C++ : Map Tutorial Part 1: Usage Detail with examples
In this article we see how & why to use std::map in c++.
std::map Introduction
std::map is an associative container that store elements in key-value pair.
Benefits of using std::map :
- It stores only unique keys and that too in sorted order based on its assigned sorting criteria.
- As keys are in sorted order therefore searching element in map through key is very fast i.e. it takes logarithmic time.
- In std::map there will be only one value attached with the every key.
- std::map can be used as associative arrays.
- It might be implemented using balanced binary trees.
Lets see an example,
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#include <iostream> #include <map> #include <string> #include <iterator> int main() { std::map<std::string, int> mapOfWords; // Inserting data in std::map mapOfWords.insert(std::make_pair("earth", 1)); mapOfWords.insert(std::make_pair("moon", 2)); mapOfWords["sun"] = 3; // Will replace the value of already added key i.e. earth mapOfWords["earth"] = 4; // Iterate through all elements in std::map std::map<std::string, int>::iterator it = mapOfWords.begin(); while(it != mapOfWords.end()) { std::cout<<it->first<<" :: "<<it->second<<std::endl; it++; } // Check if insertion is successful or not if(mapOfWords.insert(std::make_pair("earth", 1)).second == false) { std::cout<<"Element with key 'earth' not inserted because already existed"<<std::endl; } // Searching element in std::map by key. if(mapOfWords.find("sun") != mapOfWords.end()) std::cout<<"word 'sun' found"<<std::endl; if(mapOfWords.find("mars") == mapOfWords.end()) std::cout<<"word 'mars' not found"<<std::endl; return 0; } |
Output:
earth :: 4
moon :: 2
sun :: 3
Element with key ‘earth’ not inserted because already existed
word ‘sun’ found
word ‘mars’ not found
Creating std::map objects
Creating a std::map of words i.e.
Key = Word (std::string)
Value = Word’s frequency count (int)
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std::map<std::string, int> mapOfWords; |
As no external sorting criteria for key(std::string) is specified in above std::map, therefore it will use default key sorting criteria i.e operator < and all elements will be arranged inside std::map in alphabetical sorted order of keys.
Inserting data in std::map :
Inserting data using insert member function,
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mapOfWords.insert(std::make_pair("earth", 1)); mapOfWords.insert(std::make_pair("moon", 2)); |
We can also insert data in std::map using operator [] i.e.
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mapOfWords["sun"] = 3; |
Different between operator [] and insert function:
If specified key already existed in map then operator [] will silently change its value where as insert will not replace already added key instead it returns the information i.e. if element is added or not. e.g.
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mapOfWords["earth"] = 4; // Will replace the value of already added key. |
Where as for insert member function,
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mapOfWords.insert(std::make_pair("earth", 1)).second |
will return false.
Iterating through all std::map elements:
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std::map<std::string, int>::iterator it = mapOfWords.begin(); while(it != mapOfWords.end()) { std::cout<<it->first<<" :: "<<it->second<<std::endl; it++; } |
Each entry in std::map<std::string, int> is std::pair<std::string, int> therefore through iterator,
key can be accessed by it->first and value by it->second .
Searching element in std::map by key
find member function of std::map can be used to search element in std::map by key. If specified key is not present then it returns the std::map::end else an iterator to the searched element.
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iterator find (const key_type& k); //e.g. if(mapOfWords.find("sun") != mapOfWords.end()) std::cout<<"word 'sun' found"<<std::endl; if(mapOfWords.find("mars") == mapOfWords.end()) std::cout<<"word 'mars' not found"<<std::endl; |
Searching element in std::map by Value
To search element in std::map by value we need to iterate through all of the elements and check for the passed value and return i.e.
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#include <iostream> #include <map> #include <string> #include <iterator> std::map<std::string, int>::iterator serachByValue(std::map<std::string, int> & mapOfWords, int val) { // Iterate through all elements in std::map and search for the passed element std::map<std::string, int>::iterator it = mapOfWords.begin(); while(it != mapOfWords.end()) { if(it->second == val) return it; it++; } } int main() { std::map<std::string, int> mapOfWords; // Inserting data in std::map mapOfWords.insert(std::make_pair("earth", 1)); mapOfWords.insert(std::make_pair("moon", 2)); mapOfWords["sun"] = 3; std::map<std::string, int>::iterator it = serachByValue(mapOfWords, 3); if(it != mapOfWords.end()) std::cout<<it->first<<" :: "<<it->second<<std::endl; return 0; } |
Output:
sun :: 3
Deleting data from std::map
std::map’s erase member function is used to delete the element in std::map i.e.
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void erase (iterator position); size_type erase (const key_type& k); void erase (iterator first, iterator last); |
Code example,
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#include <iostream> #include <map> #include <string> #include <iterator> int main() { std::map<std::string, int> mapOfWords; mapOfWords.insert(std::make_pair("earth", 1)); mapOfWords.insert(std::make_pair("moon", 2)); mapOfWords["sun"] = 3; // Erasing By iterator std::map<std::string, int>::iterator it = mapOfWords.find("moon"); mapOfWords.erase(it); // Erasing By Key mapOfWords.erase("earth"); return 0; } |
Other Map related articles are,
1.) std::map Usage Detail with examples
3.) std::map & User defined class objects as keys
5.) How to Iterate over a map in C++
7.) Iterate a map in reverse order
8.) Check if a key exists in a Map
12.) Erase by Value or callback
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Thanks for the clear demonstration.
Thanks For appreciating.
How do I do something straightforward like get the value associated with at looked-up key and assign it to a variable?
Method 1 :
If you are sure that key exists in map then directly access using [] operator i.e.
value = mapOfWords[key];
Method 2 :
If you are not sure that key exists in map then,
auto it = mapOfWords.find(key);
if(it != mapOfWords.end())
{
value = it->second;
}
What if the key value type doesn’t have a default operator< ?
Then you can pass custom comparators. Checkout following articles for complete examples,
std::map and Comparators
std::map and User define objects