CODEWITHSUNDEEP
c++stl
june210
june210

Reputation: 35

Looping over vector of vectors and appending to a new vector

I want to loop over vect, call a function on each element, perform an operation between both the results and append that to a new vector. Is there a better way to do this?

std::vector<int> vect = {{1,2},{3,4}};
std::vector<double> out(vect.size());
for (int i = 0; i < vect.size(); i++){
       double v = somefunction(vect[i][0]) - somefunction(vect[i][1]);
       out.push_back(v);
    }

Upvotes: 2

Views: 89

Answers (2)

lubgr
lubgr

Reputation: 38277

This is what std::transform can be used for.

#include <algorithm>

std::vector<double> out(vect.size());

std::transform(vect.cbegin(), vect.cend(), out.begin(), [](const auto& inner)
    { return somefunction(inner.front()) - somefunction(inner.back()); });

This assumes that vect is a container of size 2 with front() and back() member functions that do what one would expect them to do.

Upvotes: 5

Fareanor
Fareanor

Reputation: 6805

Assuming your vect is a std::vector <std::vector <double>> as it seems to be, you could simply write:

std::vector <double> out;
for(size_t i = 0; i < vect.size(); ++i)
{
    out.push_back(someFunction(vect[i][0]) - someFunction(vect[i][1]));
}

or

std::vector <double> out;
for(auto & v : vect)
{
    out.push_back(someFunction(v[0]) - someFunction(v[1]));
}

You don't have to initialize out as you did because push_back() will add another element at the end of what you already have added in the construction.

Note that if vect only contains vectors of size two, you can use std::pair instead, for example:

std::vector<std::pair<double, double>> vect;

And replace the previous loop by:

std::vector <double> out;
for(size_t i = 0; i < vect.size(); ++i)
{
    out.push_back(someFunction(vect[i].first) - someFunction(vect[i].second));
}

or

std::vector <double> out;
for(auto & v : vect)
{
    out.push_back(someFunction(v.first) - someFunction(v.second));
}

Upvotes: 1

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