What is a Map? How would I create and use one in C++?
- 7It's a device for looking things up, ironically, a bit like Google...anon– anon2009-06-22 13:17:43 +00:00Commented Jun 22, 2009 at 13:17
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3 Answers
If you mean std::map, it stores pairs of values. In each pair, the first value is called the key, and can be used to quickly look up the associated other value.
You can write:
std::map<std::string, int> ages; ages["Fred"] = 52; ages["Sue"] = 31; std::cout << "Fred's age is " << ages["Fred"] << std::endl; answered Jun 22, 2009 at 13:14
Daniel Earwicker
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What is a map - it's a data structure that holds pairs of related values. Each pair consists of a key and a value. Every key in a map must be unique, but different keys can have the same values. Maps are often used for caching or implementing lookup tables, much like a dictionary (which is actually what maps are called in some languages).
As for how to implement a map in c++, short answer is - you don't. You use std::map or one of its variants.
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Map is collection type end it is implemented in C++ in the STL (Standard Template Library) end here is the official explanation from the library documentation.
Map is a Sorted Associative Container that associates objects of type Key with objects of type Data. Map is a Pair Associative Container, meaning that its value type is pair. It is also a Unique Associative Container, meaning that no two elements have the same key. Map has the important property that inserting a new element into a map does not invalidate iterators that point to existing elements. Erasing an element from a map also does not invalidate any iterators, except, of course, for iterators that actually point to the element that is being erased.
Example
struct ltstr { bool operator()(const char* s1, const char* s2) const { return strcmp(s1, s2) < 0; } }; int main() { map<const char*, int, ltstr> months; months["january"] = 31; months["february"] = 28; months["march"] = 31; months["april"] = 30; months["may"] = 31; months["june"] = 30; months["july"] = 31; months["august"] = 31; months["september"] = 30; months["october"] = 31; months["november"] = 30; months["december"] = 31; cout << "june -> " << months["june"] << endl; map<const char*, int, ltstr>::iterator cur = months.find("june"); map<const char*, int, ltstr>::iterator prev = cur; map<const char*, int, ltstr>::iterator next = cur; ++next; --prev; cout << "Previous (in alphabetical order) is " << (*prev).first << endl; cout << "Next (in alphabetical order) is " << (*next).first << endl; } Here is the complete documentation of the Map type in STL.
I hope this help.
