at() is a new method for std::map in C++11. Rather than insert a new default constructed element as operator[] does if an element with the given key does not exist, it throws a std::out_of_range exception. (This is similar to the behaviour of at() for deque and vector.) Because of this behaviour it makes sense for … Read more
You can achieve this like following : map<string, int>::iterator it; for (it = symbolTable.begin(); it != symbolTable.end(); it++) { std::cout << it->first // string (key) << ‘:’ << it->second // string’s value << std::endl; } With C++11 ( and onwards ), for (auto const& x : symbolTable) { std::cout << x.first // string (key) << … Read more
Use map::find and map::end: if (m.find(“f”) == m.end()) { // not found } else { // found }
Personally, I would use the map, as its use implies a data lookup – using a switch usually indicates a difference in program behavior. Furthermore modifying the data mapping is easier with a map than with a switch. If performance is a real issue, profiling is the only way to get a usable answer. A … Read more
I don’t know of any existing function for that, but you can roll your own from something similar to std::merge implementation to have linear complexity: template<class Map, class Merger> void merge(Map& dest, const Map& source, Merger merger) { auto it1 = dest.begin(); auto it2 = source.begin(); auto&& comp = dest.value_comp(); for (; it1 != dest.end() … Read more
The unordered_map container has a reserve method because it is implemented using buckets, and not a tree as in map. A bucket is: a slot in the container’s internal hash table to which elements are assigned based on the hash value of their key. Buckets are numbered from 0 to (bucket_count-1). (source) A single bucket … Read more
You can’t efficiently search for substring, but you can for prefix: #include <iostream> #include <map> #include <string> #include <algorithm> using namespace std; typedef map<string, string> TStrStrMap; typedef pair<string, string> TStrStrPair; TStrStrMap::const_iterator FindPrefix(const TStrStrMap& map, const string& search_for) { TStrStrMap::const_iterator i = map.lower_bound(search_for); if (i != map.end()) { const string& key = i->first; if (key.compare(0, search_for.size(), … Read more
You can use boost::multi_index to create a bidirectional map – you can use either value of the pair as a key to do a quick lookup.
Use a custom comparator when the default order doesn’t do it for you. You pass it as the third template parameter ( that’s normally defaulted to std::less<KeyType> ). In your case, you can use std::greater: std::map<int, int, std::greater<int> > m; Example code: #include <map> #include <iostream> #include <functional> int main() { std::map<int, int, std::greater<int>> m … Read more
Update (Back to the future): with C++11 range-based for loops – std::map<Key, Value> m { … /* initialize it */ … }; for (const auto &p : m) { std::cout << “m[” << p.first << “] = ” << p.second << ‘\n’; }