EDIT: Please note, I'm NOT asking why multimap can't contain duplicate keys.
What's the rationale behind multimap allowing duplicate key-value pairs? (not keys)
#include <map>
#include <string>
#include <iostream>
int
main(int argc, char** argv)
{
std::multimap<std::string, std::string> m;
m.insert(std::make_pair("A", "B"));
m.insert(std::make_pair("A", "B"));
m.insert(std::make_pair("A", "C"));
std::cout << m.size() << std::endl;
return 0;
}
This printed 3, which somewhat surprised me, I expected multimap to behave like a set of pairs, so I was expecting 2.
Intuitively, it's not consistent with C++ std::map behaviour, where insert does not always change the map (as opposed to operator[]).
Is there a rationale behind it, or it's just arbitrary?
Multimap only has a predicate ordering the keys. It has no method to determine whether the values are equal. Is value "A" a duplicate of value "a"? Without a second predicate for the values, there's no telling. Therefore, it doesn't even make sense to talk about duplicate values in a multimap.
If you would like a container that stores pairs, and enforces the unique-ness of both parts of the pair, look at boost::multi_index_container. It's very flexible, but takes a load of arguments as a result.
EDIT: This answer does not answer the current question anymore. I'll keep it as it is because it got upvoted a lot so it must be useful for some.
The multi in multimap stands for the fact that the same key can occur multiple times.
The standard puts no limit on the type used as value, so one cannot assume that operator==() is defined. Because we don't want the result of your code depend on whether the operator==() is defined or not, it is never used.
std::multimap is not a replacement for std::map. As you noticed, it behaves differently when the same key is inserted multiple times. If you want std::map's behaviour, use std::map.
There is also a std::multiset.
The rational: sometimes one would like to keep all old entries for the same key around as well. [TBD: Insert some example here]
Personally, I barely ever use std::multimap. If I want multiple entries for the same key, I usually rely on std::map<std::vector<T> >.
The values are allowed to be duplicates because they are not required to be comparable to each other. The container cannot do anything with the values besides copy them in. This enables types like multimap< int, my_class >.
If duplicate key-value pairs are undesirable, then use set< pair< T, U > > and use lower_bound to find the first match to a given key.
As you know, multimap allows to have multiple keys. Since it does not place any constraints on values comparability, it is unable to check, if values haven't been doubled.
If you want to have some dictionary data structure which allows for duplicate keys, but not key-value pairs, you would have to ensure that values are comparable.
Let's say we have a game of some sort, where there is 2D world of sqaure fields, and you can put items on fields. You can have multimap<Field, Item>, which will allow you to keep two identical items on the field. Items don't have to be comparable here.
My reasoning is multimap is based on the Key lookup/insertion and not on the value. So whether the value on duplicate keys is the same or not does not play a part when elements are being inserted.
23.3.2 Class template multimap
1 A multimap is a kind of associative
container that supports equivalent
keys (possibly containing multiple
copies of the same key value) and
provides for fast retrieval of values
of another type T based on the keys.
"multimap" is meant to support 'multiple' keys unlike simple "map". Since it allows multiple keys, it won't bother for their values, so it shows 3 elements in your example. The other difference is, one can not have operator [] for multimap.
A use of duplicate [map,value] pairs is to count the number of occurrences of say a word on a page of a book, be it no times, thus no entry in the multimap for that word, be it once with one entry, or more than once with the number of occurrences in multimap for make_pair(word, page_number). It was more by accident that design that I found this usage.
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Use cases of std::multimap
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I've just started learning STL containers, and I cannot understand why std::multimap exists. With std::map, we can access values by user-defined keys, but with std::multimap we cannot do that as the latter does not even have an overloaded operator[] and the same key can be mapped to several different values. To me, this looks like std::multimap is essentialy just something like std::multiset<std::pair<K, V>> with the Compare function operating on the K and we lose the main feature of a map which is the ability to access elements by key (as I see it). I've found this post, but still couldn't comprehend the usecases given here. Could someone please give me several examples when we would use std::multimap?
First note that std::map::operator[] is a little quirky. It is not the way to access elements in the map. Instead std::map::operator[] potentially inserts an element into the map and then returns a reference to either the element that was already present before or to the newly inserted. This may seem like splitting hairs, but the difference matters. The way to access a mapped_value given a key in a std::map is std::map::find. std::multimap has a find as well. No big difference with respect to that.
std::map::at has not counterpart in std::multimap because std::map::at returns a reference to the mapped_value for the given key, but in the multimap there can be more than one mapped_value for the same key, so it isnt obvious what a std::multimap::at should return if it existed. Finding and accessing elements can be done with find for both maps.
A std::multimap<K,V> can be compared to a std::map<K,std::vector<V>> but with the interface you'd expect when you want to map more than a single value to the same key. For example std::multimap iterators lets you iterate all key-value pairs in the multimap in one go. With the map of vectors you'd have to use the maps iterators and the vectors iterators. Using the map of vectors with standard algorithms is rather cumbersome.
Further, std::multimap::count returns the number of elements for a given key. With the map of vectors you would have to first find the vector for given key and call its size. This list is not complete, but I hope the difference gets clear.
One example for a multimap could be inhabitants of houses. In the same house lives more than one person. If you want to map street number to person you could use a multimap.
More generally, if you have a collection / container of elements and you want to divide them into distinct groups, you can use a multimap. A common use of std::map (or std::unordered_map) is to count frequencies, eg:
std::map<int,int> freq;
for (const auto& x : some_container) {
++freq[ x % 3 ];
}
This counts how many elements of some_container are divisible by 3, without remainder, with remainder 1, or with remainder 2. If you want to know the elements rather than only count them you can use a std::multimap.
In CPP primer or other websites I have found the language of count (from map STL) definition very vague and misleading:
Searches the container for elements with a key equivalent to k and returns the number of matches
Now what I have studied so far is that key is singular and so is the mapped value - the mapped value can be changed through assignment.
So doesn't it just returns whether the container contains the key or not? Rather than the count? Where am I wrong in understanding the concept?
A std::map's count() will always return either 0 or 1.
But the C++ library has other associative containers that might very well have multiple values for the same key. Like std::multimap and std::multiset. And by a very lucky coincidence they also have a count() method that may actually return values greater than 1.
But what this allows you to do is metaprogramming by developing templates that can use any associative container, one that may or may not be unique. All your template needs to do is use count() to determine how many values exist in the container with the given key, and the end result can be used with either std::map or std::multimap. It won't care in the slightest. In both cases, your template will get the right answer: the number of values in the container with the given key.
According to cplusplus.com
Because all elements in a map container are unique, the function can only return 1 (if the element is found) or zero (otherwise).
It may be a little bit stupid question, but assuming the std::map defined as follows:
std::map<int, int> m;
Is there any way to have more than one value stored and be accessible for a single key?
Motivation of asking:
std::map has methods like count() and equal_range() that get a key as parameters, that kind of give a sense that there is more than one value may be specified by a single key.
Those methods exist so as to provide a common interface with other associative containers that do allow multiple values per key (such as std::multimap which is exactly what you're looking for).
This makes implementing algorithms generically (i.e. with templates) much easier than it would otherwise be, and nothing of value is lost by designing it this way.
It's true that, in the case of std::map, count() can only give you zero or one (unless you're using transparent keys, which are a whole other kettle of fish).
C++20 will introduce std::map::contains(), which is more or less a check that count() == 1 — this seems to have been intended to address concerns that the function count() is kind of a weird thing to have for a std::map specifically.
Is there any way to have more than one value stored and be accessible for a single key?
Not with std::map, these objects store only one value per key, but std::multimap can store a variable number of values per key.
Similarities between both types (e.g. std::map::count, std::multimap::count) are due to establishing similar interfaces between STL containers.
No. std::map is designed to have one value per key.
If you want multiple values for one key, you should use std::multimap.
I'm having a hard time wrapping my head around unordered_maps and unordered_multimaps because my test code isn't producing what I've been told to expect.
std::unordered_map<string, int> names;
names.insert(std::make_pair("Peter", 4));
names.insert(std::make_pair("George", 4));
names.insert(std::make_pair("George", 4));
When I iterate through this list, I get one instance of George first, then Peter.
1) It's my understanding unordered_maps do not allow multiple keys to map to one value, and that multimaps due. Is this true?
2) Why can Peter and George coexist at a value of 4? What is happening to the second George? And for that matter, why is George appearing first when I iterate from begin() to end() if this is unordered?
3) What is the underlying representation of an unordered map vs. unordered multimap?
4) Is there a way to insert keys into either map without providing a value? E.g. have the compiler create its own hash function that I don't need to worry about when I retrieve keys and look for collisions?
I'll make it short:
No. Multi... refers to keys. A (non-multi)map can't have multiple equivalent keys with differeny values, ie. per key there is at most one value. A multi map can. The same holds for the unordered versions.
Peter != George, which is why they have different key and may very well have the same value.
A hashmap.
Use sets.
In your example the second insertion for George using a (non-multi) is skipped as the same key was previously inserted.
You want to use unordered_multimap to have several keys that are the same.
Since this is unordered you can't really hope to have any particular order, because it depends on the hash function.
If you want order in which you insert things, you need to use std::vector. Even normal maps, which are supposed to be ordered imply the comparison order, and not the order in which you insert things, for example string "AB" comes before "BB", because "A" is less than "B".
To insert without providing a value you need a set, and not a map.
The underlying structure of "unordered_" things is hashtable.
I've need a solution, which will store a non unique key - values pairs. I don't want to repeat keys (space efficiency) and I want to focus on lookup speed (efficiency of inserting a new data is less important). I will use std::multimap here. But I will have to lookup for keys, which meets some range criteria.
The most complex example:
Key is a string, values are not important.
I'd like to find out all values, which keys starts with "Lol".
Or I'd like to find out all values, which keys "are between" "bar" and "foo".
Can I do it with a multimap? My second thought is to use sorted vector, which will point to vectors of values. Something like that:
std::vector<std::string, std::vector<T>> sorted_vec;
Then I can easily meet search criteria. But I really care about a performance of lookups. Is it a right approach?
Yes, you can use std::multimap.
And to complete your range bases query, you can use the two algorithms std::lower_bound and std::upper_bound in the header file algorithm.
Or I'd like to find out all values, which keys "are between" "bar" and "foo".
If you have Boost, I would recommend to use boost::flat_map (it just header-only library) - it maintains sorted vector inside. Generally, it has better lookup than std::map due to better compactness/locality.
Otherwise use
vector<pair<string,value>>
or
vector<pair<string,vector<value>>> //(for re-use of keys)
// instead of pair, you may use just struct if you concerned with lexicographical compare
plus std::sort
plus std::equal_range/lower_bound
You should use a Patricia trie.
There's one in libstdc++, though it's nonstandard.
If you're going to stick with standard containers, I think your sorted vector solution is best.