What's the simplest way to create an STL - identity map?

Question

I'd like to initialize a map - object "id" with identities from 0 to n-1, i.e.

 id[0] = 0
 id[1] = 1
 .
 .
 id[n-1] = n-1

Is there a simple way - a one-liner, a method inside the map-object, simply something really simple - that does that?

Answer 1

You could use the

template <class InputIterator>
map(InputIterator f, InputIterator l,
    const key_compare& comp)

form of the constructor, but you'd need to build an InputIterator that worked as a generator function over the range you want. That'd be a whole lot more typing than just using a for loop.

Answer 2

What is wrong with

for(unsigned int i = 0; i < n; ++i)
    id[i] = i;

Answer 3

It seems a little weird to use a map with the key being a simple index. Are you sure you cannot use a vector? Doing so, you could use boost::counting_iterator to fill the vector:

std::vector<int> v(boost::counting_iterator<int>(0), 
                   boost::counting_iterator<int>(N-1));
// v is filled with integers from 0 to N-1

However, I'm not sure whether this is a huge gain compared to the simple for loop.

Answer 4

Look out for std::iota as part of the numeric library (C++0x feature):

template <ForwardIterator Iter, HasPreincrement T>
requires OutputIterator<Iter, const T&>
void iota(Iter first, Iter last, T value);