Thread: Compare function for qsort

  1. #1
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    Compare function for qsort

    While looking for information on how to use the qsort function here on the board, I came across this:
    Code:
    int compare(void const *name1, void const *name2)
    {
        char *const *x = (char *const *)name1;
        char *const *y = (char *const *)name2;
    
        return strcmp(*x, *y);
    }
    I know the local variables are there just to make the function more readable but what I don't understand is why x and y are declared as char *const *. I know strcmp takes two const *char as its parameters but when I tried to declare x and y as const *char, the program won't sort the lines correctly. Thanks.

  2. #2
    Registered User white's Avatar
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    depends on what you are comparing for doubles I use the following::
    Code:
    int dcmp(const void* a, const void* b)
    {
    	const D *x = a, *y = b;
    	if ( *x > *y )
    	{return 1;}
    	if ( *x < *y )
    	{return -1;}
    	return 0;
    }
    ----------------

  3. #3
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    >but what I don't understand is why x and y are declared as char *const *
    I would've thought you could've just done this.
    Code:
    int compare(void const *name1, void const *name2)
    {
        const char *x = (const char *)name1;
        const char *y = (const char *)name2;
    
        return strcmp(x, y);
    }
    I have no idea why they used a double pointer.

  4. #4
    End Of Line Hammer's Avatar
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    @white: You don't need to return -1, 0, 1 for use in qsort. You can return <0, 0, >0. This means you can simplify your code to:
    return (*x - *y);
    You can go further, and avoid the use of local variables completely.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  5. #5
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    swoopy, it won't work if I change the code to what you suggested

  6. #6
    End Of Line Hammer's Avatar
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    post your code then. (a compilable sample, showing the problem)
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  7. #7
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    int compare(const void *name1, const void *name2)
    {
        char *const *x = (char *const *)name1;
        char *const *y = (char *const *)name2;
    
        return strcmp(*x, *y);
    }
    
    int main(void)
    {
        char *names[] = {"Meg", "John", "Susan", "Charlotte", "Mario"};
        int nelements = sizeof(names) / sizeof(*names);
    
        char **p = names;
    
        qsort(names, nelements, sizeof *names, compare);
    
        while (nelements-- > 0)
            printf("%s\n", *p++);
    
        return 0;
    }
    This works perfectly. Now try changing the declarations of x and y in function compare to anything but that...

  8. #8
    End Of Line Hammer's Avatar
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    Sorry, didn't read your post properly first time! Here's your answer:
    qsort passes a pointer to the array element, not a copy of the array element. Your array elements are type char*, therefore a pointer to that would be char**.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  9. #9
    and the hat of int overfl Salem's Avatar
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    > return (*x - *y);
    Except you risk arithmetic underflow and the probable wrong answer which results.
    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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