How to convert vector iterator to int in C++

问题

I am looking for an element in a C++ vector, and when I find it, I want to get found element's index in a numerical form(integer, float).

My naive attempt is this :

int x;
int index;
vector<int> myvector;
vector<int>::iterator it;
it = find(myvector.begin(), myvector.end(), x);  
index = (int) * it;

This code is giving error. Can you tell me how I can convert iterator to int(if possible), or can you tell me how I can get found element's index in other way? Thanks.

回答1:

You need to use standard function std::distance

index = std::distance( myvector.begin(), it );

if ( index < myvector.size() )
{
    // do something with the vector element with that index
}

Try always to use std::distance even with random access iterators. This function is available in the new and old C++ Standards.

回答2:

If you want the index of the found element, then that's the distance from the start of the sequence:

index = it - myvector.begin();

or, since C++11,

index = std::distance(myvector.begin(), it);

which will work with any forward iterator type, not just random-access ones like those from a vector.

回答3:

You just dereference the iterator like this

index = *it;

However you should first see if you actually found something

it = find(myvector.begin(), myvector.end(), x);  
if (it != myvector.end())
{
    index = *it;
}

To find the index in that the match was found, you can use subtraction of the found pointer from the start of the vector.

it = find(myvector.begin(), myvector.end(), x);  
if (it != myvector.end())
{
    index = it - myvector.begin();  // Index from start of vector
}

Also, hopefully in your actual code you defined x, as in the snippet you showed x is uninitialized so this will result in undefined behavior.

来源：https://stackoverflow.com/questions/26995725/how-to-convert-vector-iterator-to-int-in-c

标签

c++

vector

iterator