Using std::unordered_map

Introduction

std::unordered_map is just an associative container. It works on keys and their maps. Key as the names goes, helps to have uniqueness in the map. While the mapped value is just a content that is associated with the key. The data types of this key and map can be any of the predefined data type or user-defined.

Remarks

As the name goes, the elements in unordered map are not stored in sorted sequence. They are stored according to their hash values and hence, usage of unordered map has many benefits such as it only takes O(1) to search any item from it. It is also faster than other map containers. It is also visible from the example that it is very easy to implement as the operator ( [] ) helps us to directly access the mapped value.

Declaration and Usage

As already mentioned you can declare an unordered map of any type. Let's have a unordered map named first with string and integer type.

unordered_map<string, int> first; //declaration of the map 
first["One"] = 1; // [] operator used to insert the value 
first["Two"] = 2;
first["Three"] = 3;
first["Four"] = 4;
first["Five"] = 5;

pair <string,int> bar = make_pair("Nine", 9); //make a pair of same type
first.insert(bar); //can also use insert to feed the values

Some Basic Functions

unordered_map<data_type, data_type> variable_name; //declaration

variable_name[key_value] = mapped_value; //inserting values

variable_name.find(key_value); //returns iterator to the key value

variable_name.begin(); // iterator to the first element

variable_name.end(); // iterator to the last + 1 element


2017-06-13
2017-06-13
C++ Pedia
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