qsort and structures

This is a discussion on qsort and structures within the C Programming forums, part of the General Programming Boards category; If I have a structure with two different datatypes as fields of the structure, ie: Code: typedef struct { int ...

  1. #1
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    qsort and structures

    If I have a structure with two different datatypes as fields of the structure, ie:
    Code:
    typedef struct {
    int value;
    char *name;
    } item;
    Now I have an array of these items and I want to sort them using qsort first by value, then by name. (ie, if there are two elements that have the same value, I would then sort those two elements by their name)

    How would I approach this using qsort? Thanks.

  2. #2
    Registered User alex's Avatar
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    Hi!

    start by writing a function that does the comparison:

    Code:
    int mycomp(const void *p1, const void *p2)
    {
       const item *item1=p1, *item2=p2;
       int result;
    
       /* set result to: */
       /*  0 - if item1 and item2 are equal */
       /* <0 - if item1 should come before item2 */
       /* >0 - if item1 should come after item2 */
    
       return result;   
    }
    hope this helps

    alex

  3. #3
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    Is there a way to use that function on two different fields (with same data type, ie: int)? I don't want to rewrite a function that looks almost exactly the same for each field that I have.

  4. #4
    and the hat of wrongness Salem's Avatar
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    > I think you can't use qsort() easily in this case.
    Of course you can, dead easy

    *borrows Alex's code*
    Code:
    int mycomp(const void *p1, const void *p2) {
       const item *item1=p1, *item2=p2;
       if ( item1->value < item2->value ) {
          return -1;
       } else
       if ( item1->value > item2->value ) {
          return +1;
       } else {
          return strcmp( item1->name, item2->name );
       }
    }
    
    // blah
    item array[10];
    qsort( array, 10, sizeof(item), mycomp );

  5. #5
    Registered User alex's Avatar
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    theweirdo: no, you can't write one function to handle multiple fields, at least there is no neat way to do it. If you really want you can use a hack: a global variable that gives the offset of the field relative to the start of the struct.

    vVv: I think using qsort is easier than your solution. Also: mystrcmp("test", "testing") = 0.

    alex

  6. #6
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    no global variables

    first off, global variables are a bad idea. Only use them sparenly.

    The best solution would be to use a class but c doesn't support them. This might be one of those times you might want to look at c++ instead of c.

    If you want to use a class, make the class called qsort and define qsort as having multiple return types for a member of the class.

    This would be the best solution.

    on second thought.

    try using a union as the return type for function qsort()(c style)
    Last edited by wildman6801; 02-13-2002 at 11:13 PM.

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