c++ sort doesn't use custom function

Question

I'm trying to sort

std::vector< std::vector< std::string> > perm;

I am attempting to use std::sort from the algorithm header

I called it with

std::sort( perm.begin(), perm.end(), sortPerms);

This is my sort function:

bool sortPerms (const std::vector<std::string> &i, const std::vector<std::string> &j) {
  for(unsigned int x = 0; x < i.size(); x++) {
    if(i[x] != j[x])
      return false;
  }
  //both are equal
  return true;
}

The purpose of sorting is to then call std::unique to obtain a vector with unique values. When I compile with gcc in cygwin, I get no errors but I have repeats, and when I compile with visual studio 2010, I get an error that operator< is not defined. I stepped through and it is attempting to use its own sort function, and not the one that I have defined.

I'm not sure how to fix this, any suggestions?

Other details: It is guaranteed that all vectors will be of same size. It's purpose is a vector of every permutation of an original vector of strings. Each string is a command, and I am looking for all of the different ways these commands can be shuffled. So I need to strip the duplicates.

Can you point to the line in which you have the error, please? — Grzegorz
– Grzegorz, Commented Oct 25, 2012 at 0:27
How do you expect to sort something based solely on a concept of being unequal? — Kerrek SB
– Kerrek SB, Commented Oct 25, 2012 at 0:29
Its an assertion inside of <algorithm> that states that operator< is undefined. I'm not sure why that's an issue if I supplied my own comparator — clark
– clark, Commented Oct 25, 2012 at 0:33
I don't see supplied operator but a callback function to std::sort(). I guess on Windows it uses LEFT < RIGHT, where LEFT is in your case std::vector< std::string >. You may need to declare structure with an operator< inside that would derive from std::vector< std::string > and work through it. — Grzegorz
– Grzegorz, Commented Oct 25, 2012 at 0:36
@Grzegorz: Why a new type that derives std::vector<std::string>? Just make a free ("global") operator< like sane people! bool operator<(const std::vector<std::string>& a, const std::vector<std::string>& b); — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Oct 25, 2012 at 0:37

Dave S · Accepted Answer · 2012-10-25 00:46:53Z

3

Your sort function should return when a < b, not a != b.

Also, by default, std::vector<> should already support lexicographic comparison via operator<, which should do what you're expecting (compare each element in turn, with the first non-equivalent elements used for the comparison). See http://en.cppreference.com/w/cpp/container/vector/operator_cmp

Your current sortPerms function looks like what you would pass to std::unique, but that has different behavior than a predicate used for std::sort.

edited Oct 25, 2012 at 0:46

answered Oct 25, 2012 at 0:32

Dave S

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1 Comment

clark Over a year ago

sweet, thanks. I wasn't sure if vector would have a function of such, thanks. Also, I don't think that a < b, comparing the actual commands, would work in this case for grouping like std::vector<std::string>'s together. But I'm aware that's usually how its done. Thanks again

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