Prime Number

Prime number



A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. It means a prime number is only divisible by 1 and itself, and it start from 2. The smallest prime number is 2.

****************************************
        Prime Number
*****************************************/
#include< stdio.h>
#include< conio.h>
void main()
{
  int i,no;
  clrscr();
  printf("Enter any number: ");
  scanf("%d",&no);
  if(no==1)
  {
  printf("Smallest Prime number is 2");
  }
  for(i=2;i< no;i++)
  {
  if(no%i==0)
  {
  printf("Not Prime number");
  break;
  }
  }
 if(no==i)
 {
 printf("Prime number");
 }
  getch();
}

Output

Enter any number: 10
Not Prime Number

Program Explanation

First enter any number if this number is 1 then show message "Smallest Prime number is 2".

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

In above code we first perform for loop. First initilize 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.

 if(no==i)
 {
 printf("Prime number");
 }

Here if value of i is equal to Given number then print "Prime number", In this code "i" have same value equal to given number because for loop conditon is "i < no" at that time it false when i==no. So this time "i" have same value to given number. And prime number is divisible by 1 and itself, So here we check no==i.

Print next Prime number

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

/***********************************************
               Next Prime number
************************************************/
#include< stdio.h>
#include< conio.h>
void main()
{
  int i,j=2,num;
  clrscr();
  printf("Enter any number: ");
  scanf("%d",&num);
  printf("Next Prime number: ");
  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();
}

Output

Enter any number: 10
Next Prime Number 11