Question

In the book it explains:

ptr = &a /* set ptr to point to a */

*ptr = a /* '*' on left:set what ptr points to */

They seem the same to me, aren’t they?

Answer 1

No. The first one changes the pointer (it now points at a). The second one changes the thing that the pointer is pointing at.

Consider:

int a = 5;
int b = 6;

int *ptr = &b;

if (first_version) {
    ptr = &a;
    // The value of a and b haven't changed.
    // ptr now points at a instead of b
}
else {
    *ptr = a;
    // The value of b is now 5
    // ptr still points at b
}