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Dec 10, 2012 · std::unordered_map store elements using hash table. Therefore, elements will not be stored in any sorted order. They will be stored in arbitrary order . Memory Usage : Memory usage is more in unordered_map as compared to map because unordered_map need space for storing hash table too.
In C++, however, this is not so: std::map is a sorted associative container; std::unordered_map is a hash-table based associative container introduced in C++11; So, in order to clarify the guarantees on ordering. In C++03: std::set, std::multiset, std::map and std::multimap are guaranteed to be ordered according to the keys (and the criterion ...
Jul 8, 2009 · You might combine a std::vector with a std::tr1::unordered_map (a hash table). Here's a link to Boost's documentation for unordered_map. You can use the vector to keep track of the insertion order and the hash table to do the frequent lookups. If you're doing hundreds of thousands of lookups, the difference between O (log n) lookup for std::map ...
Jan 2, 2017 · No. It will iterate based on the sorted order, not the order that you inserted elements. In the case of std::string, it sorts in lexicographic order (alphabetic order). If you want to iterate based on the insertion order, you're better off using a sequence container, such as a std::vector or a std::list.
Jan 22, 2013 · 20. std::map does that itself. You don't have to do anything. By default it sorts the keys in the increasing order. If you want it to do sorting in decreasing order, then pass std::greater<T> as third template argument to std::map. std::map<int, X> m1; //sorts key in increasing order.
Feb 20, 2011 · 6. You can't sort a std::map this way, because a the entries in the map are sorted by the key. If you want to sort by value, you need to create a new std::map with swapped key and value. map<long, double> testMap; map<double, long> testMap2; // Insert values from testMap to testMap2.
Jan 28, 2011 · You can use the iterator that is returned by the begin() method of the map template: std::map<K, V> myMap; std::pair<K, V> firstEntry = *myMap.begin() But remember that the std::map container stores its content in an ordered way. So the first entry is not always the first entry that has been added.
Take into account that value_type for std::map is defined the following way. typedef pair<const Key, T> value_type. Thus in my example p is a const reference to the value_type where Key is std::string and T is int. Also it would be better if the function would be declared as. void output( const map<string, int> &table );
Aug 19, 2016 · 126. As already mentioned, map allows to iterate over the elements in a sorted way, but unordered_map does not. This is very important in many situations, for example displaying a collection (e.g. address book). This also manifests in other indirect ways like: (1) Start iterating from the iterator returned by find(), or (2) existence of member ...
However, to account for an empty map, you should adapt the return line as follows (or similar): return it == mymap.end() ? -1 : it->second; Note 2: As also mentioned by Lance Diduck, you should be passing the map by const reference to your getMin() function. The way you did it, you are creating an unnecessary copy of the whole map. Code on Ideone
