通过地图C ++反向进行STL循环

Question

const map<player, int>::iterator beginning = game->begin(); 
const map<player, int>::iterator inserted = game->find(player(x, y));
map<player, int>::iterator left = inserted;
while(left != beginning)
{
//some operations here
--left;
}

As you, see I want to loop through the map in reverse, accessing all the elements before the one i've just inserted. The problem is, with the construction above it doesn't access the first element and I have no idea why. Any suggeestions? Is game->begin() the first element? If so, how to solve it?

Answer 1

You can use rbegin() and rend() .

See here http://www.cplusplus.com/reference/map/map/rbegin/ .

Answer 2

game.begin() is iterator to the first element.

you can start from the end of map by assigning the iterator to rbegin

map<player, int>::reverse_iterator it = game.rbegin()

for (it = game.rbegin(); it != game.rend(); it++)
{
}

Answer 3

The following will access all the elements before the one just inserted (not including the one just inserted). It also protects against the situation where there are no elements 'before' the inserted element:

if (left != beginning) { 
    do {
        --left;
        //some operations here
    } while (left != beginning);
}

Answer 4

The condition in the loop is wrong. You are asking to skip the loop when the iterator is begin , but you actually want to execute it in that case.

There are different things that you can do, the one that will cause the lesser impact in your code is probably going to be changing the loop to be infinite and adding the exit condition before changing the iterator:

while (true) {
   // do something
   if (it == begin()) break;
   --it;
}

Alternatively you can use reverse iterators (which you can initialize with your current iterator) and test against rend() .