C Program to Find Prime Numbers in a Given Range

This is a C program to find prime numbers in a given range.

Problem Description

The program takes the range and finds all the prime numbers between the range and also prints the number of prime numbers.

Problem Solution

1. Take the range of numbers between which you have to find the prime numbers as input.
2. Check for prime numbers only on the odd numbers between the range.
3. Also check if the odd numbers are divisible by any of the natural numbers starting from 2.
4. Print the prime numbers and its count.
5. Exit.

Program/Source Code

Here is source code of the C program to calculate the prime numbers in a given range. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

  1. #include <stdio.h>
  2. #include <stdlib.h>
  4. void main()
  5. {
  6.     int num1, num2, i, j, flag, temp, count = 0;
  8.     printf("Enter the value of num1 and num2 \n");
  9.     scanf("%d %d", &num1, &num2);
  10.     if (num2 < 2)
  11.     {
  12.         printf("There are no primes upto %d\n", num2);
  13.         exit(0);
  14.     }
  15.     printf("Prime numbers are \n");
  16.     temp = num1;
  17.     if ( num1 % 2 == 0)
  18.     {
  19.         num1++;
  20.     }
  21.     for (i = num1; i <= num2; i = i + 2)
  22.     {
  23.         flag = 0;
  24.         for (j = 2; j <= i / 2; j++)
  25.         {
  26.             if ((i % j) == 0)
  27.             {
  28.                 flag = 1;
  29.                 break;
  30.             }
  31.         }
  32.         if (flag == 0)
  33.         {
  34.             printf("%d\n", i);
  35.             count++;
  36.         }
  37.     }
  38.     printf("Number of primes between %d & %d = %d\n", temp, num2, count);
  39. }
Program Explanation

1. User must take the range as input and it is stored in the variables num1 and num2 respectively.
2. Initially check whether num2 is lesser than number 2.If it is, then print the output as “there are no prime numbers”.
3. If it is not, then check whether num1 is even.If it is even, then make it odd by incrementing the num1 by 1.
4. Using for loop starting from num1 to num2, check whether the current number is divisible by any of the natural numbers starting from 2.Use another for loop to do this.Increment the first for loop by 2, so as to check only the odd numbers.
5. Firstly initialize the variables flag and count to zero.
6. Use the variable flag to differentiate the prime and non-prime numbers and use the variable count to count the number of prime numbers between the range.
7. Print the prime numbers and variable count separately as output.

Runtime Test Cases
Enter the value of num1 and num2
70 85
Prime numbers are
Number of primes between 70 and 85 = 4
Enter the value of num1 and num2
0 1
There are no primes upto 1

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If you wish to look at other example programs on Mathematical Functions, go to C Programming Examples on Mathematical Functions. If you wish to look at programming examples on all topics, go to C Programming Examples.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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