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 ?

Answer 1

I am a beginner so here is a beginners answer. The if in the for loop gives i which can then be used however needed such as Numbers[i] in another vector. Most is fluff for examples sake, the for/if really says it all.

int main(){
vector<string>names{"Sara", "Harold", "Frank", "Taylor", "Sasha", "Seymore"};
string req_name;
cout<<"Enter search name: "<<'\n';
cin>>req_name;
    for(int i=0; i<=names.size()-1; ++i) {
        if(names[i]==req_name){
            cout<<"The index number for "<<req_name<<" is "<<i<<'\n';
            return 0;
        }
        else if(names[i]!=req_name && i==names.size()-1) {
            cout<<"That name is not an element in this vector"<<'\n';
        } else {
            continue;
        }
    }

Answer 2

Nobody mentioned the general version as a function for any std::vector<T> so here it is (in one ugly line) :

#include <algorithm>

template <typename T> long long int indexOf(const std::vector<T> &vector , const T &data){

   return (std::find(vector.begin(), vector.end(), data) != vector.end()) //Is it in the vector ?
           ?
           std::distance(vector.begin(), std::find(vector.begin(), vector.end(), data)) //Yes, take the distance (C++11 and above, see the previous answers)
           : -1; //No, return -1
}

And without the ternary operator :

#include <algorithm>

template <typename T> long long int indexOf(const std::vector<T> &vector , const T &data){

     if((std::find(vector.begin(), vector.end(), data) != vector.end())){
         return std::distance(vector.begin(), std::find(vector.begin(), vector.end(), data));
     }
     else{return -1;}
}

It returns either the index or -1 if the element is not in the vector.

---

Note that this may be very slow for big vectors, as it computes two times std::find.

Answer 3

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);
}

Answer 4

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_));