qsort problem

This is a discussion on qsort problem within the C Programming forums, part of the General Programming Boards category; I was trying to understand pointers to functions and came across qsort program can you guys tell me how the ...

  1. #1
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    qsort problem

    I was trying to understand pointers to functions and came across qsort program

    can you guys tell me how the compare function "record_compare" works
    given the function
    Code:
    typedef struct {
    int key;
    } Record
    and

    Code:
    int record_compare (void cons *a, void const *a)
    { return ( ((Record *)a)->key - ((Record *)b)->key);
    }

  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    The function takes in two void * constants. As you can not dereference a void point you have to cast it to the correct type.

    So (Record *)a casts a as a Record pointer. -> is the short hand to dereference a point to a structure or union and get a member.

    The function needs to return 0 if they are equal, greater then 0 if the first one is greater, or less then 0 if the second one is greater. Since you are really comparing two integer values by subtracting the second from the first you get those ranges.
    Last edited by Thantos; 05-24-2004 at 02:59 PM.

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    very neatly put, thantos.
    even a fish would not have been in danger had it kept its mouth shut.

  4. #4
    Just Lurking Dave_Sinkula's Avatar
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    >Since you are really comparing two integer values by subtracting the second from the first you get those ranges.

    ...except that this technique will fail to produce correct results if there is integer overflow.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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