looping statements in C ( while, do while , for)
Loops in C
Loops are used in programming to repeat a specific block of statements until some end condition is met.
Types of Loops in C
Depending on the position of the control statement in a program, looping in C is classified into two types:
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Entry controlled loop
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Exit controlled loop
1. Entry Controlled Loop (Pre-checking Loop)
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In an entry controlled loop, the condition is checked before executing the loop body.
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If the condition is false, the loop body may not execute at all.
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Also called a pre-checking loop.
2. Exit Controlled Loop (Post-checking Loop)
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In an exit controlled loop, the condition is checked after executing the loop body.
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The loop body executes at least once.
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Also called a post-checking loop.
Important Note
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Control conditions must be well defined and specified, otherwise the loop may execute infinitely.
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A loop that never stops and executes statements repeatedly is called an:
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Infinite loop
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Also known as an Endless loop
-
Looping Statements in C
There are three looping statements in C programming:
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while loop — (Entry controlled)
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do...while loop — (Exit controlled)
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for loop — (Entry controlled)
while Loop in C
The while loop is an entry-controlled looping statement.
Syntax
Explanation
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The testExpression checks whether the condition is true or false before each iteration.
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The while loop first evaluates the test expression.
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If the test expression is true (nonzero), the statements inside the loop body are executed.
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After execution, the test expression is evaluated again.
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This process continues until the test expression becomes false.
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When the test expression is false, the while loop terminates.
#include <stdio.h>
int main()
{
int number;
long long factorial;
printf("Enter an integer: ");
scanf("%d",&number);
factorial= 1;
//loop terminates when number is less than or equal to 0
while(number > 0)
{
factorial*= number;
--number;
}
printf("Factorial=%lld", factorial);
return 0;
}
do...while Loop in C
The do...while loop is similar to the while loop with one important difference:
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The body of the do...while loop executes once before checking the test expression.
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Hence, the do...while loop always runs at least once.
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It is an exit-controlled loop.
Syntax
Explanation
-
The code block (loop body) inside the braces is executed first.
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Then, the test expression is evaluated.
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If the test expression is true (nonzero), the loop body executes again.
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This process continues until the test expression becomes false (0).
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When the test expression is false, the do...while loop terminates.
// Program to add numbers until user enters zero
#include <stdio.h>
int main()
{
double number, sum = 0;
//loop body is executed at least once
do
{
printf("Enter a number: ");
scanf("%lf",&number);
sum+= number;
}
while(number!= 0.0);
printf("Sum= %.2lf",sum);
return 0;
}
| while | do-while |
|---|---|
| Condition is checked first then statement(s) is executed. | Statement(s) is executed atleast once, thereafter condition is checked. |
| It might occur statement(s) is executed zero times, If condition is false. | At least once the statement(s) is executed. |
| No semicolon at the end of while. while(condition) | Semicolon at the end of while. while(condition); |
| If there is a single statement, brackets are not required. | Brackets are always required. |
| Variable in condition is initialized before the execution of loop. | variable may be initialized before or within the loop. |
| while loop is entry controlled loop. | do-while loop is exit controlled loop. |
| while(condition) { statement(s); } | do { statement(s); } while(condition); |
for Loop in C
The for loop is an entry-controlled looping statement.
Syntax
Explanation
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The initialization statement is executed only once at the beginning of the loop.
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Then, the test expression is evaluated:
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If the test expression is false (0), the for loop terminates.
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If the test expression is true (nonzero), the statements inside the loop body execute.
-
-
After executing the loop body, the update statement runs.
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This process repeats until the test expression becomes false.
Note
The for loop is commonly used when the number of iterations is known in advance.
// Program to calculate the sum of first n natural numbers
// Positive integers 1,2,3...n are known as natural numbers
#include <stdio.h>
int main()
{
int num, count, sum = 0;
printf("Enter a positive integer: ");
scanf("%d",&num);
// for loop terminates when n is less than count
for(count = 1; count <= num; ++count)
{
sum+= count;
}
printf("Sum= %d", sum);
return 0;
}
Which Loop to Select?
Selecting the right loop can be challenging. To choose an appropriate loop, follow these steps:
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Analyze the problem and determine whether it requires a pre-test loop or a post-test loop.
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If a pre-test is required, use a while loop or a for loop.
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If a post-test is required, use a do-while loop.
Summary
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Looping is one of the key concepts in any programming language.
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A block of loop control statements in C executes repeatedly until the condition becomes false.
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Loops are of two types:
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Entry-controlled loops
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Exit-controlled loops
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C programming provides:
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while loop
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do-while loop
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for loop
-
-
for and while loops are entry-controlled loops.
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do-while is an exit-controlled loop.
Sample programs ( all university questions)
Print factors of a number
#include <stdio.h>
main()
{
int n,i;
printf("Enter the number...\n");
scanf("%d",&n);
printf("Factors of the number\n");
for(i=1;i<=n;i++)
if(n%i==0)
printf("%d\n",i);
}
Note: an efficient way is to go up to n/2
Check whether the given number is prime or not
#include <stdio.h>
#include <math.h>
main()
{
int n,flag=1,i;
printf("enter a number n >1 for primality test\n");
scanf("%d",&n);
for(i=2;i<=sqrt(n);i++)
if(n%i==0)
{ flag=0;break;}
if(flag==1)
printf("%d is prime \n",n);
else
printf("%d is not prime \n",n);
}
Print Fibonacci Series up to 100
#include <stdio.h>
main() {
int a=0,b=1,c=1;
while(c<100)
{
printf("%d, ", c);
c = a+b;
a= b;
b=c;
}
printf("\n");
}
Program to reverse a number
#include <stdio.h>
main()
{
int n, rev = 0, digit;
printf("Enter an integer: ");
scanf("%d", &n);
while (n != 0) {
digit= n % 10;
rev = rev * 10 + digit;
n /= 10;
}
printf("Reversed number = %d\n", rev);
}
printing the sin series x^1/1!-x^3/3!+x^5/5!.......x^20/20!
#include <stdio.h>
main()
{
int i,sign=1;
for(i=1;i<=20;i+=2)
{
printf("x^%d/%d!",i,i);
sign=-sign;
if(sign==-1)
printf("-");
else
printf("+");
}
}
Check for perfect number ( Eg: 6,28,496 etc..)
#include <stdio.h>
#include <string.h>
int main()
{
int n,i,s=0;
printf("Enter the number\n");
scanf("%d",&n);
for(i=1;i<=n/2;i++)
{
if(n%i==0)
s=s+i;
}
if(s==n)
printf("%d is a perfect number \n",n);
else
printf("%d is a not a perfect number \n",n);
}
Write a C program to find the sum of first and last digit of a number.
#include <stdio.h>
main()
{
int n, fdig,ldig;
printf("Enter an integer: ");
scanf("%d", &n);
ldig=n%10;
while (n != 0) {
fdig=n%10;
n /= 10;
}
printf("Sum of first and last digit = %d\n", fdig+ldig);
}
Write C program to convert the given decimal number into binary number.( university question)
#include <stdio.h>
main()
{
int i,n,bit,bin=0,place=1;
printf("Enter the number:");
scanf("%d",&n);
while(n!=0)
{
bit=n%2;
bin=bin+bit*place;
n=n/2;
place=place*10;
}
printf("Binary=%d\n",bin);
}
2)Print the odd numbers between 0 and 50 in the reverse order ( use while)
3)Print the series 5,10,15,….,100 (using for loop)
4)Generate the series 1, 2, 4 ,7, 11 ,16....n ( use while...read n)
5)Generate the Fibonacci series 0 1 1 2 3 5 8…..n ( use do while..read n)
6)Find the factorial of a number ( use for statement do not use built in factorial() function)
7)Print the powers of a number upto 10th power. ( Eg: if n=2 then the program should print 2^0, 2^1, 2^2,2^3,…..,2^10 use for loop)
8)Print the multiplication table of a given number. ( use while)
9)Print the numbers from 1 to 10 and their natural logarithms as a table ( use for)
10)Find the sum of the digits of a number.( use while-university question)
11)Check whether the given 3 digit number is an Armstrong number. (use do while Eg:153,370,371,407) ( uq)
12)Find the factors of a number ( use for)
13)Check whether the given number is prime or not ( use for)
14) Find the GCD or HCF of two numbers ( use Euclid algorithm and while loop)
15) Reverse a number (use while)
16)Find the numbers between 10 and 1000 that are divisible by 13 but not divisible by 3
( use for)
17)Find the sum of series 1-x^2/2+x^4/4-x^6/6........x^n/n ( use while)
18)Check whether the given number is a Krishnamurti number( Krishnamurti Number: It is a number which is equal to the sum of the factorials of all its digits.
For example : 145 = 1! + 4! + 5! = 1 + 24 + 120 = 145)
19)Read N numbers and find the biggest and smallest. ( university question)
20)Read N numbers and find the sum of all odd and even numbers separately.(university question)
21)Count the number of digits in a number.(university question)
22)Write a C program to input a list of n numbers. Calculate and display the sum of cubes of each value in the list.(university question)
23)Write a C program to count the number of zeros and negative terms in a given set of n numbers.
24)Write a C program to input a list of n numbers. calculate and display the average of numbers. Also display the cube of each value in the list.( university question)
25)read mark of n students and print the average mark.Also print the number of students who scored more than 80% mark (mark out of 50)
26)Find the sum s=1+2/3!+3/5!+4/7!... n
27)Write a C program to evaluate the series x-x^3/3!+x^5/5!-x^7/7!....n(uq)
29)Print the pattern 101010....using for loop(uq)
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