对的 Map 中的 lower_bound 和 upper_bound

Question

I am trying to understand the lower_bound and upper_bound in map of pairs from here .

The definitions are like below:

lower_bound: In map of pairs lower_bound() for pair(x, y) will return an iterator pointing to the pair whose first value is greater than or equals x and second value is greater than or equals to y. If the above-mentioned criteria are not met, then it returns an iterator which points to the pair {map.size(), 0} .

upper_bound: In map of pairs upper_bound() for pair(x, y) will return an iterator pointing to the pair whose first value is greater than x and second value is greater than y. If the above-mentioned criteria are not met, then it returns an iterator which points to the pair {map.size(), 0} .

Now, let's consider the below example.

map<pair<int, int>, int> mp;

mp.insert({ { 2, 3 }, 8 });
mp.insert({ { 2, 5 }, 5 });
mp.insert({ { 7, 1 }, 3 });
mp.insert({ { 9, 3 }, 1 });
mp.insert({ { 5, 0 }, 3 });

We have above map of pairs.

Now, if I want to find the lower bound of pair {2, 4} , the result is {2, 5} which seems fine to me according to the definition. The definition says that " lower_bound will return an iterator pointing to the pair whose first value is greater than or equals x and second value is greater than or equals to y ". So, in this case 2 is equal to 2 and 5 is greater than 4.

But, if I want to find the upper bound of pair {2,2} , the result is {2, 3} which seems to me wrong according to the definition. The definition says that " upper_bound will return an iterator pointing to the pair whose first value is greater than x and second value is greater than y " But in the above case, first value is equal to x. According to the definition, the upper bound of {2,2} should be {9, 3} where 9 is greater than 2 and 3 is greater than 2.

I think I missed something here. Can anyone please help me out here?

Answer 1

You are storing this by key value in an ordered map. The ordering of the maps' keys will be as follows:

<2, 3> | <2, 5> | <5, 0> | <7, 1> | <9, 3> 

See here.

So, the actual std::map::upper_bound reference says the following:

iterator upper_bound( const Key& key );

Returns an iterator pointing to the first element that is greater than key.

From this it is easy to see, the first key in the list that is greater than <2, 2> is <2, 3> (for the call mp.upper_bound(std::make_pair(2, 2)) ).

Answer 2

For upper_bound greater means greater according to the type of the key in the map. The key in this case is a pair of ints and the operator< for a pair of ints is used for this comparison. According to the definition of this operator if the first element of the pair is the same, the second elements are compared, so indeed {2, 3} is greater than {2, 2} .

Here is link to the gnu implementation for reference.