Sorting a vector in descending order within two ranges

Yury :

Say I have a vector of integers:

std::vector<int> indices;
for (int i=0; i<15; i++) indices.push_back(i);

Then I sort it in descending order:

sort(indices.begin(), indices.end(), [](int first, int second) -> bool{return indices[first] > indices[second];})
for (int i=0; i<15; i++) printf("%i\n", indices[i]);

This produces the following:

14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

Now I want to have the numbers 3, 4, 5, and 6 to be moved to the end, and keep the descending order for them (preferably without having to use sort for the second time). I.e., here is what I want:

14
13
12
11
10
9
8
7
2
1
0
6
5
4
3

How should I modify the comparison function of the std::sort to achieve that?

NutCracker :

Your comparison function is wrong since the values you get as first and second are the elements of the std::vector. Therefore, there is no need to use them as indices. So, you need to change

return indices[first] > indices[second];

to

return first > second;

Now, regarding the problem you try to solve...

You can leave 3, 4, 5 and 6 out of comparison with other elements and still compare it with each other:

std::sort(
    indices.begin(), indices.end(),
    [](int first, int second) -> bool {
        bool first_special = first >= 3 && first <= 6;
        bool second_special = second >= 3 && second <= 6;
        if (first_special != second_special)
            return second_special;
        else
            return first > second;
    }
);

Demo