Please!! need help in outputing the even numbers of myvector into a file

[JOHN786]

Hi guys,
I am new c++ learner, I have stuck in copying the even values of myvector into a outfile stream, the program prints the required result on screen but when you see the created file it doesn't pop up the even values of the vector.
I would appreciated if anyone could help me out in this regard. Below is my coding.

#include <iostream>
#include <vector>
#include <fstream>
#include <iterator>

using namespace std;


void even_outputfile(vector <int> &numb,int);


int main()
{

int i, size=20;
vector <int> myvector;
for (i= 0; i<size; i++)
{
myvector.push_back(i);
cout<<"[ "<<myvector[i]<<" ]";
}
std:cout<<endl<<"\n";

even_outputfile(myvector,size);

return 0;
}


void even_outputfile(vector <int> &numb, int size)
{
int i;
for(i=0; i<size; i++)//loop to read even elements of the vector
{
if(numb[i]%2!=1)
{
std::cout<<"[ "<<numb[i]<<" ]";// prints the copied vector to the screen
}
}
// date type for output file
std::ofstream output_file;

//create an outfile name sorted
output_file.open("even.txt");

//copy data from vector to a file
ostream_iterator<int> output_iterator(output_file, " ");

std::copy(numb.begin(), numb.end(), output_iterator);
}

[48T7M4Gy]

http://www.cplusplus.com/reference/algorithm/copy_if/

[integralfx]

int i, size = 20;
Move i within the for loop as it is only used there. Like this: for (int i = 0; i < size; i++)
Also, make size const, to prevent any accidental modifications.

std:cout << endl << "\n";
Missing a colon.
You don't need to prefix cout with std, when you're using namespace std
What's your purpose of mixing endl and '\n' ? If in doubt, prefer to use '\n', to avoid the unnecessary call to std::flush().

void even_outputfile(vector<int>& numb, int size)
You don't need to have the parameter size. vectors keep track of this by themselves.
http://www.cplusplus.com/reference/vector/vector/size/

    int i;
    for (i = 0; i < size; i++) //loop to read even elements of the vector 

Again, prefer to keep i within the loop.

    //copy data from vector to a file
    ostream_iterator<int> output_iterator(output_file, " ");

    std::copy(numb.begin(), numb.end(), output_iterator);

This will copy everything in numb to your output file.
Try outputting to the file, within the for loop, just like how you printed every even number.

[48T7M4Gy]

Good point, copy_if only creates a new filtered vector and of course doesn't output to a file. No real advantage by making a new vector in this case.

[JLBorges]

#include <iostream>
#include <vector>
#include <fstream>
#include <algorithm>
#include <iterator>

std::ostream& copy_even( const std::vector<int>& vec, std::ostream& stm )
{
    // http://www.stroustrup.com/C++11FAQ.html#lambda
    static const auto is_even = [] ( int v ) { return v%2 == 0 ; } ;

    std::copy_if( vec.begin(), vec.end(), std::ostream_iterator<int>( stm, " " ), is_even ) ;
    return stm ;
}

int main()
{
    const std::vector<int> numbers { 0, 5, 8, 2, 3, 7, 6, 4, 9, 2, 5, 6, 7, 4, 4, 7, 2, 5 } ;

    copy_even( numbers, std::cout ) << '\n' ; // copy even numbers to stdout

    std::ofstream file( "even.txt" ) ;
    copy_even( numbers, file << "the numbers written to file are: " ) << '\n' ; // copy even numbers to file
}

http://coliru.stacked-crooked.com/a/c8f59d756cc2fa66

[48T7M4Gy]

I guess now the second point I can acknowledge is <algorithm> functionality (combined with IO stream's) is extremely powerful.