Sort vectors by last elements

Question

Have a "vector of vectors" that looks something like this

3 1 2 0 77
0 3 1 2 44
1 0 3 2 29
3 0 1 2 49

I would like to sort them according to the last element in every row so that it would look like this in the end

1 0 3 2 29 
0 3 1 2 44
3 0 1 2 49
3 1 2 0 77

Of course my real example is a lot more complex... but this is basically what I need to get done. Right now I use this snippet which seems to sort according to the first elements.

vector<vector<int>>population;
partial_sort( population.begin(),population.begin()+10, population.end() );

Answer 1

You can use std::sort with a function (or functor object) that provides a strict weak ordering for vectors. I.e. you define a vector-less-than function that orders two vectors correctly, something like this (off the top of my head). Edit: after comments, added checking for one or two empty vectors, which does make things trickier.

bool CustomVectorCompare(const std::vector<int> &i_lhs, const std::vector<int> &i_rhs)
  {
  if(i_rhs.empty())
    return false; // If right side is empty, left can only be equal or larger

  if(i_lhs.empty())
    return true;  // Consider an empty vector to be "smaller" 
                  // than any non-empty vector.       

  return i_lhs.back() < i_rhs.back();
  }

  std::sort(population.begin(), population.end(), CustomVectorCompare);

Answer 2

Use a simple std::sort and pass a functor that compares only the vector's last elements.

Partial_sort rearranges the elements in the range [first, last) so that they are partially in ascending order. Specifically, it places the smallest middle - first elements, sorted in ascending order, into the range [first, middle). The remaining last - middle elements are placed, in an unspecified order, into the range [middle, last).

Answer 3

You can pass along a comparator as the fourth argument to std::partial_sort or std::sort, so just write a function object with a call operator taking two vector arguments that compare your vectors the way you want to.