Question

For example, let’s say I want to pass the values (1,2),(2,3),(3,4), etc. into a function and have it return a list of numbers, whatever they may be, i.e. 1, 3, 5, 3, 6 after some operations. What is the best way to achieve this result in C++? After moving from python it seems a lot more difficult to do it here, any help?

Answer 1

I’m not saying this is the best way but I think it is pretty good, also from the memory-copying prospective, note I avoid returning a vector (expensive since it invokes the operator= implicitly):

#include <vector>

using namespace std;

/**
 * Meaningful example: takes a vector of tuples (pairs) values_in and returns in
 * values_out the second elements of the tuple whose first element is less than 5
 */
void less_than_5(const vector<pair<int, int> >& values_in, vector<int>& values_out) {
    // clean up the values_out
    values_out.clear();

    // do something with values_in
    for (vector<pair<int, int> >::iterator iter = values_in.begin(); iter != values_in.end(); ++iter) {
        if (iter->first < 5) {
            values_out.push_back(iter->second);
        }
    }

    // clean up the values_out (again just to be consistent :))
    values_out.clear();

    // do something with values_in (equivalent loop)
    for (int i = 0; i < values_in.size(); ++i) {           
        if (values_in[i].first < 5) {
            values_out.push_back(values_in[i].second);
        }
    }        

    // at this point values_out contains all second elements from values_in tuples whose 
    // first is less than 5
}