129 lines
2.7 KiB
Markdown
129 lines
2.7 KiB
Markdown
---
|
|
title: Map
|
|
---
|
|
|
|
## Introduction of map
|
|
|
|
`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.
|
|
|
|
## Benefits of using 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 `map` there will be only one value attached with the every key.
|
|
* `map` can be used as associative arrays.
|
|
* It might be implemented using balanced binary trees.
|
|
|
|
Here is an example:
|
|
|
|
```cpp
|
|
#include <iostream>
|
|
#include <map>
|
|
|
|
using namespace std;
|
|
|
|
int main (){
|
|
map<char,int> first;
|
|
|
|
//initializing
|
|
first['a']=10;
|
|
first['b']=20;
|
|
first['c']=30;
|
|
first['d']=40;
|
|
|
|
map<char, int>::iterator it;
|
|
for(it=first.begin(); it!=first.end(); ++it){
|
|
cout << it->first << " => " << it->second << '\n';
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
```
|
|
a => 10
|
|
b => 20
|
|
c => 30
|
|
d => 40
|
|
```
|
|
|
|
## Creating map object
|
|
` map<string, int> myMap; `
|
|
|
|
## Get Size
|
|
Get size of map with size function
|
|
```
|
|
map<int, int > myMap;
|
|
myMap[100] = 3
|
|
count << "size of map is " << myMap.size() << '\n';
|
|
```
|
|
|
|
Output:
|
|
```
|
|
size of map is 1
|
|
```
|
|
|
|
## Insertion
|
|
Inserting data with insert member function.
|
|
|
|
```cpp
|
|
myMap.insert(make_pair("earth", 1));
|
|
myMap.insert(make_pair("moon", 2));
|
|
```
|
|
|
|
We can also insert data in std::map using operator [] i.e.
|
|
|
|
`myMap["sun"] = 3;`
|
|
|
|
If "sun" is already mapped before, this action will override the value mapped to key.
|
|
|
|
## Erase
|
|
Erasing data with erase function
|
|
|
|
```
|
|
map<int, int > myMap;
|
|
myMap[10] = 1000;
|
|
cout << "before erase, size of map is " << myMap.size() << '\n';
|
|
myMap.erase(10);
|
|
cout << "after erase, size of map is " << myMap.size() << '\n';
|
|
```
|
|
|
|
Output:
|
|
```
|
|
before erase, size of map is 1
|
|
after erase, size of map is 0
|
|
```
|
|
|
|
## Accessing map value
|
|
|
|
To access map values, simply call Map[key]. For example:
|
|
```
|
|
map<string, int > M;
|
|
M["abc"] = 1;
|
|
M["def"] = 2;
|
|
cout << "value of abc is " << M["abc"] << '\n';
|
|
cout << "value of def is " << M["def"] << '\n';
|
|
```
|
|
|
|
Output:
|
|
```
|
|
value of abc is 1
|
|
value of def is 2
|
|
```
|
|
|
|
## Accessing map elements
|
|
|
|
To access map elements, you have to create iterator for it. Here is an example as stated before.
|
|
```cpp
|
|
map<char, int>::iterator it;
|
|
for(it=first.begin(); it!=first.end(); ++it){
|
|
cout << it->first << " => " << it->second << '\n';
|
|
}
|
|
```
|
|
|
|
Here you can learn more about map: <a href="http://www.cplusplus.com/reference/map/map/map/" target="_blank">cpluspluc_map</a>
|
|
|
|
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>
|
|
|
|
|