How can i sort c++ vector array by class property

Question

I have a vector array of type Element. How can I now sort the array in Ascending order, based on the distance of each Element's Center value from 100?

In this case the resulting Elements vector array would result in the vector array being sorted something like this...

Elements sorted:

Elements.push_back(Element(77));
Elements.push_back(Element(128));
Elements.push_back(Element(20));
Elements.push_back(Element(-370));
Elements.push_back(Element(-489));

code:

#include "stdafx.h"
#include <iostream>
#include <vector>

using namespace std;


class Element
{
    public:
        int Center;

        Element(int center)
        {
            Center = center;
        }
};

int main(int)
{
    cout << "Start...\n";
    vector<Element> Elements;

    // add test objects
    Elements.push_back(Element(20));
    Elements.push_back(Element(-370));
    Elements.push_back(Element(128));
    Elements.push_back(Element(77));
    Elements.push_back(Element(-489));
    //cout << "value of a: " << Elements.size() << endl;

    for (vector<Element>::size_type i = 0; i != Elements.size(); i++) {
        cout << "value of a: " << Elements[i].Center << endl;
    }

    return 0;
}

Use [`std::sort`](http://en.cppreference.com/w/cpp/algorithm/sort) with a lambda expression that does the comparison that you want. — Michael, Feb 17 '16 at 15:53
sorted for closest to 100? 128 is 28 away from 100 and 77 is 23. Why does 128 come before 77. — Floris Velleman, Feb 17 '16 at 15:53
Unrelated but be aware of and use [initializer lists](http://en.cppreference.com/w/cpp/language/initializer_list) (i.e., `Element(int center) : Center(center) {}`) — James Adkison, Feb 17 '16 at 15:56
If so, it seems the sorted order of `75` and `125` are undefined. Perhaps a tie breaker of an ascending ordering should be used as well? — James Adkison, Feb 17 '16 at 16:10

score 3 · Accepted Answer · edited Feb 17 '16 at 16:16

3

Here is an example from here:

// sort using a lambda expression 
    std::sort(s.begin(), s.end(), [](int a, int b) {
        return b < a;   
    });

You provide a lambda that gives the logic about what criteria you are sorting with. In your case, the criteria will handle the Center variable. a and b are any two elements in your vector that are being compared.

In your case, yours will look something like this (I'm not at a compiler at the moment to test, and the specifics of your sort logic may be different than I understand):

std::sort(Elements.begin(), Elements.end(), [](Element a, Element b) {
     return abs(100 - b.Center) > abs(100 - a.Center);   
});

edited Feb 17 '16 at 16:16

JokerMartini

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answered Feb 17 '16 at 15:56

johnbakers

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what do you mean by this? (not at a compiler at the moment): – JokerMartini Feb 17 '16 at 16:02
1

This shows how to do a standard ascending sort but doesn't sort "based on the distance of each Element's Center value from 100?" as asked by the OP. – James Adkison Feb 17 '16 at 16:03
`a.Center` does not work for an `int` type. – James Adkison Feb 17 '16 at 16:04
@JamesAdkison what do you mean about a.Center not being an int? – johnbakers Feb 17 '16 at 16:05
In your code `a` is of type `int` therefore `a.Center` is not valid (i.e., `[](int a, int b) { ... }`). I'm pretty sure you meant `Element` instead of `int` for `a` and `b`. – James Adkison Feb 17 '16 at 16:06
@JamesAdkison sorry for the typo – johnbakers Feb 17 '16 at 16:08
Pay attention to the fact that `(100 - b.Center)` and `(100 - a.Center)` must be absolute values, otherwise 128 will come before 77. Also you must switch order of b and a (or add >), as it is now it's the opposite order of OP wants – hlscalon Feb 17 '16 at 16:13
yeah i caught that just now. Thank you – JokerMartini Feb 17 '16 at 16:13
@old_mountain I'm just showing the general idea. As noted in the answer, the OP can run with this and create the exact logic he wants for his sort. – johnbakers Feb 17 '16 at 16:14
@johnbakers thank you very much for your help on this john, its a huge help! – JokerMartini Feb 17 '16 at 16:15

score 0 · Answer 2 · answered Feb 17 '16 at 16:14

Considering the fact that your example is wrong (77 should come before 128) you might want to use std::sort like this:

std::sort(Elements.begin(), Elements.end(), [](const Element& a, const Element& b) {
     return std::abs(100 - a.Center) < std::abs(100 - b.Center);   
});

The problem here is that if you were to have: 75 and 125 it will put 75 first (if that was inserted first), as pointed out by James Adkison.

Live example

How can i sort c++ vector array by class property

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