Consideration on vectors

Question

I have two vector: locs_current_all and sc_current_peaks_pos (both filled with position values), some elements are in common. In vector sc_current_peaks_pos there are the indexes that I want to erase from vector sc_current_peaks_pos.

So here my code.

1st method: try to copy in the vector non_short_circuit_current only the different elements between
locs_current_all and sc_current_peaks_pos.

vector<int> non_short_circuits_current;
int j_nsc=0;
for(int i=0;i<=sc_current_peaks_pos.size()-1;i++){

    for(int k=j_nsc;k<=locs_current_all.size()-1;k++){
        j_nsc++;
        if(sc_current_peaks_pos[i]==locs_current_all[k])
            non_short_circuits_current.push_back(locs_current_all[k]);

           //else break ;

    }

}

2nd method: erase from vector locs_current_all the elements inside sc_current_peaks_pos.

vector<int> non_short_circuits_current;
std::for_each(std::begin(sc_current_peaks_pos), std::end(sc_current_peaks_pos), [&](int indexOpp){
 non_short_circuits_current.erase(locs_current_all [indexOpp]);
 });// erase wants more parameters!!!

All the two methods fail. Thanks in advance for your help.

possible duplicate of [Erasing elements from a vector](http://stackoverflow.com/questions/347441/erasing-elements-from-a-vector) — Csq, Jun 25 '14 at 11:08
@Csq in the example posted from you, he wants to erase just one element and all its occurrances....in my case is not a single value, but a vector of values to erase. — CecchinoSMI, Jun 25 '14 at 11:11
In the future, please change names in pasted code to simpler ones - right now, the code is quite unreadable. — More Axes, Jun 25 '14 at 11:17
@MoreAxes Yes you are right....but now where is your solution? — CecchinoSMI, Jun 25 '14 at 11:21
Frankly, I am confused by the wording of the question. "In vector sc_... there are indexes that I want to erase from vector sc_..." - do you mean that you want to remove certain indexes from sc_... or that sc_... contains indices into itself that you want removed? Or is there another vector that contains the indices into sc_... that are to be removed? — More Axes, Jun 25 '14 at 11:26
Do you care about the order of the inputs and/or results? If not, you can `std::sort` both vectors, then iterate over them together - if the iterators point to the same value advance them both, if the candidate value is less than the to-erase value (or the latter is at `end()`) then copy/retain it, otherwise advance the to-erase iterator. — Tony Delroy, Jun 25 '14 at 11:29
@MoreAxes If you look at the comments to Vlad From Moskow below, there is an example of what I want to do. — CecchinoSMI, Jun 25 '14 at 12:57

Vlad from Moscow · Accepted Answer · 2014-06-25T13:24:19.467

If I have understood your task correctly then try something similar

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <ctime>


int main() 
{
    const int N = 20;
    std::srand( ( unsigned )std::time( 0 ) );

    std::vector<int> v1( N );
    std::vector<int> v2( N );

    std::generate( v1.begin(), v1.end(), [] { return std::rand() % N; } );
    std::generate( v2.begin(), v2.end(), [] { return std::rand() % N; } );

    for ( int x : v1 ) std::cout << x << ' ';
    std::cout << std::endl;

    for ( int x : v2 ) std::cout << x << ' ';
    std::cout << std::endl;

    auto is_present = [&]( int x )
    { 
        return std::find( v2.begin(), v2.end(), x ) != v2.end();
    };

    v1.erase( std::remove_if( v1.begin(), v1.end(), is_present ), v1.end() );

    for ( int x : v1 ) std::cout << x << ' ';
    std::cout << std::endl;

    for ( int x : v2 ) std::cout << x << ' ';
    std::cout << std::endl;

    return 0;
}

The output is

15 4 9 9 11 13 14 16 19 5 6 14 11 2 5 0 6 0 9 12 
16 18 5 14 10 1 5 1 19 18 10 15 2 19 4 5 4 10 13 16 
9 9 11 6 11 0 6 0 9 12 
16 18 5 14 10 1 5 1 19 18 10 15 2 19 4 5 4 10 13 16

EDIT: If the vectors are ordered then you could use std::binary_search instead if std::find as it is used in my sample.

Ok...the problem is quite different. Suppose that vector v2 is different (in dimension) from vector v1. The values inside v2 are content inside vector v1 and I want to erase these from v1. — CecchinoSMI, Jun 25 '14 at 12:53
@CecchinoSMI You are right. v1 erases all values that also are present in v2. It is unimportant what sizes the vectors have. — Vlad from Moscow, Jun 25 '14 at 13:03

score 0 · Answer 2 · edited May 23 '17 at 12:32

I was busy answering this and my internet connection died, so I'm not going to post any more code samples, the previous answer has some nice code for you to work with; but I thought it might be worthwhile noting that:

std::remove() does not physically delete or erase elements from the container It only moves elements within the container.
vector.erase() deletes elements from a container, since the container itself will have enough knowledge to remove elements from itself (i.e. because this varies between containers, the methods need to be members).
Once elements in a vector are erased (or inserted), it could potentially invalidate any copies of an iterators you may have made, so it is generally a good idea to assume all of the are invalid and re-get them (unless you are absolutely sure that they will be still valid). Basically if you are looping from the vec.begin() to vec.end() and you erase an element before the end of the loop, you are already in trouble.
The "erase remove" pattern is fairly common when erasing elements in the vector, of course you'll need to define your custom criteria is for the deletion is to be.
If they are sorted sequences, the std library may already have some algorithms you may want to use.

Consideration on vectors

2 Answers2