114

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 !

Grzegorz Krukowski
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Nour
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4 Answers4

183

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_));
Sergey Kalinichenko
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  • Sorry, I can't see @Bob__ 's comments, maybe deleted? I'm wondering why `ptrdiff_t` is better than `size_t` since `ptrdiff_t ` would raise a warning of comparison between signed and unsigned integer – Hiraku Oct 17 '17 at 05:43
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    @Hiraku He did delete his comment. He suggested using `ptrdiff_t` because it lets you store the distance between any pair of iterators into the same container, even in situations when the result is negative. If we use `size_t` we must be careful not to subtract a larger iterator from a smaller iterator. – Sergey Kalinichenko Oct 17 '17 at 10:49
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    To be more precise, you should add "#include " (for using std::find). Author of question omitted this "include" too. – Grag2015 Oct 19 '18 at 21:19
107

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);
}
juanchopanza
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    why not use const-iterators? – dani Feb 13 '16 at 09:33
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    For beginners: To use `const_iterator`, use cbegin() and cend(). I.e., `auto it = std::find(Names.cbegin(), Names.cend(), old_name_);`, then `auto` would deduce to const_iterator, not a normal iterator. – starriet Oct 06 '22 at 11:00
0

I'm pretty sure pointer arithmetic applies the same way here as in arrays.

int getVecPos(std::vector<string> vec, string element){
    int i;
    for(i = 0; i < vec.size(); i++){
        if(vec[i] == element){
            break;
        }
    }
    if(i == vec.size()){
        std::cout<<"No such element as "<<element<<" found. Please enter again: ";
        std::cin>>element;
        i = getVecPos(vec, element);
    }
        
    return i;
}

You can add it to the vec.begin() to get an iterator that you can use for the other vector functions or you can use for finding the index of an element in the array. Example:-

int getVecPos(std::vector<string>, string);//This was the function from earlier.
std::vector<string> UpdateHealables(std::vector<string> healables, string healing_item){
    auto it = healables.begin() + getVecPos(healables, healing_item);
    healables.erase(it);
    return healables;
}
Hoen
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-3

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;
        }
    }
javer
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