Sort vector of vectors

问题:

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.

回答1:

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);
});

回答2:

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.

回答3:

#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/