使用 qsort 对 C 中的结构数组进行排序

Question

I tried to sort an array of Products by its unit price but the result doesn't work.

typedef struct {
    int supply;
    int totalPrice;
    double unitPrice;
} Product;

int comparator(const void * a, const void * b) {
    return (*(Product*) b).unitPrice - (*(Product*) a).unitPrice;
}

int main() {
    Product m[3] = {
        {18, 75, 4.17},
        {15, 72, 4.80},
        {10, 45, 4.50}
    };
    qsort(m, 3, sizeof(Product), comparator);
    for (int i = 0; i < 3; i++) {
        printf("unitPrice=%f\n", m[i].unitPrice);
    }
}

Answer 1

comparator is broken. It subtracts two double values and returns an int . Any fraction in the subtraction result is lost, so numbers less than one unit apart will be considered equal.

Fix it to return a non-zero number if the items differ.

Answer 2

Returning a double value from a function which returns int type value, will result in implicit conversion of double to int and the fractional part will be discarded from returned value.

If you are using gcc compiler, try compiling with -Wconversion option, the compiler will give warning:

 warning: implicit conversion turns floating-point number into integer: 'double' to 'int' [-Wfloat-conversion]
    return (*(Product*) b).unitPrice - (*(Product*) a).unitPrice;
    ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

You can do:

int comparator(const void * a, const void * b) {
    double first = (*(Product*) a).unitPrice;
    double second = (*(Product*) b).unitPrice;

    if (second > first) {
        return 1;
    } else if (second < first) {
        return -1;
    }
    return 0; // second == first
}