22

I have 

    vector<vector<int>> vec

in my c++ app.

Every vector of integers as an element of "big" vector has 4 INT values. I want to sort vec basing on third value of it's content vectors of ints (I mean every "inside" vector third element) - is it possible?

EDIT

Let's say I've got a function 

COST(vector<int>)

which calculates me some value based on my vector values - can I use it in comparation parameter too? It'd help me a lot more.

pawel
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    just define your own compare operator that uses the value of the 3rd element. see here for details: http://stackoverflow.com/questions/1380463/sorting-a-vector-of-custom-objects – thang Jan 19 '13 at 22:26
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    Keep in mind lambdas are an option as well. – chris Jan 19 '13 at 22:26
  • Hmm, all your inner `vector`s contain 4 `int`s, and each int has a special meaning? Sounds like you'd rather want to place objects of a class in that vector? – lethal-guitar Jan 19 '13 at 22:30
  • return `cost(a) < cost(b)` in your comparator – RiaD Jan 19 '13 at 23:03

4 Answers4

53

Sure it is. std::sort can take a third parameter which is the comparison function to use when sorting. For example, you could use a lambda function:

std::vector<std::vector<int>> vec;
// Fill it

std::sort(vec.begin(), vec.end(),
          [](const std::vector<int>& a, const std::vector<int>& b) {
  return a[2] < b[2];
});

Alternatively, you can pass anything else callable with signature bool(const std::vector<int>&, const std::vector<int>&), such as a functor or function pointer.

Response to edit: Simply apply your COST function to a and b:

std::sort(vec.begin(), vec.end(),
          [](const std::vector<int>& a, const std::vector<int>& b) {
  return COST(a) < COST(b);
});
Joseph Mansfield
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    remark that the 3rd parameter is a strict weak ordering that returns true if first parameter should be placed before second parameter. this translates to a less than (i.e. a < b). this is in case OP wants to sort in descending order. – thang Jan 19 '13 at 22:37
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    I get an error when there is just one element in the vector of vectors, since `b` will then be null and `b[2]` will throw `reference binding to null pointer of type 'int'` Any remedies, anyone?? – Dane Aug 28 '20 at 07:00
4

If you want to compare the two vectors by cost, try this:

bool predicate(const std::vector<int>& a, const std::vector<int>& b)
{
    return COST(a) < COST(b);
}

Notes:

  • The above works with C++98, too, I'm not sure about how widespread the use of C++11 is and whether you have a compliant compiler. Otherwise, you can of course use a lambda expression, too, as sftrabbit suggested.
  • You don't say what COST returns, I simply assumed some sortable value like float or long.
  • I hope you don't copy the vector when passing it to COST(), that would be horribly inefficient.
  • COST suggests a macro, like all UPPERCASE_NAMES. Don't use macros. Don't use macro names for functions.
Ulrich Eckhardt
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3

sort(vec.begin(), vec.end(), comp);

where comp is:

static bool comp(const vector<int>& vec1, const vector<int>& vec2){
    return vec1[2] < vec2[2];
}
mecdeality
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1
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <ctime>

using namespace std;

// This makes the sort be according to column 2 and ascending
bool sortFunc( const vector<int>& p1,
           const vector<int>& p2 ) {
 return p1[1] < p2[1];
 }

int main() {

  srand(time(NULL));

  // Creates and initializes 10 x 4 vector
  vector< vector<int> > vec;
  for( int i=0; i<10; i++ ) {
   vector<int> tmpVec;
   for( int j=0; j<2; j++ ) {
  tmpVec.push_back( rand()%10 );
   }
   vec.push_back( tmpVec );
  }

  // Print out the pre-sorted vector
 cout << "Pre-sorting state:" << endl;
  for( int i=0; i<vec.size(); i++ ) {
   for( int j=0; j<vec[i].size(); j++ ) {
  cout << vec[i][j] << " ";
  }
cout << endl;
}
  cout << endl;

  // Do the sorting according to column 2
  sort(vec.begin(), vec.end(), sortFunc);

  // Print out the post-sorted vector
   cout << "Post-sorting state:" << endl;
   for( int i=0; i<vec.size(); i++ ) {
    for( int j=0; j<vec[i].size(); j++ ) {
  cout << vec[i][j] << " ";
    }
   cout << endl;
   }

  return 0;
  }

source: https://shihho.wordpress.com/2012/11/28/sort_with_vectors/

borgomeister
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