command int cannot calculate large number ?

Question

i wanted to find largest prime factor of 600851475143, and i wrote a code that can calculate prime factor of 13195 or less numbers. but my code cannot calculate 600851475143 this numbers largest prime factor why is that ?
here is my code:

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

int main( ){

    int n, a, b;
    printf( "enter a number=> " );
    scanf( "%d", &n );
    for ( a = n; a >= 2; a-- ){
        for ( b = 2; b <= a; b++ ){
            if ( a % b == 0 ){break;}
            else if ( b == a - 1 ){
             if( n % a == 0 ){
             printf("here is the largest prime number => %d\n", a);
                return 0;
                }
            }
        }
    }
    return 0;
}

What makes you think that there is no limit to how big a number can be expressed as an `int`? — dandan78, Apr 24 '14 at 16:25

score 1 · Accepted Answer · edited May 23 '17 at 12:11

1

Here 600851475143 is too big to hold for int data type. Use long long instead of int.So,change the following lines in your code to this:

long long n, a, b;

scanf( "%lld", &n );

printf("here is the largest prime number => %lld\n", a);

Checkout the link for details : Range of values in C Int and Long 32 - 64 bits

edited May 23 '17 at 12:11

Community

1
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answered Apr 24 '14 at 16:21

Ashraful Haque

549
2
10

yea it seems working but my algorithm seems bad, it will take almost 1 weak to calculate :)thanks – user3569411 Apr 24 '14 at 16:37

score 0 · Answer 2 · edited May 23 '17 at 12:05

0

That value is larger than the maximum value of the int data type. You will likely need a 64 bit INT value.

Are types like uint32, int32, uint64, int64 defined in any stdlib header?

edited May 23 '17 at 12:05

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answered Apr 24 '14 at 16:16

Marshall Tigerus

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command int cannot calculate large number ?

2 Answers2