrl + Break" keys.



a: 1
a: 2
...
...
a: 10
a: 11
...
...




Infinite loops are mostly unintentionally created as a result of programming bug. Even if the looping keyword doesnt specify the termination condition, the loop has to be terminated with the break keyword.

In the programming context, the term “nesting” refers to enclosing a particular programming element inside another similar element. For example, nested loops, nested structures, nested conditional statements, etc.

Nested Loops

When a looping construct in C is employed inside the body of another loop, we call it a nested loop (or, loops within a loop). Where, the loop that encloses the other loop is called the outer loop. The one that is enclosed is called the inner loop.

General Syntax of Nested Loops

The general form of a nested loop is as follows −

Outer loop {
   Inner loop {......}...}

C provides three keywords for loops formation − while, do-while, and for. Nesting can be done on any of these three types of loops. That means you can put a while loop inside a for loop, a for loop inside a do-while loop, or any other combination.

The general behaviour of nested loops is that, for each iteration of the outer loop, the inner loop completes all the iterations.

Nested For Loops

Nested for loops are very common. If both the outer and inner loops are expected to perform three iterations each, the total number of iterations of the innermost statement will be “3 * 3 = 9”.

Example: Nested for Loop

Take a look at the following example −

#include <stdio.h>intmain(){int i, j;// outer loopfor(i =1; i <=3; i++){// inner loopfor(j =1; j <=3; j++){printf("i: %d j: %d\n", i, j);}printf("End of Inner Loop \n");}printf("End of Outer Loop");return0;}

Output

When you run this code, it will produce the following output −

i: 1 j: 1
i: 1 j: 2
i: 1 j: 3
End of Inner Loop

i: 2 j: 1
i: 2 j: 2
i: 2 j: 3
End of Inner Loop

i: 3 j: 1
i: 3 j: 2
i: 3 j: 3
End of Inner Loop

End of Outer loop

Explanation of Nested Loop

Now let’s analyze how the above program works. As the outer loop is encountered, “i” which is the looping variable for the outer loop is initialized to 1. Since the test condition (a <= 3) is true, the program enters the outer loop body.

The program reaches the inner loop, and “j” which is the variable that controls the inner loop is initialized to 1. Since the test condition of the inner loop (j <= 3) is true, the program enters the inner loop. The values of “a” and “b” are printed.

The program reaches the end of the inner loop. Its variable “j” is incremented. The control jumps to step 4 until the condition (j <= 3) is true.

As the test condition becomes false (because “j” becomes 4), the control comes out of the inner loop. The end of the outer loop is encountered. The variable “i” that controls the outer variable is incremented and the control jumps to step 3. Since it is the start of the inner loop, “j” is again set to 1.

The inner loop completes its iteration and ends again. Steps 4 to 8 will be repeated until the test condition of the outer loop (i <= 3) becomes false. At the end of the outer loop, “i” and “j” have become 4 and 4 respectively.

The result shows that, for each value of the outer looping variable, the inner looping variable takes all the values. The total lines printed are “3 * 3 = 9”.

Nesting a While Loop Inside a For Loop

Any type of loop can be nested inside any other type. Let us rewrite the above example by putting a while loop inside the outer for loop.

Example: Nested Loops (while Loop Inside for Loop)

Take a look at the following example −

#include <stdio.h>intmain(){int i, j;// outer for loopfor(i =1; i <=3; i++){// inner while loop
  j =1;while(j &lt;=3){printf("i: %d j: %d\n", i, j);
     j++;}printf("End of Inner While Loop \n");}printf("End of Outer For loop");return0;}</code></pre>


Output



i: 1 j: 1
i: 1 j: 2
i: 1 j: 3
End of Inner While Loop

i: 2 j: 1
i: 2 j: 2
i: 2 j: 3
End of Inner While Loop

i: 3 j: 1
i: 3 j: 2
i: 3 j: 3
End of inner while Loop

End of outer for loop




Programmers use nested loops in a lot of applications. Let us take a look at some more examples of nested loops.



C Nested Loops Examples



Example: Printing Tables



The following program prints the tables of 1 to 10 with the help of two nested for loops.



#include <stdio.h>intmain(){int i, j;printf("Program to Print the Tables of 1 to 10 \n");// outer loopfor(i =1; i <=10; i++){// inner loopfor(j =1; j <=10; j++){printf("%4d", i*j);}printf("\n");}return0;}



Output



Run the code and check its output −



Program to Print the Tables of 1 to 10
1    2    3    4    5    6    7    8    9   10
2    4    6    8   10   12   14   16   18   20
3    6    9   12   15   18   21   24   27   30
4    8   12   16   20   24   28   32   36   40
5   10   15   20   25   30   35   40   45   50
6   12   18   24   30   36   42   48   54   60
7   14   21   28   35   42   49   56   63   70
8   16   24   32   40   48   56   64   72   80
9   18   27   36   45   54   63   72   81   90
   10   20   30   40   50   60   70   80   90  100




Example: Printing Characters Pyramid



The following code prints the increasing number of characters from a string.



#include <stdio.h>#include <string.h>intmain(){int i, j, l;char x[]="TutorialsPoint";
   l =strlen(x);// outer loopfor(i =0; i < l; i++){// inner loopfor(j =0; j <= i; j++){printf("%c", x[j]);}printf("\n");}return0;}



Output



When you run this code, it will produce the following output −



T
Tu
Tut
Tuto
Tutor
Tutori
Tutoria
Tutorial
Tutorials
TutorialsP
TutorialsPo
TutorialsPoi
TutorialsPoin
TutorialsPoint




Example: Printing Two-Dimensional Array



In this program, we will show how you can use nested loops to display a two-dimensional array of integers. The outer loop controls the row number and the inner loop controls the columns.



#include <stdio.h>intmain(){int i, j;int x[4][4]={{1,2,3,4},{11,22,33,44},{9,99,999,9999},{10,20,30,40}};// outer loopfor(i=0; i<=3; i++){// inner loopfor(j=0; j <=3; j++){printf("%5d", x[i][j]);}printf("\n");}return0;}



Output



When you run this code, it will produce the following output −



    1    2    3    4
   11   22   33   44
9   99  999 9999
   10   20   30   40

The do-while loop is one of the most frequently used types of loops in C. The do and while keywords are used together to form a loop. The do-while is an exit-verified loop where the test condition is checked after executing the loop’s body. Whereas the while loop is an entry-verified. The for loop, on the other hand, is an automatic loop.

Syntax of do while Loop

The syntax of do-while loop in C is −

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

How do while Loop Works?

The loop construct starts with the keword do. It is then followed by a block of statements inside the curly brackets. The while keyword follows the right curly bracket. There is a parenthesis in front of while, in which there should be a Boolean expression.

Now let’s understand how the while loop works. As the C compiler encounters the do keyword, the program control enters and executes the code block marked by the curly brackets. As the end of the code block is reached, the expression in front of the while keyword is evaluated.

If the expression is true, the program control returns back to the top of loop. If the expression is false, the compiler stops going back to the top of loop block, and proceeds to the immediately next statement after the block. Note that there is a semicolon at the end of while statement.

Flowchart of do while Loop

The following flowchart represents how the do-while loop works −

Since the expression that controls the loop is tested after the program runs the looping block for the first time, the do-while loop is called an “exit-verified loop”. Here, the key point to note is that a do-while loop makes sure that the loop gets executed at least once.

The while keyword implies that the compiler continues to execute the ensuing block as long as the expression is true. However, since the condition sits at the end of the looping construct, it is checked after each iteration (rather than before each iteration as in the case of a while loop).

The program performs its first iteration unconditionally, and then tests the condition. If found to be true, the compiler performs the next iteration. As soon as the expression is found to be false, the loop body will be skipped and the first statement after the while loop will be executed.

Let us try to understand the behaviour of the while loop with a few examples.

Example of do while Loop

The following program prints the Hello world message five times.

#include <stdio.h>intmain(){// local variable definitionint a =1;// while loop executiondo{printf("Hello World\n");
  a++;}while(a &lt;=5);printf("End of loop");return0;}</code></pre>


Output



Here, the do-while loop acts as a counted loop. Run the code and check its output −



Hello World
Hello World
Hello World
Hello World
Hello World
End of loop




The variable "a" that controls the number of repetitions is initialized to 1. The program enters the loop unconditionally, prints the message, increments "a" by 1.



As it reaches the end of the loop, the condition in the while statement is tested. Since the condition "a <= 5" is true, the program goes back to the top of the loop and re-enters the loop.



Now "a" is 2, hence the condition is still true, hence the loop repeats again, and continues till the condition turns false. The loop stops repeating, and the program control goes to the step after the block.



Now, change the initial value of "a" to 10 and run the code again. It will produce the following output −



Hello World
End of loop 




This is because the program enters the looping block unconditionally. Since the condition before the while keyword is false, hence the block is not repeated for the next time. Hence, the do-while loop takes at least one iteration as the test condition is at the end of the loop. For this reason, do-while loop is called an "exit-verified loop".



Difference Between while and do while Loops



The loops constructed with while and do-while appear similar. You can easily convert a while loop into a do-while loop and vice versa. However, there are certain key differences between the two.



The obvious syntactic difference is that the do-while construct starts with the do keyword and ends with the while keyword. The while loop doesn't need the do keyword. Secondly, you find a semicolon in front of while in case of a do-while loop. There is no semicolon in while loops.



Example



The location of the test condition that controls the loop is the major difference between the two. The test condition is at the beginning of a while loop, whereas it is at the end in case of a do-while loop. How does it affect the looping behaviour? Look at the following code −



#include <stdio.h>intmain(){// local variable definitionint a =0, b =0;// while loop executionprintf("Output of while loop: \n");while(a <5){
  a++;printf("a: %d\n", a);}printf("Output of do-while loop: \n");do{
  b++;printf("b: %d\n",b);}while(b &lt;5);return0;}</code></pre>


Output



Initially, "a" and "b" are initialized to "0" and the output of both the loops is same.



Output of while loop:
a: 1
a: 2
a: 3
a: 4
a: 5

Output of do-while loop:
b: 1
b: 2
b: 3
b: 4
b: 5




Now change the initial value of both the variables to 3 and run the code again. There's no change in the output of both the loops.



Output of while loop:
a: 4
a: 5

Output of do-while loop:
b: 4
b: 5




Now change the initial value of both the variables to 10 and run the code again. Here, you can observe the difference between the two loops −



Output of while loop:

Output of do-while loop:
b: 11




Note that the while loop doesn't take any iterations, but the do-while executes its body once. This is because the looping condition is verified at the top of the loop block in case of while, and since the condition is false, the program doesn't enter the loop.



In case of do-while, the program unconditionally enters the loop, increments "b" to 11 and then doesn't repeat as the condition is false. It shows that the do-while is guaranteed to take at least one repetition irrespective of the initial value of the looping variable.



The do-while loop can be used to construct a conditional loop as well. You can also use break and continue statements inside a do-while loop.

In C, while is one of the keywords with which we can form loops. The while loop is one of the most frequently used types of loops in C. The other looping keywords in C are for and do-while.

The while loop is often called the entry verified loop, whereas the do-while loop is an exit verified loop. The for loop, on the other hand, is an automatic loop.

Syntax of C while Loop

The syntax of constructing a while loop is as follows −

while(expression){statement(s);}

The while keyword is followed by a parenthesis, in which there should be a Boolean expression. Followed by the parenthesis, there is a block of statements inside the curly brackets.

Flowchart of C while Loop

The following flowchart represents how the while loop works −

How while Loop Works in C?

The C compiler evaluates the expression. If the expression is true, the code block that follows, will be executed. If the expression is false, the compiler ignores the block next to the while keyword, and proceeds to the immediately next statement after the block.

Since the expression that controls the loop is tested before the program enters the loop, the while loop is called the entry verified loop. Here, the key point to note is that a while loop might not execute at all if the condition is found to be not true at the very first instance itself.

The while keyword implies that the compiler continues to execute the ensuing block as long as the expression is true. The condition sits at the top of the looping construct. After each iteration, the condition is tested. If found to be true, the compiler performs the next iteration. As soon as the expression is found to be false, the loop body will be skipped and the first statement after the while loop will be executed.

Example of while Loop in C

The following program prints the “Hello World” message five times.

#include <stdio.h>intmain(){// local variable definitionint a =1;// while loop executionwhile(a <=5){printf("Hello World \n");
  a++;}printf("End of loop");return0;}</code></pre>


Output



Here, the while loop acts as a counted loop. Run the code and check its output −



Hello World
Hello World
Hello World
Hello World
Hello World
End of loop




Example Explanation



The variable "a" that controls the number of repetitions is initialized to 1, before the while statement. Since the condition "a <= 5" is true, the program enters the loop, prints the message, increments "a" by 1, and goes back to the top of the loop.



In the next iteration, "a" is 2, hence the condition is still true, hence the loop repeats again, and continues till the condition turns false. The loop stops repeating, and the program control goes to the step after the block.



Now, change the initial value of "a" to 10 and run the code again. Now the output will show the following −



End of loop




This is because the condition before the while keyword is false in the very first iteration itself, hence the block is not repeated.



A "char" variable represents a character corresponding to its ASCII value. Hence, it can be incremented. Hence, we increment the value of the variable from "a" till it reaches "z".



Using while as Conditional Loop



You can use a while loop as a conditional loop where the loop will be executed till the given condition is satisfied.



Example



In this example, the while loop is used as a conditional loop. The loop continues to repeat till the input received is non-negative.



#include <stdio.h>intmain(){// local variable definition char choice ='a';int x =0;// while loop executionwhile(x >=0){(x %2==0)?printf("%d is Even \n", x):printf("%d is Odd \n", x);printf("\n Enter a positive number: ");scanf("%d",&x);}printf("\n End of loop");return0;}



Output



Run the code and check its output −



0 is Even

Enter a positive number: 12
12 is Even

Enter a positive number: 25
25 is Odd

Enter a positive number: -1

End of loop




While Loop with break and continue



In all the examples above, the while loop is designed to repeat for a number of times, or till a certain condition is found. C has break and continue statements to control the loop. These keywords can be used inside the while loop.



Example



The break statement causes a loop to terminate −



while(expr){......if(condition)break;...}



Example



The continue statement makes a loop repeat from the beginning −



while(expr){......if(condition)continue;...}



More Examples of C while Loop



Example: Printing Lowercase Alphabets



The following program prints all the lowercase alphabets with the help of a while loop.



#include <stdio.h>intmain(){// local variable definitionchar a ='a';// while loop executionwhile(a <='z'){printf("%c", a);
  a++;}printf("\n End of loop");return0;}</code></pre>


Output



Run the code and check its output −



abcdefghijklmnopqrstuvwxyz
End of loop




Example: Equate Two Variables



In the code given below, we have two variables "a" and "b" initialized to 10 and 0, respectively. Inside the loop, "b" is decremented and "a" is incremented on each iteration. The loop is designed to repeat till "a" and "b" are not equal. The loop ends when both reach 5.



#include <stdio.h>intmain(){// local variable definitionint a =10, b =0;// while loop executionwhile(a != b){
  a--;
  b++;printf("a: %d b: %d\n", a,b);}printf("\n End of loop");return0;}</code></pre>


Output



When you run this code, it will produce the following output −



a: 9 b: 1
a: 8 b: 2
a: 7 b: 3
a: 6 b: 4
a: 5 b: 5

End of loop




while Vs. do while Loops



The do-while loop appears similar to the while loop in most cases, although there is a difference in its syntax. The do-while is called the exit verified loop. In some cases, their behaviour is different. Difference between while and do-while loop is explained in the do-while chapter of this tutorial.

Most programming languages including C support the for keyword for constructing a loop. In C, the other loop-related keywords are while and do-while. Unlike the other two types, the for loop is called an automatic loop, and is usually the first choice of the programmers.

The for loop is an entry-controlled loop that executes the statements till the given condition. All the elements (initialization, test condition, and increment) are placed together to form a for loop inside the parenthesis with the for keyword.

Syntax of for Loop

The syntax of the for loop in C programming language is −

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

Control Flow of a For Loop

Here is how the control flows in a “for” loop −

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.

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 control jumps to the next statement just after the “for” loop.

After the body of the “for” loop executes, the control flow 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.

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 the condition). After the condition becomes false, the “for” loop terminates.

Flowchart of for Loop

The following flowchart represents how the for loop works −

Developers prefer to use for loops when they know in advance how many number of iterations are to be performed. It can be thought of as a shorthand for while and do-while loops that increment and test a loop variable.

The for loop may be employed with different variations. Let us understand how the for loop works in different situations.

Example: Basic for Loop

This is the most basic form of the for loop. Note that all the three clauses inside the parenthesis (in front of the for keyword) are optional.

#include <stdio.h>intmain(){int a;// for loop executionfor(a =1; a <=5; a++){printf("a: %d\n", a);}return0;}

Output

Run the code and check its output −

a: 1
a: 2
a: 3
a: 4
a: 5

Initializing for Loop Counter Before Loop Statement

The initialization step can be placed above the header of the for loop. In that case, the init part must be left empty by putting a semicolon.

Example

#include <stdio.h>intmain(){int a =1;// for loop executionfor(; a <=5; a++){printf("a: %d\n", a);}return0;}

Output

You still get the same output −

a: 1
a: 2
a: 3
a: 4
a: 5

Updating Loop Counter Inside for Loop Body

You can also put an empty statement in place of the increment clause. However, you need to put the increment statement inside the body of the loop, otherwise it becomes an infinite loop.

Example

#include <stdio.h>intmain(){int a;// for loop executionfor(a =1; a <=5;){printf("a: %d\n", a);
  a++;}return0;}</code></pre>


Output



Here too, you will get the same output as in the previous example −



a: 1
a: 2
a: 3
a: 4
a: 5




Using Test Condition Inside for Loop Body



You can also omit the second clause of the test condition in the parenthesis. In that case, you will need to terminate the loop with a break statement, otherwise the loop runs infinitely.



Example



#include <stdio.h>intmain(){int a;// for loop executionfor(a =1;; a++){printf("a: %d\n", a);if(a ==5)break;}return0;}



Output



On executing this code, you will get the following output −



a: 1
a: 2
a: 3
a: 4
a: 5




Using for Loops with Multiple Counters



There may be initialization of more than one variables and/or multiple increment statements in a for statement. However, there can be only one test condition.



Example



#include <stdio.h>intmain(){int a, b;// for loop executionfor(a =1, b =1; a <=5; a++, b++){printf("a: %d b: %d a*b: %d\n", a, b, a*b);}return0;}



Output



When you run this code, it will produce the following output −



a: 1 b: 1 a*b: 1
a: 2 b: 2 a*b: 4
a: 3 b: 3 a*b: 9
a: 4 b: 4 a*b: 16
a: 5 b: 5 a*b: 25




Decrement in for Loop



You can also form a decrementing for loop. In this case, the initial value of the looping variable is more than its value in the test condition. The last clause in the for statement uses decrement operator.



Example



The following program prints the numbers 5 to 1, in decreasing order −



#include <stdio.h>intmain(){int a;// for loop executionfor(a =5; a >=1; a--){printf("a: %d\n", a);}return0;}



Output



Run the code and check its output −



a: 5
a: 4
a: 3
a: 2
a: 1




Traversing Arrays with for Loops



For loop is well suited for traversal of one element of an array at a time. Note that each element in the array has an incrementing index starting from "0".



Example



#include <stdio.h>intmain(){int i;int arr[]={10,20,30,40,50};// for loop executionfor(i =0; i <5; i++){printf("a[%d]: %d\n", i, arr[i]);}return0;}



Output



When you run this code, it will produce the following output −



a[0]: 10
a[1]: 20
a[2]: 30
a[3]: 40
a[4]: 50




Example: Sum of Array Elements Using for Loop



The following program computes the average of all the integers in a given array.



#include <stdio.h>intmain(){int i;int arr[]={10,20,30,40,50};int sum =0;float avg;// for loop executionfor(i=0; i<5; i++){
  sum += arr&#91;i];}
   avg =(float)sum /5;printf("Average = %f", avg);return0;}



Output



Run the code and check its output −



Average = 30.000000




Example: Factorial Using for Loop



The following code uses a for loop to calculate the factorial value of a number. Note that the factorial of a number is the product of all integers between 1 and the given number. The factorial is mathematically represented by the following formula −



x!=1*2*...* x




Here is the code for computing the factorial −



#include <stdio.h>intmain(){int i, x =5;int fact =1;// for loop executionfor(i=1; i<= x; i++){
  fact *= i;}printf("%d != %d", x, fact);return0;}</code></pre>


Output



When you run this code, it will produce the following output −



5! = 120




The for loop is ideally suited when the number of repetitions is known. However, the looping behaviour can be controlled by the break and continue keywords inside the body of the for loop. Nested for loops are also routinely used in the processing of two dimensional arrays.

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, dowhile 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.

Example: Loops in C

To understand the need of loops in a program, consider the following snippet −

#include <stdio.h>intmain(){// local variable definitionint 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);return0;}

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 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>intmain(){// local variable definitionint a =1;while(a <=100){printf("a: %d\n", a);
  a++;}return0;}</code></pre>


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.



Parts of C Loops



To constitute a loop, the following elements are necessary −




Looping statement (while, dowhile 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 dowhile 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
1
while loopRepeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
2
for loopExecutes a sequence of statements multiple times and abbreviates the code that manages the loop variable.
3
do-while loopIt is more like a while statement, except that it tests the condition at the end of the loop body.
4
nested loopsYou 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
1
break statementTerminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
2
continue statementCauses the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
3
goto statementTransfers 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.



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>intmain(){for(;;){printf("This loop will run forever. \n");}return0;}



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.