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

Answer 1

The brute force way would be to iterate through all the integers smaller than the sought number and check the number is divisible by each of them. However this algorithm is extremely inefficient, but improving upon the simple idea, we can come up with an algorithm:

#include <stdio.h>

#include <stdlib.h>

#include <math.h>

/*

*

*/

int main(int argc, char** argv) {

int checked = 72; //put your number that you want to check

int temp = 2; //the temporary variable to check against

char isPrime = 1; //intially assume it is prime

if (checked != 1 checked != 2) {

while (temp <= sqrt((double) checked)) { //only have to check up to the square root of checked

if (checked % temp 0) {

temp += 1; //only have to check odd numbers } else {

temp += 2; } } }

if (isPrime) {

printf("Checked is prime!\n"); } else {

printf("Checked is not prime!\n"); }

return (EXIT_SUCCESS);

}

or a better edited version here: http://img689.imageshack.us/img689/8164/primecodec.png