I wanted to develop a function which takes a vector of any type and returns a map (similar to Counter in python)

Question

template<class T>
map<T,int> counter(vector<T> a)
{
    map<T,int> b;
    pair<map<T,int>::iterator ,bool> pos; //This line shows compiler message
    for(int i=0;i<a.size();++i)
    {
        pos=b.insert(make_pair(a[i],1));
        if(!(pos.second))
        {
            b[pos.first -> first]++;
        }
    }
    return b;
}

This is code using templates.

map<int,int> counter(vector<int> a)
{
    map<int,int> b;
    pair<map<int,int>::iterator ,bool> pos;
    for(int i=0;i<a.size();++i)
    {
        pos=b.insert(make_pair(a[i],1));
        if(!(pos.second))
        {
            b[pos.first -> first]++;
        }
    }
    return b;
}

This is the function if I rewrite for integers alone. Integers code is working fine and first one is not. Can you explain me why it is happening and How can I modify it so I works for every data type?

Without seeing what errors you get (when posting questions about build error always include the full build log, complete and unedited), but my guess is that you should read [Where and why do I have to put the “template” and “typename” keywords?](http://stackoverflow.com/questions/7923369/when-is-the-typename-keyword-necessary) — Some programmer dude, Dec 11 '15 at 08:49

score 2 · Answer 1 · answered Dec 11 '15 at 09:30

When you say "the first isn't working fine" are you implying that it doesn't compile due to some funky error about iterator? Inside a template dependent names have to be marked as types using typename:

pair<map<T, int>::iterator, bool> pos;

The name iterator is a nested name inside std::map<T, int>. You'd just use

pair<typename map<T, int>::iterator, bool> pos;

instead.

Personally, I'd be inclined to entirely side-step the issue an implement the function like this:

template<class T>
std::map<T, int> counter(std::vector<T> const& a)
{
    std::map<T, int> result;
    std::for_each(a.begin(), a.end(), [&](T const& value){ ++result[value]; });
    return result;
}

This implementation takes advantage of the fact that operator[]() inserts new elements with a value initialized value (i.e., using T()). Put differently, when there is on object inside the map, it will return a reference to a zero initialized int.

score 0 · Answer 2 · edited May 23 '17 at 12:07

0

map<T,int>::iterator is a dependent name, i.e. it is dependent on a template parameter. You need to tell the compiler that it always names a type so that it can be parsed correctly:

pair<typename map<T,int>::iterator ,bool> pos;
//   ^^^^^^^^

See this question for more information about the typename keyword and when you need to use it.

edited May 23 '17 at 12:07

Community

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answered Dec 11 '15 at 08:48

TartanLlama

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I wanted to develop a function which takes a vector of any type and returns a map (similar to Counter in python)

2 Answers2