129 lines
2.7 KiB
Markdown
129 lines
2.7 KiB
Markdown
---
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title: Map
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---
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## Introduction of map
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`map` is an associative container that store elements in key-value pair. Just like in `Java` we have collection, associative array in PHP and so on.
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## Benefits of using map
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* It stores only unique keys and that too in sorted order based on its assigned sorting criteria.
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* As keys are in sorted order therefore searching element in map through key is very fast i.e. it takes logarithmic time.
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* In `map` there will be only one value attached with the every key.
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* `map` can be used as associative arrays.
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* It might be implemented using balanced binary trees.
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Here is an example:
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```c++
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#include <iostream>
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#include <map>
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using namespace std;
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int main (){
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map<char,int> first;
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//initializing
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first['a']=10;
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first['b']=20;
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first['c']=30;
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first['d']=40;
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map<char, int>::iterator it;
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for(it=first.begin(); it!=first.end(); ++it){
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cout << it->first << " => " << it->second << '\n';
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}
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return 0;
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}
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```
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Output:
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```
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a => 10
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b => 20
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c => 30
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d => 40
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```
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## Creating map object
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` map<string, int> myMap; `
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## Get Size
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Get size of map with size function
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```
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map<int, int > myMap;
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myMap[100] = 3
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count << "size of map is " << myMap.size() << '\n';
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```
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Output:
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```
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size of map is 1
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```
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## Insertion
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Inserting data with insert member function.
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```c++
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myMap.insert(make_pair("earth", 1));
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myMap.insert(make_pair("moon", 2));
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```
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We can also insert data in std::map using operator [] i.e.
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`myMap["sun"] = 3;`
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If "sun" is already mapped before, this action will override the value mapped to key.
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## Erase
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Erasing data with erase function
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```
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map<int, int > myMap;
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myMap[10] = 1000;
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cout << "before erase, size of map is " << myMap.size() << '\n';
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myMap.erase(10);
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cout << "after erase, size of map is " << myMap.size() << '\n';
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```
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Output:
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```
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before erase, size of map is 1
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after erase, size of map is 0
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```
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## Accessing map value
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To access map values, simply call Map[key]. For example:
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```
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map<string, int > M;
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M["abc"] = 1;
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M["def"] = 2;
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cout << "value of abc is " << M["abc"] << '\n';
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cout << "value of def is " << M["def"] << '\n';
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```
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Output:
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```
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value of abc is 1
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value of def is 2
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```
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## Accessing map elements
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To access map elements, you have to create iterator for it. Here is an example as stated before.
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```c++
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map<char, int>::iterator it;
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for(it=first.begin(); it!=first.end(); ++it){
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cout << it->first << " => " << it->second << '\n';
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}
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```
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Here you can learn more about map: <a href="http://www.cplusplus.com/reference/map/map/map/" target="_blank">cpluspluc_map</a>
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N.B: All code in example are in C++11 version. You can learn more about C++ version <a href="http://en.cppreference.com/w/cpp/compiler_support" target="_blank">Here</a>
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