C++ Erase vector element by value rather than by position? [duplicate]

289
votes
vector<int> myVector;

and lets say the values in the vector are this (in this order):

5 9 2 8 0 7

If I wanted to erase the element that contains the value of "8", I think I would do this:

myVector.erase(myVector.begin()+4);

Because that would erase the 4th element. But is there any way to erase an element based off of the value "8"? Like:

myVector.eraseElementWhoseValueIs(8);

Or do I simply just need to iterate through all the vector elements and test their values?

4
c++vectorstleraseerase-remove-idiom
@BenVoigt: your question is quite arrogant - clearly the guy cannot answer it, what YOU should have done is create an answer that covers all the cases you mention. - slashmais
@slashmais: Oh nonsense, my clarifying question was quite simple and doesn't require an expert programmer to answer. And there's no way I could cover all possible values of "what do you want to do?" for all three of the cases. Just for the case of "no matching elements" possible behaviors include "nothing", "throw an exception", "return an error", "exit the process (possibly via assert())", "log a message to std::cerr"... and even those aren't exhaustive. No, the asker of the question needs to state the error handling policy, and whether finding no matches even is an error. - Ben Voigt
... case of QED. i think - slashmais

4 Answers

493
votes

How about std::remove() instead:

#include <algorithm>
...
vec.erase(std::remove(vec.begin(), vec.end(), 8), vec.end());

This combination is also known as the erase-remove idiom.

120
votes

You can use std::find to get an iterator to a value:

#include <algorithm>
std::vector<int>::iterator position = std::find(myVector.begin(), myVector.end(), 8);
if (position != myVector.end()) // == myVector.end() means the element was not found
    myVector.erase(position);
13
votes

You can not do that directly. You need to use std::remove algorithm to move the element to be erased to the end of the vector and then use erase function. Something like: myVector.erase(std::remove(myVector.begin(), myVector.end(), 8), myVec.end());. See this erasing elements from vector for more details.

1
votes

Eric Niebler is working on a range-proposal and some of the examples show how to remove certain elements. Removing 8. Does create a new vector.

#include <iostream>
#include <range/v3/all.hpp>

int main(int argc, char const *argv[])
{
    std::vector<int> vi{2,4,6,8,10};
    for (auto& i : vi) {
        std::cout << i << std::endl;
    }
    std::cout << "-----" << std::endl;
    std::vector<int> vim = vi | ranges::view::remove_if([](int i){return i == 8;});
    for (auto& i : vim) {
        std::cout << i << std::endl;
    }
    return 0;
}

outputs

2
4
6
8
10
-----
2
4
6
10