[c++] How do I erase an element from std::vector<> by index?

I have a std::vector<int>, and I want to delete the n'th element. How do I do that?

std::vector<int> vec;

vec.push_back(6);
vec.push_back(-17);
vec.push_back(12);

vec.erase(???);

the fastest way (for programming contests by time complexity() = constant)

can erase 100M item in 1 second;

    vector<int> it = (vector<int>::iterator) &vec[pos];
    vec.erase(it);

and most readable way : vec.erase(vec.begin() + pos);

If you have an unordered vector you can take advantage of the fact that it's unordered and use something I saw from Dan Higgins at CPPCON

template< typename TContainer >
static bool EraseFromUnorderedByIndex( TContainer& inContainer, size_t inIndex )
{
    if ( inIndex < inContainer.size() )
    {
        if ( inIndex != inContainer.size() - 1 )
            inContainer[inIndex] = inContainer.back();
        inContainer.pop_back();
        return true;
    }
    return false;
}

Since the list order doesn't matter, just take the last element in the list and copy it over the top of the item you want to remove, then pop and delete the last item.

The erase method will be used in two ways:

1. Erasing single element:

   vector.erase( vector.begin() + 3 ); // Deleting the fourth element
   

2. Erasing range of elements:

   vector.erase( vector.begin() + 3, vector.begin() + 5 ); // Deleting from fourth element to sixth element
   

I suggest to read this since I believe that is what are you looking for.https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom

If you use for example

 vec.erase(vec.begin() + 1, vec.begin() + 3);

you will erase n -th element of vector but when you erase second element, all other elements of vector will be shifted and vector sized will be -1. This can be problem if you loop through vector since vector size() is decreasing. If you have problem like this provided link suggested to use existing algorithm in standard C++ library. and "remove" or "remove_if".

Hope that this helped

template <typename T>
void remove(std::vector<T>& vec, size_t pos)
{
    std::vector<T>::iterator it = vec.begin();
    std::advance(it, pos);
    vec.erase(it);
}

It may seem obvious to some people, but to elaborate on the above answers:

If you are doing removal of std::vector elements using erase in a loop over the whole vector, you should process your vector in reverse order, that is to say using

for (int i = v.size() - 1; i >= 0; i--)

instead of (the classical)

for (int i = 0; i < v.size(); i++)

The reason is that indices are affected by erase so if you remove the 4-th element, then the former 5-th element is now the new 4-th element, and it won't be processed by your loop if you're doing i++.

Below is a simple example illustrating this where I want to remove all the odds element of an int vector;

#include <iostream>
#include <vector>

using namespace std;

void printVector(const vector<int> &v)
{
    for (size_t i = 0; i < v.size(); i++)
    {
        cout << v[i] << " ";
    }
    cout << endl;
}

int main()
{    
    vector<int> v1, v2;
    for (int i = 0; i < 10; i++)
    {
        v1.push_back(i);
        v2.push_back(i);
    }

    // print v1
    cout << "v1: " << endl;
    printVector(v1);
    
    cout << endl;
    
    // print v2
    cout << "v2: " << endl;
    printVector(v2);
    
    // Erase all odd elements
    cout << "--- Erase odd elements ---" << endl;
    
    // loop with decreasing indices
    cout << "Process v2 with decreasing indices: " << endl;
    for (int i = v2.size() - 1; i >= 0; i--)
    {
        if (v2[i] % 2 != 0)
        {
            cout << "# ";
            v2.erase(v2.begin() + i);
        }
        else
        {
            cout << v2[i] << " ";
        }
    }
    cout << endl;
    cout << endl;
    
    // loop with increasing indices
    cout << "Process v1 with increasing indices: " << endl;
    for (int i = 0; i < v1.size(); i++)
    {
        if (v1[i] % 2 != 0)
        {
            cout << "# ";
            v1.erase(v1.begin() + i);
        }
        else
        {
            cout << v1[i] << " ";
        }
    }
    
    
    return 0;
}

Output:

v1:
0 1 2 3 4 5 6 7 8 9

v2:
0 1 2 3 4 5 6 7 8 9
--- Erase odd elements ---
Process v2 with decreasing indices:
# 8 # 6 # 4 # 2 # 0

Process v1 with increasing indices:
0 # # # # #

Note that on the second version with increasing indices, even numbers are not displayed as they are skipped because of i++

Actually, the erase function works for two profiles:

• Removing a single element

  iterator erase (iterator position);
  

• Removing a range of elements

  iterator erase (iterator first, iterator last);
  

Since std::vec.begin() marks the start of container and if we want to delete the ith element in our vector, we can use:

vec.erase(vec.begin() + index);

If you look closely, vec.begin() is just a pointer to the starting position of our vector and adding the value of i to it increments the pointer to i position, so instead we can access the pointer to the ith element by:

&vec[i]

So we can write:

vec.erase(&vec[i]); // To delete the ith element

The erase method on std::vector is overloaded, so it's probably clearer to call

vec.erase(vec.begin() + index);

when you only want to erase a single element.

here is one more way to do this if you want to delete a element by finding this with its value in vector,you just need to do this on vector.

vector<int> ar(n);
ar.erase(remove(ar.begin(), ar.end()), (place your value here from vector array));

it will remove your value from here. thanks

If you work with large vectors (size > 100,000) and want to delete lots of elements, I would recommend to do something like this:

int main(int argc, char** argv) {

    vector <int> vec;
    vector <int> vec2;

    for (int i = 0; i < 20000000; i++){
        vec.push_back(i);}

    for (int i = 0; i < vec.size(); i++)
    {
        if(vec.at(i) %3 != 0)
            vec2.push_back(i);
    }

    vec = vec2;
    cout << vec.size() << endl;
}

The code takes every number in vec that can't be divided by 3 and copies it to vec2. Afterwards it copies vec2 in vec. It is pretty fast. To process 20,000,000 elements this algorithm only takes 0.8 sec!

I did the same thing with the erase-method, and it takes lots and lots of time:

Erase-Version (10k elements)  : 0.04 sec
Erase-Version (100k elements) : 0.6  sec
Erase-Version (1000k elements): 56   sec
Erase-Version (10000k elements): ...still calculating (>30 min)

How about this?

void squeeze(vector<int> &v)
{
    int j = 0;
    for (int i = 1; i < v.size(); i++)
        if (v[i] != v[j] && ++j != i)
            v[j] = v[i];
    v.resize(j + 1);
}

Erase an element with index :

vec.erase(vec.begin() + index);

Erase an element with value:

vec.erase(find(vec.begin(),vec.end(),value));

To delete an element use the following way:

// declaring and assigning array1 
std:vector<int> array1 {0,2,3,4};

// erasing the value in the array
array1.erase(array1.begin()+n);

For a more broad overview you can visit: http://www.cplusplus.com/reference/vector/vector/erase/

The previous answers assume that you always have a signed index. Sadly, std::vector uses size_type for indexing, and difference_type for iterator arithmetic, so they don't work together if you have "-Wconversion" and friends enabled. This is another way to answer the question, while being able to handle both signed and unsigned:

To remove:

template<class T, class I, class = typename std::enable_if<std::is_integral<I>::value>::type>
void remove(std::vector<T> &v, I index)
{
    const auto &iter = v.cbegin() + gsl::narrow_cast<typename std::vector<T>::difference_type>(index);
    v.erase(iter);
}

To take:

template<class T, class I, class = typename std::enable_if<std::is_integral<I>::value>::type>
T take(std::vector<T> &v, I index)
{
    const auto &iter = v.cbegin() + gsl::narrow_cast<typename std::vector<T>::difference_type>(index);

    auto val = *iter;
    v.erase(iter);

    return val;
}