C program to print prime numbers upto a given number - My CS Tutorial

How to write a C program to print prime numbers upto a given number?

In this tutorial we will learn how to write a C program to print prime numbers upto a given number as if we will enter a number like 10 the output of this program is
2  3  5  7

Explanation:-

1. The Program starts with initializing
   • num → For storing the number to get all prime numbers between 1 and num
   • i,j → temporary variables
   • count → To count the number ofdivisors

2. printf("Enter a number \n");
   scanf("%d",&num);

   To take value from user.Let us assume it be 8(num=8)
3. The loop iterates from i=1 to 8 one by one.Then count is made zero for every iteration.
4. Now j will iterate from 1 to i and checks if any prime number is there between 1 and i. For example: if it is 5th iteration then i =5
   • Iteration 1: Then the for loop j starts as j=1,j → true then loop continues
     • if i%j → 5%1==0 which is true,
     • then count=1
   • Iteration 2: Then the for loop j starts as j=2,j → true then loop continues
     • if i%j → 5%2==0 which is false,
     • then count remains same i.e.count=1;
   • Iteration 3: Then the for loop j starts as j=3,j → true then loop continues
     • if i%j → 5%3==0 which is false,
     • then count remains same i.e.count=1;
   • Iteration 4: Then the for loop j starts as j=4,j → true then loop continues
     • if i%j → 5%4==0 which is false,
     • then count remains same i.e.count=1;
   • Iteration 5: Then the for loop j starts as j=5,j → true then loop continues
     • if i%j → 5%5==0 which is true,
     • then count=2
   • Iteration 6: Then the for loop j starts as j=6,j → false then loop terminates
   • Now if count==2 → true then this will be the prime number so this number is printed i.e i=5 is printed.
5. Like the above iteration it checks each andevery value of i whether it is prime or notand if it is prime it will print else not.

C program code

#include<stdio.h>

#include<conio.h>

void main()

{

  int num,i,count=0,j;

  printf("\n Enter a number: \n ");

  scanf("%d",&num);

  printf("\n The Prime numbers upto %d are:\n ",num);

  for(i=1;i<=num;i++)

  {

    count=0;

    for(j=1;j<=i;j++)

    {

      if(i%j==0)

      {

        count++;

      }

    }

    if(count==2)

    {

      printf("%d\n ",i);

    }

  }

  getch();

}

OUTPUT 1:

OUTPUT 2:

In first output,the value of number is 15

After testing conditions

The prime numbers upto 15 are

2, 3, 5, 7, 11, 13

In second output,the value of number is 20

After testing conditions

The prime numbers upto 20 are

2, 3, 5, 7, 11, 13, 17, 19

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Created by-- HARSH CHAUHAN