尝试对结构数组使用选择排序

Question

I've got an array of structs, and I'm trying to sort it by price using selection sort. I'm consistently getting a ECX_BAD_ACCESS signal.

This is what I've got:

#include <iostream>
#include <string>
using namespace std;

So thing is just a struct with a name and a price. The price is what I want to sort the structs by

struct Thing
{
    std::string name;
    double price;
};

Right now, the main method just checks to make sure that the sortThing function is working.

Thing *sortThing(Thing input[], int size);

int main ()
{
    Thing *stuff = new Thing[3];

    stuff[0].name = "middle";
    stuff[0].price = 2.00;
    stuff[1].name = "last";
    stuff[1].price = 3.00;
    stuff[2].name = "first";
    stuff[2].price = 1.00;

    stuff = sortThing(stuff, 3);

    cout << stuff[0].name + " " + stuff[1].name + " " + stuff[2].name;

    return 0;
}

This is where the problem is. It complains about the line where I assign input[maxIndex].name to output[i].name, saying: "Program Received 'EXC_BAD_ACCESS'"

Thing *sortThing(Thing input[], int size)
{
    Thing *output = new Thing[size];
    Thing nullThing;
    nullThing.name = "&nullThing";
    int min = 0;
    int minIndex;

    for (int i = 0; i < size; i++)
    {

        for (int j = 0; j < size; j++)
        {
            if (input[j].name.compare("&nullThing") != 0 && input[j].price <= min) minIndex = j;
        }
        output[i].name = input[minIndex].name;
        output[i].price = input[minIndex].price;
        input[minIndex] = nullThing;
        minIndex = 0;
        min = 0;
    }

    return output;
}

Any help is appreciated

Answer 1

I think your loop for determining minIndex is incorrect... Since all prices are >= 0, you will never set minIndex , and since you never initialized it, it's value can be anything. Most likely in your case the value is outside the range of the array and effectively having an array out of bounds error.

Might want to look into some sorting algorithms.

Answer 2

Debugging 101: here's the first thing to do. Change:

int minIndex;

to:

int minIndex = -42;

(temporarily) then output the value of minIndex immediately after the inner loop.

This will show you that the value is not being changed.

The reason it's not being changed is the initial setting of min . Unless you're giving your product away for free, or paying people to take it away, prices will always be greater than zero and the second part of the if statement will always be false.

Quick fix, set min initially to INT_MAX (and also where you reset it at the end of the outer loop).