Question

here is code snippet

void F (int a, int *b)
{
 a = 7 ;
 *b = a ;
 b = &a ;
 *b = 4 ;
 printf("%d, %d\n", a, *b) ;
}
int main()
{
 int m = 3, n = 5;
 F(m, &n) ;
 printf("%d, %d\n", m, n) ;
 return 0;
}

answer

4 4 
3 7

I see how 4 4 was computed, I don’t get how they got 3 7 (I do understand how 3 is computed, it is not changed since it was not passed by reference)

Thanks !

Answer 1

At the start of main, we have

m=3   n=5  // int m = 3, n = 5;

then we call F(m, &n), passing m by value, and n by pointer
such that

m = 3   n = 5
a = 3   b->n   // F(m, &n);

Now, inside F(), we assign 7 to a:

m = 3   n = 5
a = 7   b->n     // a = 7

then we assign a (=7) to the memory address pointed by b (->n)

m = 3   n = 7
a = 7   b->n     // *b = a;

next we change b, so that now b points to a:

m = 3   n = 7
a = 7   b->a     // b = &a;

and then we assign 4 to the memory address pointed by b (->a)

m = 3   n = 7
a = 4   b->a     // *b = 4;

printing a (=4) and *b (->a =4)

and printing m (=3) and n (=7) outside the function