How do I erase an element from std::vector<> by index?

问题

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(???);

回答1:

To delete a single element, you could do:

std::vector<int> vec;

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

// Deletes the second element (vec[1])
vec.erase(vec.begin() + 1);

Or, to delete more than one element at once:

// Deletes the second through third elements (vec[1], vec[2])
vec.erase(vec.begin() + 1, vec.begin() + 3);

回答2:

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.

回答3:

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);
}

回答4:

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
   

回答5:

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

回答6:

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.

回答7:

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)

回答8:

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/

回答9:

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

回答10:

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;
}

回答11:

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

回答12:

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);
}

回答13:

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);

来源：https://stackoverflow.com/questions/875103/how-do-i-erase-an-element-from-stdvector-by-index