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A prime number is a whole number or a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 7 is prime number because it has no positive divisors other than 1 and 7. 6 is not a prime number because it has positive divisors such as 1, 2, 3, 6.

- 0 and 1 are not considered as a prime numbers.
- 2 is the only even prime number.

Let us work through a program to find whether the number is prime or not.

c-prime-number-1.c

#include <stdio.h> int main() { int num, temp, i, j = 0; printf("Enter a number to check if it is prime : "); scanf("%d",&num); for ( i = 2; i <= num/2; i++ ) { temp = num%i; if(temp == 0) j++; } if(num > 1) { if ( j < 1) printf("%d is a prime number",num); else printf("%d is not a prime number",num); } return 0; }

Enter a number to check if it is a prime number: 111 111 is not a prime number

Logically prime numbers are divisible by 1 and by itself, so we have to neglect those numbers. Let us consider a prime number say 101, after half of this number i.e) 50.5 approx 50. only one number is cable of dividing 101 which is by itself. So we will consider the numbers from 2 to 50. Now the idea is straight forward, if any of those numbers from 2 to 50 will divide 101. Then clearly 101 is not a prime number. This is a logic we used in for loop condition.

Let us work through a program to find whether the number is prime or not.

c-prime-number-2.c

#include <stdio.h> int main() { int num, a; printf("Enter a number to check if it is prime : "); scanf("%d",&num); for ( a = 2;a <= num-1;a++ ) { if ( num%a == 0 ) { printf("%d is not a prime number\n", num); break; } } if ( a == num ) printf("%d is a prime number",num); return 0; }

Enter a number to check if it is a prime number: 7 7 is a prime number

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