QT std::sort function is not sorting correctly

Ayush Gupta

I have below structure.

struct test
{
uint id;
QString name;
};

I have below map which I am inserting element.
map<uint, test> maptest1;
maptest1.insert ( 1, {1,"Ram"));
maptest2.insert( 2, { 1,"Shyam"));
maptest3.insert(3, {2,"Mohan"));

Now I am trying to sort this map based on ID of test struct.

I did below copied all things in vector

vector<test> test1;
for(const test& a : maptest.values())
{
test1.push_back(a);
}

Now I am sorting the vector using below function
std::sort(test1.begin() , test2.begin(), [this]( const test& test1 , const test& test2 ) { return test1.id < test2.id});

But sorting does not works fine for those who have same id in test (like ram and shyam).

I am working with some more large numbers of such instances this is just as an example.

Can anyone help me

J.Hilk

hi, @Ayush-Gupta

why don't you modify your test lambda then?

std::sort(test1.begin() , test2.begin(), [this]( const test& test1 , const test& test2 )->bool { 
    if(test1.id == test2.id) {
        //case same ID -> your other compare method
        return other;
    }
    return test1.id < test2.id
});

Ayush Gupta

@J-Hilk I did like this but not works
std::sort(test1.begin() , test2.begin(), [this]( const test& test1 , const test& test2 )->bool {
if(test1.id == test2.id) {
return false;
}
return test1.id < test2.id
});

VRonin

what you want is probably just to use std::stable_sort instead.
test1.begin() , test2.begin() What is this? Normally it's test1.begin() , test1.end()
for(const test& a : maptest.values()) test1.push_back(a); As far as I remember std::map diesn't have a values() method. whatever you are using, the values method propably already returns a sortable unidimensional container of the values, there is no need to copy them over to a 3rd container

Ayush Gupta

@VRonin That is typo I am using
test1.begin() , test2.end().
Why you think std::stable_sort will work instead of sort?

aha_1980

@Ayush-Gupta said in QT std::sort function is not sorting correctly:

test1.begin() , test2.end()

Which is wrong too. Both most be from the same data structure.

Regards

VRonin

@Ayush-Gupta said in QT std::sort function is not sorting correctly:

Why you think std::stable_sort will work instead of sort?

The only difference between sort and stable_sort is how they handle items with the same value. stable_sort keeps them in the same relative order as the original vector

J.Hilk

actually, didn't see the error @VRonin mentioned, with that fixed, it works

for example:

QVector<test> maptest1;
    maptest1.append( {1,"Ram"});
    maptest1.append( { 1,"Shyam"});
    maptest1.append( {2,"Mohan"});

    std::sort(maptest1.begin() , maptest1.end(), [=]( const test& test1 , const test& test2 )->bool {
        if(test1.id == test2.id) {
            //case same ID -> your other compare method
            return test1.name.at(0) > test2.name.at(0);
        }
        return test1.id < test2.id;
    });
    for(test a: maptest1){
        qDebug() << a.id << a.name;
    }
//output:
// 1 "Shyam"
//1 "Ram"
//2 "Mohan"

Ayush Gupta

@VRonin You are right stable_sort works perfect for me.
All many thanks for your help.

aha_1980

@Ayush-Gupta So please mark this topic as SOLVED now. Thanks!