Remove an element from a vector by value - C++ [duplicate]

Question

If I have

vector<T> list

Where each element in the list is unique, what's the easiest way of deleting an element provided that I don't know if it's in the list or not? I don't know the index of the element and I don't care if it's not on the list.

Countertype

Why the name of a vector container is list .. It could be confusing, a lot (for someone, that reads your code) – Lely

@KirilKirov, I don't think list is a reserved keyword in C++? Even so, it's just an example. – Countertype

It is a container type, just as vector is. And both are pretty different :) – Lely

is this a homework question or are you trying to solve a specific problem? if the latter describing the problem may give you alternative solutions, if the former you should tag it as homework – Clytemnestra

If all elements in your container must be unique and you don't need to use its index, you probably should use std::set<T> instead of vector. – Citrange

Answer 1

You could use the Erase-remove idiom for std::vector

Quote:

std::vector<int> v; 
// fill it up somehow
v.erase(std::remove(v.begin(), v.end(), 99), v.end()); 
// really remove all elements with value 99

---

Or, if you're sure, that it is unique, just iterate through the vector and erase the found element. Something like:

for( std::vector<T>::iterator iter = v.begin(); iter != v.end(); ++iter )
{
    if( *iter == VALUE )
    {
        v.erase( iter );
        break;
    }
}

Lely

I would note that you can use find_if() instead of straight iterations. – Citrange

Your 2nd answer .. I wouldn't do it this way. To extend to delete ALL occurrences of VALUE would trip people up in 2 possible places: iter is invalidated after .erase(), and ++iter would fail immediately. Would have to write iter=v.erase(iter); instead. Now if you do it that way, an extra ++iter will be committed after an erasure, skipping elements. To fix this you should always iterate backwards through the vector when deleting elements. Using a numerical index will fix these problems. for( int i = v.size()-1; i >= 0; i-- ) if( v[i]==VALUE ) v.erase( v.begin()+i ) ; – Fourhanded

I know, but I said "to extend to delete" – Fourhanded

@Fourhanded - true, but this is another answer, does not make my answer wrong. – Lely

Answer 2

Based on Kiril's answer, you can use this function in your code :

template<typename T>
inline void remove(vector<T> & v, const T & item)
{
    v.erase(std::remove(v.begin(), v.end(), item), v.end());
}

And use it like this

remove(myVector, anItem);

Sukkah

Answer 3

If occurrences are unique, then you should be using std::set<T>, not std::vector<T>.

This has the added benefit of an erase member function, which does what you want.

See how using the correct container for the job provides you with more expressive tools?

#include <set>
#include <iostream>

int main()
{
   std::set<int> notAList{1,2,3,4,5};

   for (auto el : notAList)
      std::cout << el << ' ';
   std::cout << '\n';

   notAList.erase(4);

   for (auto el : notAList)
      std::cout << el << ' ';
   std::cout << '\n';
}

// 1 2 3 4 5
// 1 2 3 5

Live demo

Magus

A vector is used for a reason, e.g. preserving the insert order. so If occurrences are unique, then you should be using std::set<T>, not std::vector<T> is nowhere valid. Let's just focus on the OP's settings. – Demon

Answer 4

From c++20

//LIKE YOU MENTIONED EACH ELEMENT IS UNIQUE
std::vector<int> v = { 2,4,6,8,10 };

//C++20 UNIFORM ERASE FUNCTION (REMOVE_ERASE IDIOM IN ONE FUNCTION)
std::erase(v, 8); //REMOVES 8 FROM VECTOR

Now try

std::erase(v, 12);

Nothing will happen, the vector remains intact.

Odd