Thread: pointers

  1. #1
    Registered User
    Join Date
    Jul 2004


    Hello, this is a problem from a written homework assignment:
    i have to draw out a stack model for this

    void foo(int **i, int **j){
    int *temp;
    if (**i < **j) {
        temp = *i;
        *i = *j;
         *j = temp;
    int main() {
    int a = 3;
    int b = 4;
    int *x, *y;
    x = &a;
    y = &b;
    foo(&x, &y);
    printf("a = %d\n", a);
    printf("b = %d\n", b);
    printf("*x = %d\n", *x);
    printf("*y = %d\n", *y);
    As a hint, he gave us the output:
    *x = 4
    *y = 3

    I have a = 3, b = 4 and x points to a(3) and y points to b(4). however, when foo is called, i dont understand the double pointers. Somehow, judging by the output, the x and y pointers are switched using i and j. any help would be appreicated

  2. #2
    Registered User Micko's Avatar
    Join Date
    Nov 2003
    Double pointer is a variable like any other. What is significant is that it's store an adress of pointer variable.
    for example if you want to swap two integer variables with function you need to use two pointer to type int ( pass by reference). What if you want to swap two pointers? You just need to to use two double pointer to type int and so on.
    Key is to think about pointer variable and also double pointer like like an ordinary variable.

  3. #3
    Registered User
    Join Date
    Jul 2004
    hmm ok, but does i point to x or does i point to what x points to?

  4. #4
    Handy Andy andyhunter's Avatar
    Join Date
    Dec 2004
    Ok this is basically your assigment chain
    a = 3;
    x = &a;
    i = &x;
    So we can see that a is equal to 3.
    We can see that x points to a.
    We can see that i points to x which points to a.

    So if I had x and wanted the value stored by a, I would dereference it once, aka
    *x = 3;
    So if I had i and wanted to get what x was I would dereference it once, aka
    *i = x;
    However we know that x points to a, so to get to a from i we would need to dereference it twice, aka
    **i = a = 3;
    edit - You could look at it like this, we know:
    *x = a
    *i = x
    So some simple substitution:
    *(*i) = a;
    Which is where we wound up above: **i = a;
    Last edited by andyhunter; 02-13-2005 at 05:08 PM.
    i don't think most standard compilers support programmers with more than 4 red boxes - Misplaced

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  5. #5
    Registered User
    Join Date
    Jul 2004
    ok thanks

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