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Write a C program to check whether a given number is prime or not

2 min 13 sec

prime number is a number greater than 1 with only two factors – themselves and 1. 

A prime number cannot be divided by any other numbers without leaving a remainder.

An example of a prime number is 13. It can only be divided by 1 and 13. Dividing a prime number by another number results in numbers left over e.g. 13 ÷ 6 = 2 remainder 1.

15 is not an example of a prime number because it can be divided by 5 and 3 as well as by itself and 1.

C program to check whether a given number is prime or not.

int main()
    int number = 7;
    char flag = 0;
    for(int i = 2; i <= (number/2); i++)
      if(number % i == 0)
         flag = 1;

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

return 0;


7 is a prime number.

In this program, a loop is iterated from 2 to number/2. In each iteration, using the condition (number % i) it is checked whether a given number is perfectly divisible by i. 

If a number is perfectly divisible by i, then the flag is set to 1 and break. After iteration, we will check whether a given number is 1 or not. if it is 1, the number is considered as a non-prime number else, if the flag is 0, then the given number is a prime number else the number is not a prime number.

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