This post will discuss how to print out all keys and values from a std::map or std::unordered_map in C++.
Maps are associative containers whose elements are key-value pairs. Using a map, we can associate a value with some key and later retrieve that value using the same key. The retrieval operation in a map is very fast. There are several ways in C++ to print out all pairs present on the map:
1. Using range-based for-loop
The recommended approach in C++11 is to use the new range-based for-loops for printing the map pairs, as shown below:
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2 4 6 8 10 12 14 16 18 20 22 | #include <unordered_map> template<typenameK,typenameV> { std::cout<<'{'<<pair.first<<': '<<pair.second<<'}n'; } intmain() std::unordered_map<int,char>m={ {2,'B'}, }; } |
Output:
{3: C}
{1: A}
{2: B}
2. Using std::for_each function
Another simple solution is to use std::for_each. It takes an iterator to the beginning and end of the map, and applies a specified function on every pair within that range. The specified function may be a unary function, a lambda expression, or an object of a class overloading the () operator.
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 | #include <unordered_map> { voidoperator()(conststd::pair<K,V>&p){ } voidprint(conststd::pair<K,V>&p){ } template<typenameK,typenameV> { std::for_each(m.begin(), [](conststd::pair<int,char>&p){ }); // or pass an object of a class overloading the ()operator // std::for_each(m.begin(), m.end(), print<K, V>); { {1,'A'}, {3,'C'} } |
Output:
{3: C}
{1: A}
{2: B}
3. Using Iterator
C Program To Print World Map
We can also use iterators to iterate a map and print its pairs. It is recommended to use the const_iterator from C++11, as we’re not modifying the map pairs inside the for-loop.
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2 4 6 8 10 12 14 16 18 20 22 | #include <unordered_map> template<typenameK,typenameV> { std::cout<<'{'<<(*it).first<<': '<<(*it).second<<'}n'; } intmain() std::unordered_map<int,char>m={ {2,'B'}, }; } |
Output:
{3: C}
{1: A}
{2: B}
4. Operator<< Overloading
To get std::cout to accept the map object, we can overload the << operator to recognize an ostream object on the left and a map object on the right, as shown below:
2 4 6 8 10 12 14 16 18 20 22 24 | #include <unordered_map> template<typenameK,typenameV> conststd::unordered_map<K,V>&m){ os<<'{'<<p.first<<': '<<p.second<<'}n'; returnos; { {1,'A'}, {3,'C'} } |
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Output:
{3: C}
{1: A}
{2: B}
C Program To Print World Map Without
That’s all about printing out all keys and values from a map in C++.
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