How to use range-based for() loop with std::map?

In C++, a map is defined by an element storage container in the form of key-value pairs where each key is unique and mapped with a respective value. While range-based for loop is a very simple approach to iterate over elements in the container. In this article, we will learn the usage of range-based for loop over std::map in C++.

Example Scenario

Input:
myMap: { {1, "Ravi"}, {2, "Tom"} }
Output:
1: Ravi
2: Tom

What is std::map?

The std::map is an associative container that stores key-value pairs in sorted order. The maps are used for fast lookup, insertion, and deletion based on keys.

Following is the basic syntax of map in C++ STL:

map<key_type, value_type, comp> m;

Here,

• key_type: This is data type of key.
• value_type: This is data type of value.
• comp: This is optional if you want to use then use it as it compares two keys of sorting.
• m: This is the name assigned to map.

Range-based for Loop

A range-based for loop allows you to iterate over elements of a container or array directly.

Below is the basic syntax of range-based for loop:

for ( range_declaration : range_expression ) {
   // block of code
}

Example to Use Range-based for() Loop with std::map

In this example, we use the map data structure, which is a part of the C++ STL. A map stores key-value pairs, where each key is unique. In this case, we insert characters as keys and strings as values. These key-value pairs form are printed with the help of range-based for loop:

#include<iostream>
#include<map>

int main() {
   
   // Declaration of map 
   // where each key is a char and each value is a string
   std::map<char, std::string> my_map;
   
   // insert key-value pairs into the map
   my_map.insert(std::pair<char, std::string>('A', "Apple"));
   my_map.insert(std::pair<char, std::string>('B', "Ball"));
   my_map.insert(std::pair<char, std::string>('C', "Cat"));
   my_map.insert(std::pair<char, std::string>('D', "Dog"));
   my_map.insert(std::pair<char, std::string>('E', "Elephant"));
   my_map.insert(std::pair<char, std::string>('F', "Fox"));
   my_map.insert(std::pair<char, std::string>('G', "Ghost"));
   my_map.insert(std::pair<char, std::string>('H', "Horse"));
   my_map.insert(std::pair<char, std::string>('I', "India"));
   my_map.insert(std::pair<char, std::string>('J', "Jungle"));
   
   // range-based for loop
   for(auto& key_val : my_map) {
      std::cout << "The " << key_val.first << " is pointing to: " << key_val.second << std::endl;
   }
return 0;
}

The above code produces the following result:

The A is pointing to: Apple
The B is pointing to: Ball
The C is pointing to: Cat
The D is pointing to: Dog
The E is pointing to: Elephant
The F is pointing to: Fox
The G is pointing to: Ghost
The H is pointing to: Horse
The I is pointing to: India
The J is pointing to: Jungle