Is *a+=*b same as int sum = *a+*b where sum = *a?

Question

#include <stdio.h>
#include <cmath>
void update(int *a,int *b) {
    *a = *a + *b;
    int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);
    *b = absDifference; 
}
/* 
    void update(int *a,int *b) {
    int sum = *a + *b;
    int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);
    *a = sum;
    *b = absDifference; 
} */   
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;
}

The upper function is not working on hackerrank but the commented function is working. I don't get the difference.

Answer 1

There is no difference between *a+=*b and int sum = *a+*b where sum = *a .

But! Where you put your condition check makes a lot of difference.

In first function, the *a is being added with *b and then condition check is for *a - *b which is nothing but original (or old) value of *a .

*a = *a + *b;
int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);

In second function, *a is unchanged. Only sum is being updated with *a + *b . Here the condition is checking for the original *a minus *b . Only after this condition check, *a is being updated.

int sum = *a + *b;
int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);
*a = sum;

Equivalent of first function with a intermediate sum variable, would be this:

int sum = *a + *b;
*a = sum;
int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);

And this is what not working for you.

Answer 2

Try to use this function.

int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b);
*a = *a + *b;
*b = absDifference; 

Answer 3

In your upper function *a = *a + *b; *a value had changed. So int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b); int absDifference = *a - *b > 0 ? *a - *b : -(*a - *b); this statement result not same with follow function.