Prime Number Program in C

C Program to Print Prime Number

A Prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. It is only divisible by 1 and itself, and it start from 2. The smallest prime number is 2. In other words, prime numbers can't be divided by other numbers than itself or 1. For example 2, 3, 5, 7, 11, 13, 17, 19, 23.... are the prime numbers.

Note: Zero (0) and 1 are not considered as prime numbers. Two (2) is the only one even prime number because all the numbers can be divided by 2.

Let's see the prime number program in C. In this c program, we will take an input from the user and check whether the number is prime or not.

#include<stdio.h>
#include<conio.h>

void main()
{
  int i,no;
  clrscr();
  printf("Enter any num: ");
  scanf("%d",&no);
  if(no==1)
  {
  printf("Smallest Prime no. is 2");
  }
  for(i=2;i<no;i++)
  {
  if(no%i==0)
  {
  printf("Not Prime no.");
  break;
  }
  }
 if(no==i)
 {
 printf("Prime no.");
 }
  getch();
}

Enter any num: 10
Not Prime no.

Program Explanation

First enter any number if this that is 1 then show message "Smallest Prime no. is 2".

for(i=2;i<no;i++)
  {
  if(no%i==0)
  {
  printf("Not Prime no.");
  break;
  }
  }

In above code we first perform for loop. First initialize i=2, because Every number is divisible by 1 so we start dividing given number from 2, then check condition "i" is less than number (i< no), and increment value of i (i++). Here "if(no%i==0)" is perform like : no%2==0, no%30=0, no%4==0, no%5==0, no%6==0, no%7==0,....... because every time value of i is increment until value of i is less than given number. If reminder is zero then print "Not Prime number" and control goes outside from for loop due to break keyword.

Note: break is only used within loops (for, while, etc) and switch case.

Print next prime number in C

When we enter any number this code will print next Prime number.
Example: Suppose we enter 5 then next prime number is 7.

#include<stdio.h>
#include<conio.h>

void main()
{
int i,j=2,num;
clrscr();
printf("Enter any no.: ");
scanf("%d",&num);
printf("Next Prime no.: ");
for(i=num+1; i<3000; i++)
{
for(j=2; j<i; j++)
{
if(i%j==0)
{
break;
} // if
} // for
if(i==j || i==1)
{
printf("%d\t",i);
break;
} // if
}  // outer for
getch();
}

Enter any no.: 10
Next Prime no. 11

#include<stdio.h>
#include<conio.h>

void main()
 {
  int n, i, flag = 0;
  clrscr();
  printf("Enter a positive integer: ");
  scanf("%d", &n);

  for (i = 2; i <= n / 2; ++i) 
{
    // condition for non-prime
    if (n % i == 0) 
{
      flag = 1;
      break;
    }
  }

  if (n == 1) 
{
    printf("1 is neither prime nor composite.");
  } 
  else 
{
    if (flag == 0)
      printf("%d is a prime number.", n);
    else
      printf("%d is not a prime number.", n);
  }

getch();
}

Enter a positive integer: 5
5 is a prime number.

Enter a positive integer: 8
8 is not a prime number.