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As the question states ... I don't get the point about multisets / multimaps.
So, what's the purpose?
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The multiset object uses this expression to determine both the order the elements follow in the container and whether two element keys are equivalent (by comparing them reflexively: they are equivalent if ! comp(a,b) && ! comp(b,a)). This can be a function pointer or a function object (see constructor for an example).
Multisets are part of the C++ STL (Standard Template Library). Multisets are the associative containers like Set that stores sorted values (the value is itself the key, of type T), but unlike Set which store only unique keys, multiset can have duplicate keys. By default it uses < operator to compare the keys.
A MultiSet is a data structure which stores and manipulates an unordered collection of elements which may be repeated. It is implemented as a Maple object.
The multiset::find() is a built-in function in C++ STL which returns an iterator pointing to the lower_bound of the element which is searched in the multiset container. If the element is not found, then the iterator points to the position past the last element in the set.
Some use cases:
multimap
multiset
is in essence a map with a key and a integer count.
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The most important benefit of using a multiset over a vector/list(or any other container) is the time complexity of find operation. average case time complexity for multiset is O(logn) and unordered_multiset is O(1). Same is true for multimap and ordered_multimap.
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