Sorting vector of objects by object's variable

Question

I have a vector of objects. Each of these objects has 2 fields (the values of which can be repeated), e.g:

//myClass name = myClass(x,y)
myClass obj1 = myClass(2,5);
myClass obj2 = myClass(2,4);
myClass obj3 = myClass(1,5);
myClass obj4 = myClass(3,2);
 
std::vector<myClass> myVector;
 
myVector.push_back(obj1);
myVector.push_back(obj2);
myVector.push_back(obj3);
myVector.push_back(obj4);

I want to sort the vector. First it should be sorted by 1st values. If the values of the 1st variable is the same, then should be sorted by second variable. Vector after sorting should be like that:

obj3 //(1,5)

obj2 //(2,4)

obj1 //(2,5)

obj4 //(3,2)

I have wrote this simple code with bubble sort:

for (int i = 0; i < myVector.size(); i++)
    {
        for (int j = 0; j < myVector.size() - 1; j++)
        {
            if (myVector[j].x < myVector[j + 1].x)
                std::swap(myVector[j], myVector[j + 1]);
        }
    }

Now myVector is sorted by first value, but how to sort elements, which first value it the same, by second value? Like in example?

Yannis Sauzeau · Accepted Answer · 2022-04-24 11:48:33Z

1

You can try something like that:

for (int i = 0; i < myVector.size(); i++)
    {
        for (int j = 0; j < myVector.size() - 1; j++)
        {
            if (myVector[j].x < myVector[j + 1].x)
            {
                std::swap(myVector[j], myVector[j + 1]);
            }
            else if (myVector[j].x == myVector[j + 1].x)
            {
                if (myVector[j].y < myVector[j + 1].y)
                    std::swap(myVector[j], myVector[j + 1]);
            }
        }
    }

answered Apr 24, 2022 at 11:48

Yannis Sauzeau

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Comments

A M · Accepted Answer · 2022-04-24 11:46:44Z

1

You may change your if statement to:

if ( (myVector[j].x == myVector[j + 1].x) ? (myVector[j].y < myVector[j + 1].y) : (myVector[j].x < myVector[j + 1].x)  )

answered Apr 24, 2022 at 11:46

A M

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4 Comments

Yannis Sauzeau Over a year ago

You're right but i think it's little bit complicated to understand for a novice

Mavimix Over a year ago

@YannisSauzeau Yeah, this code works, but I really don't understand how...

Yannis Sauzeau Over a year ago

look at my code above, you see that the statement std::swap(myVector[j], myVector[j + 1]); is the same on both conditions, the only difference is the statement if (myVector[j].x < myVector[j + 1].x) that becomes if (myVector[j].y < myVector[j + 1].y) when myVector[j].x == myVector[j + 1].x, so you can use ternary operator to avoid this code duplication. But it's better for you at the beginning to use comprehensive code with duplication, it's for that i propose to use you my code above.

Yannis Sauzeau Over a year ago

@armin-montigny I don't find the ternary operator difficult to understand, I use it a lot (especially with Flutter) and of course your solution is better to avoid code duplication, however this operator is often misunderstood by beginners, the proof the author of the question didn't understand your answer

Vlad from Moscow · Accepted Answer · 2022-04-24 12:22:12Z

For starters if you want to sort your vector in the ascending order then at least you need to use the if statement the following way

        if (myVector[j+1].x < myVector[j].x)
            std::swap(myVector[j], myVector[j + 1]);

instead of

        if (myVector[j].x < myVector[j + 1].x)
            std::swap(myVector[j], myVector[j + 1]);

A simple approach is to use the standard function std::tie declared in the header <tuple>.

For example

#include <tuple>

//...

for (int i = 0; i < myVector.size(); i++)
{
    for (int j = 0; j < myVector.size() - 1; j++)
    {
        if ( std::tie( myVector[j+1].x, myVector[j+1].y ) < 
             std::tie( myVector[j].x, myVector[j].y ) )
            std::swap(myVector[j], myVector[j + 1]);
    }
}

Instead of the manually written loops you could use the standard algorithm std::sort. For example

#include <tuple>
#include <vector>
#include <iterator>
#include <algorithm>

//...

std::sort( std::begin( myVector ), std::end( myVector ),
           []( const auto &obj1, const auto &obj2 )
           {
               return std::tie( obj1.x, obj1.y ) < std::tie( obj2.x, obj2.y );
           } );

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