*Pointer = - *Pointer?

Question

I've only been doing C for a few weeks so I am quite new to it.

I've seen things like

* (variable-name) = -* (variable-name) 

in lecture notes but what exactly would it do? Would it just negate the value being pointed to?

Answer 1

Yes. When you add the star it means it is pointing to the value. It is essentially saying: to the value at address (variable-name), become -1* the value of (variable-name).

If you are new to C, you may find it easier to use & instead of pointers. &'s are essentially the opposite of *. & doesn't point, it gives the address of a variable (which I find to be a simpler concept).

The following is an example which should demonstrate the use of * and &.

#include <stdio.h>

int main(void)
{
    int blah = 6;
    int *num = &blah;
    (*num) = -(*num);
    printf("%d\n", num); //Displays num
}

Answer 2

Yes it negates the value being pointed to. We cannot write operations on addresses so operations are done on deferenced pointers. Here, *d = -*c means:

*d = (-1) * (*c)

Sample code:

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    int *c;
    int *d;
    d = (int *)malloc(sizeof(int));
    c = (int *)malloc(sizeof(int));
    *c = 9;
    *d = -*c;
    printf("%d", *d);
    free(c);
    free(d);
    return 0;
}