Thread: changing arr[0] to arr[1]

  1. #1
    Registered User
    Join Date
    Aug 2011

    changing arr[0] to arr[1]

    Here's what I mean

    void dectobin (int * arr, unsigned char b)
        arr[0] = (b & 128) ? 1 : 0;
        if (b) dectobin (&arr[1], b << 1);
    &arr[1] is what I'm mostly concerned about. I tested it out in some code:
    void testarray (int arr[])
        arr[0] = 255;
    // ...
    for (int i = 0; i < 8; ++i)
    for (int i = 0; i < 8; ++i)
            cout << "[" << i << "]: " << testarr[i] << endl;
    Indeed it did what I thought it would. Inside the function arr[0] is actually arr[1] because &testarr[1] makes arr[0] == arr[1]... wait, what?? Can someone explain to me the behind the scenes? The best explanation I have is &testarr[1] sets the address of arr[0] as arr[1]. My brain is stuck in an infinite loop of sorts. BTW, yes I know what the main code is, and fairly understand bitwise operations and truth tables.
    Last edited by Darkroman; 09-01-2013 at 02:14 AM. Reason: Fixed up some typos.

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    It doesn't make any difference to testarray, it's just a pointer to an int.
    The fact that you made the parameter as int arr[] doesn't convey any extra arrayness properties.

    You could do this.
    int array[10];
    int val;
    int *p = malloc(10,sizeof(*p);
    // Then any of these
    Just make sure the function in question doesn't try to access beyond the bounds of the supplied parameter.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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