Question

Basically, I’ve been trying to complete the 3rd question on projecteuler.net. The example gives me the number 13195 which this program (writting in C) accurately returns a prime factor tree of 5 7 13 29, but when I input the question number 600851475143 nothing happens. I have also made a similar program in Python about a year ago and that solves the factor tree for 600851475143. I think it has to do with the data types I’m using but I can’t find a reliable source for information on that and how to do modulo with floats/doubles/big thingies.

Thanks,

Clement

Code:

//
//  main.c
//  Project Euler Question 3
//
//  Created by Cwbh on 2/11/13.
//  Copyright (c) 2013 Cwbh. All rights reserved.
//

#include <stdio.h>
#include <math.h>

int is_prime(int x);

int main(int argc, const char * argv[])
{
    int pft[100];
    int number;
    int pointerloc = 0;

    printf("Enter the number to find the Prime Factor Tree of: ");
    scanf("%d", &number);

    if (is_prime(number) == 0) {
        for (int i = 2; i < number; i++) {
            if (number%i == 0 && is_prime(i) == 1) {
                pft[pointerloc] = i;
                pointerloc++;
            }
        }
    }else{
        printf("You've entered a prime number to begin with!");
    }

    for (int i = 0; i < pointerloc; i++) {
        printf("%d\n",pft[i]);
    }


    return 0;
}

int is_prime(int x){
    int prime = 1;

    for (int i = 2; i < x; i++) {
        if (x%i == 0) {
            prime = 0;
            break;
        }
    }

    return prime;

}

Answer 1

It looks like int is a 32-bit type on your machine. That means 600851475143 won’t fit (the largest number representable in a 32 bit integer is 4294967295). Use a 64-bit type and you should be OK. You can use uint64_t from stdint.h, or maybe your machine has a 64-bit long or long long type.

The % operator only works on integer types in C, so trying to use it for a float or double won’t work.

Another option you have is to use a “big number” library of some kind. You can certainly write your own simple one that will suffice for Project Euler problems at this level.