map iterator doesn't have (+ n) iterator - any work around this?

[sheva29]

Is there any particular reason why you can't assign a specific position to the iterator of a map?

e.g

map <string,bool>iterator it = myMap.end() - 1;// not allowed

And is also a way around this?

Thanks!

[Disch]

Is there any particular reason why you can't assign a specific position to the iterator of a map?

maps are not built for random access, so in order to advance X spaces, it has to follow a trail of linked pointers. This means it takes O(n) time and not O(1) time like you'd except a + or - operation to take.

And is also a way around this?

You can use std::advance:

std::advance( it, X );   // it += X
std::advance( it, -X );  // it -= X 

But again note that this will have O(n) time with a map.



EDIT: fixed advance example

[sheva29]

@Disch

Super helpful, thanks so much!