C++ Erase-Remove Idiom – How to Remove Vector Elements by Index

c++erase-remove-idiomstlvector

I wanted to remove the elements of the vector based on the index, say all the even indexed elements.
I have read about the erase remove idiom but can't see how to apply it.
This is what I tried:

    vector<int> line;
    line.reserve(10);
    for(int i=0;i<10;++i)
    {
      line.push_back(i+1);
    }
    for(unsigned int i=0;i<line.size();++i)
    {
      //remove the even indexed elements
      if(i%2 == 0)
      {
        remove(line.begin(),line.end(),line[i]);
      }
    }
line.erase( line.begin(),line.end() );

This erases the entire vector. I was hoping to only remove the elements that had been marked by the remove algorithm.

Then I tried this

for(unsigned int i=0;i<line.size();++i)
    {
      //remove the even indexed elements
      if(i%2 == 0)
      {
        line.erase( remove(line.begin(),line.end(),line[i]),line.end() );
      }
    }

This again doesn't work as there is a problem while removing, the indices seem to shift whilst iterating over the vector.
What should be the correct approach to accomplish this.

Best Answer

By going from 0 to size, you'll end up skipping half of the elements because the indices change as you erase the elements. Make your for loop go from size() to 0:

for(unsigned int i = line.size(); i > 0; i--)
{

}
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