In C abs a - b does not return absolute value of negative number

Question

#include <stdio.h>
#include <cmath>

void update(int *a,int *b) {
    // Complete this function 
    (*a) = abs(*a + *b);
    (*b) = abs(*a - *b);   
}

int main() {
    int a, b;
    int *pa = &a, *pb = &b;

    scanf("%d %d", &a, &b);
    update(pa, pb);
    printf("%d\n%d", a, b);

    return 0;
}

I have the code below (from HackerRank C++ challenges). 

If I enter 4 and 5, it will return 9 and 4 as output instead of 9 and 1. 

I assumed it would be 4 - 5 = abs(-1) = 1. 

Why doesn't it operate that way?

Answer 1

On the first line, you reallocated *a, and it is now utilising that new value on the second line. 

int origa = *a;
*a = abs(origa + *b);
*b = abs(origa - *b);

Alternatively, you may use tuples to get fancy:

std::tie(*a, *b) = std::make_tuple(abs(*a + *b), abs(*a - *b));

which just computes and packs up the values first, then unpacks them after all the reads are done.