Thread: How to Qsort an array of structs by one of its members?

  1. #1
    Registered User Vespasian's Avatar
    Join Date
    Aug 2011

    How to Qsort an array of structs by one of its members?

    I am versed in qsorting struct members within a struct instance, as in the example below:

    #include <iostream>
    #include <stdlib.h>
    using namespace std;
    struct item_database {
        int data1[8]={1,4,4,3,5,6,4,4};
        int data2[8]={1,7,6,3,4,2,11,11};
    int compare (const void* element1, const void* element2)
        if(*(int*)element1 >= *(int*)element2)
            return -1;
            return 1;
    int main()
        item_database a;
        item_database* aptr = &a;
        qsort(aptr->data1,8, sizeof(int), compare);
        cout << a.data1[1] << endl;
        return 0;
    What I CANT do however is to sort an array of structs by one of its members. This is a complete different problem even if it sounds similar.

    In other words, the problem is this; Say we have a struct:

    struct item_database {
        int key;
        string token;
    Then we declare an array of this struct in the heap:
        item_database *idptr = new item_database[700000];
    and initialize its values.

    How would one go about sorting this heap array of structs by its member function "token"?

    I used the following call to qsort (standard library) and call to the compare function but it doesnt work:

        qsort (idptr, 1000, sizeof(string), compare);
    int compare (const void* a, const void* b)
        if (*(char*)a >= *(char*)b)
            return 1;
            return -1;
    Or in more lamens terms say we have the following stuct instances:
    Key: Token:
    1 Hello
    2 World
    3 How
    4 Are
    5 You

    Then I need those structs to order according to token (alphabetically, increasing or decreasing depending whats in our compare function)
    Last edited by Vespasian; 03-31-2013 at 11:16 AM.

  2. #2
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Instead of using qsort, use std::sort. Your compare function could then become:
    bool compare(const item_database& a, const item_database& b)
        return a.token < b.token;
    Or you could use a lambda instead, if possible.

    Generally, unless you have some special reason, you should prefer std::sort to qsort.
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  3. #3
    SAMARAS std10093's Avatar
    Join Date
    Jan 2011
    Nice, France
    I have the feeling that qsort is used in C. Here I have implemented it.

    As a result, I would suggest you to use std::sort. I have implement this too, here. Of course, you have to modify a bit the code for your needs. (the compar function).

    In order for you to understand how unusual is for someone to use qsort in cpp, when I saw the title of the thread I thought you had posted C code in the wrong thread.
    Code - functions and small libraries I use

    It’s 2014 and I still use printf() for debugging.

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  4. #4
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    Even your compare function for ints is wrong. It is returning -1 for items which are equal, when it must return zero in that case.
    That will cause it to sort incorrectly in some cases.

    Use use std::sort instead, as described.
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