c++ - Merging two vectors into a third by even indices from the first vector and odd indices from the second -

i'm trying merge 2 vectors third 1 copying/merging elements @ indices first, , copying/merging elements @ odd indices second.

note: both vectors 2 dimensional.

i have following logic, doesn't seem work:

void pattern::mergepatters( void ) {   cout<<"\n merge patters"<<endl;    patter_input.reserve( patter_one.size() + patter_two.size() );    for( int i=0;i<patter_one.size(); i++ ){     for( int j =0; j<patter_one[i].size(); j++){       if(j%2==0){         patter_input.push_back( patter_one[j]);       }     }       //cout<< " patter_one answers["<<i<<"]= " << answers_p1[i]<<endl;   }    for( int i=0;i<patter_two.size(); i++ ){     for( int j =0; j<patter_two[i].size(); j++){       if(j%2!=0){         patter_input.push_back( patter_two[j]);       }     }       //cout<< " patter_one answers["<<i<<"]= " << answers_p1[i]<<endl;   }  }  void pattern::printmergepatters ( void ){   cout<<"\n printmergepatters "<<endl;   ( int i=0;i<patter_input.size();i++){     for( int j =0; j<patter_input[i].size(); j++){       cout << " merge patter["<<i<<"]["<<j<<"]= " << patter_input[i][j]<<endl;     }   } } 

i suggest perform copy in 1 loop, instead of using two. may easier using iterators patter_one , patter_two arrays, i'll use indices instead.

const unsigned int total_size = patter_one.size() + patter_two.size(); patter_input.reserve(total_size); unsigned int p1_index = 0u; unsigned int p2_index = 0u; (unsigned int = 0u; < total_size; += 2u) {   patter_input.push_back(patter_one[p1_index]);   patter_input.push_back(patter_two[p2_index]);   ++p1_index;   ++p2_index; } 

notice increment of loop 2 because there 2 array elements pushed in each iteration.

edit 1: copying odd , elements
copy elements patter_one , odd elements patter_two, change index initialization values , index increments:

unsigned int p1_index = 0u; unsigned int p2_index = 1u; (unsigned int = 0u; < total_size; += 2u) {   patter_input.push_back(patter_one[p1_index]);   patter_input.push_back(patter_two[p2_index]);   p1_index += 2;   p2_index += 2; } 

the requirements need clarification, presented 2 alternatives. amount reserved may not correct depending on sizes of patter_one , patter_two vectors. above fragment copies half of patter_one , patter_two vectors, original reserve quantity may not correct.

the reserve amount copying half of patter* vectors is:

const unsigned int total_size = patter_one.size() / 2 // items                               + patter_two.size() / 2; // odd here.