CS256 - Principles of Structured Design Fall 2021

Displaying ./code/cs256su21code/jun29/isPrime.c

/*

revisiting isPrime, but using command line arguments to 
run the program

if compiling on linux, you need to compile like this

>gcc isPrime.c -lm

or if you're naming your executable

>gcc -o isPrime isPrime.c -lm

either way, you will need to include the -lm if you are on linux, 

this does not seem to be the case on macOS
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int isPrime(int n);

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

   // if the user enters too few arguments, it's good practice to print
   // a usage statement, they're usually set up like this 
	if(argc < 2){
		printf("Usage: %s <number>\n", argv[0]);
		exit(-1); //similar to return, but exits the program entirely
                // needs stdlib.h
	}

	int n = atoi(argv[1]); //atoi takes a number string and converts that to an integer

   if(isPrime(n) == 1)
      printf("%d is Prime\n", n);
   else
      printf("%d is NOT Prime\n", n);


	return 0;
}

int isPrime(int n){

   if(n <= 1) return 0;

//you can actually quit this loop at the square root of n
   for(int i = 2; i < sqrt(n); i++){
      if(n % i == 0) 
         return 0;
   }

   return 1;

}