CODEWITHSUNDEEP
c++stliterator
BeeBand
BeeBand

Reputation: 11483

Is there any reason that the STL does not provide functions to return an iterator via index?

Is there a reason that the STL does not provide functions to return an iterator into a container via an index?

For example, let's say I wanted to insert an element into a std::list but at the nth position. It appears that I have to retrieve an iterator via something like begin() and add n to that iterator. I'm thinking it would be easier if I could just get an iterator at the nth position with something like, std::list::get_nth_iterator(n).

I suspect I have misunderstood the principles of the STL. Can anyone help explain?

Thanks BeeBand

Upvotes: 3

Views: 518

Answers (4)

Michael Burr
Michael Burr

Reputation: 340168

You can use advance() from the <iterator> header:

list<foo>::iterator iter = advance(someFooList.begin(), n); 

list<foo>::iterator iter = someFooList.begin();

std::advance( iter, n);

If the iterator supports random access (like vector) it'll work quite efficiently, if it only supports increasing (or decreasing) the iterator, like list, it'll work but only as well as can be.

Upvotes: 12

Jerry Coffin
Jerry Coffin

Reputation: 490048

Generally, anything that might be costly is made a bit clumsy, so you won't do it by accident. Using your example, with a container that provides random access iterators it would be simply container.begin()+n, but for an std::list (which provides a forward iterator) you'd need to use list.begin() followed by advance().

If you want to get the Nth iterator, chances are that you just shouldn't be using std::list in the first place. Then again, if you started that sentence at "chances", it would remain nearly as true...

Upvotes: 1

nothrow
nothrow

Reputation: 16168

std::list isn't random-access container, so there is no reason for accessing n-th element. if you need this, consider using std::vector instead..

Upvotes: 2

Matt Greer
Matt Greer

Reputation: 62027

std::list is a linked list. So it does not support random access. To get to the nth position in the list, you have to start at the beginning and move through all the nodes until you arrive at n. This is pretty expensive (O(n)), and thus it's bad to have a method that does not suggest this expense. get_nth_iterator(n) implies getting the iterator that points to the nth node is cheap.

std::vector of course supports this directly with the [] operator, because the datastructure supports random access and so this is very inexpensive for it.

Upvotes: 5

Related Questions