How to get position of a certain element in strings vector, to use it as an index in ints vector?

Question

I am trying to get the index of an element in a vector of strings, to use it as an index in another vector of int type, is this possible ?

Example:

vector <string> Names; vector <int> Numbers;   ...  // condition to check whether the name exists or not if((find(Names.begin(), Names.end(), old_name_)) != Names.end())       {   // if yes         cout <<"Enter the new name."<< endl;         cin >> name;         replace(Names.begin(), Names.end(), old_name_, name);     } 

Now I want to get the position of old_name in the Names vector, to use it in accessing certain element in Numbers vector. So that I can say:

Numbers[position] = 3 ; // or whatever value assigned here. 

I tried using:

vector <string> :: const_iterator pos; pos = (find(Names.begin(), Names.end(), old_name_)) Numbers[pos] = 3; 

but obviously this doesn't work since pos is of type string !

Answer 1

To get a position of an element in a vector knowing an iterator pointing to the element, simply subtract v.begin() from the iterator:

ptrdiff_t pos = find(Names.begin(), Names.end(), old_name_) - Names.begin(); 

Now you need to check pos against Names.size() to see if it is out of bounds or not:

if(pos >= Names.size()) {     //old_name_ not found } 

vector iterators behave in ways similar to array pointers; most of what you know about pointer arithmetic can be applied to vector iterators as well.

Starting with C++11 you can use std::distance in place of subtraction for both iterators and pointers:

ptrdiff_t pos = distance(Names.begin(), find(Names.begin(), Names.end(), old_name_)); 

Answer 2

If you want an index, you can use std::find in combination with std::distance.

auto it = std::find(Names.begin(), Names.end(), old_name_); if (it == Names.end()) {   // name not in vector } else {   auto index = std::distance(Names.begin(), it); }