Using qsort to sort an array of structs in C

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);
    }
}

Editing your question to also show the output would be helpful. — Elliott
– Elliott, Commented Aug 11, 2020 at 4:53

Eric Postpischil · Accepted Answer · 2020-08-11 02:07:32Z

2

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.

answered Aug 11, 2020 at 2:07

Eric Postpischil

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Comments

H.S. · Accepted Answer · 2020-08-11 04:45:06Z

1

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
}

answered Aug 11, 2020 at 4:45

H.S.

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1 Comment

the busybee Over a year ago

It is always a code smell to compare floating point values on equality (implicitly done here) without any tolerance. I would define something like static const double TOLERANCE = 0.005; and then (second > first + TOLERANCE) and (second < first - TOLERANCE).

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