CODEWITHSUNDEEP
c++vector
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Reputation: 3075

How to copy vector column-wise?

I've a 2d vector say

vector < vector < int > > sample;

sample = 1 2 3 4 5
         6 7 8 9 0
         1 1 1 1 1
         2 2 2 2 2

Now I want to copy only the last two columns into another 2d vector like

vector < vector < int > > test;

test = 4 5
       9 0 
       1 1
       2 2

How can I do this efficiently ?

Upvotes: 3

Views: 2165

Answers (2)

Johan Kotlinski
Johan Kotlinski

Reputation: 25739

Maybe like this?

#include <algorithm>

vector<vector<int> > new_vector;
new_vector.resize(sample.size());

for (size_t i = 0; i < new_vector.size(); ++i) {
    new_vector[i].resize(2);
    copy(sample[i].end() - 2, sample[i].end(), new_vector[i].begin());
}

Upvotes: 3

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506905

I heard boost has a foreach loop

std::vector< std::vector<int> > v;
BOOST_FOREACH(std::vector<int> const &i, test) {
  v.push_back(std::vector<int>(i.end() - 2, i.end()));
}

If you haven't boost at hands, I would go with a usual for loop. But I don't think I would use nested std::vector in the first place. If you only ever have two-column rows, best use a vector of boost::array<int, 2>.

Upvotes: 3

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