Iterate over a map in C++

This post will discuss how to iterate over a map in C++.

The std::begin and std::end function returns an iterator pointing to the first element and the one-past-the-last element in the sequence. We can iterate over a map that starts with std::begin and ends at std::end, as shown below:

#include <iostream>

#include <map>

#include <string>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

for (auto it = map.begin(); it != map.end(); it++) {

std::cout << "{" << it->first << ", " << it->second << "}" << std::endl;

}

return 0;

}

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Output:

{one, 1}
{three, 3}
{two, 2}

We can also use the range-based for-loop over the iterator-based for-loop.

#include <iostream>

#include <map>

#include <string>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

for (const auto &entry: map) {

std::cout << "{" << entry.first << ", " << entry.second << "}" << std::endl;

}

return 0;

}

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Output:

{one, 1}
{three, 3}
{two, 2}

Starting with C++17, consider using the range-based for-loop with structured bindings:

#include <iostream>

#include <map>

#include <string>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

for (auto const& [key, val] : map) {

std::cout << "{" << key << ", " << val << "}" << std::endl;

}

return 0;

}

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Output:

{one, 1}
{three, 3}
{two, 2}

To iterate over the map in reverse order, consider using the range-based for-loop with a reverse adapter offered by the boost library:

#include <iostream>

#include <map>

#include <boost/range/adaptor/reversed.hpp>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

for (const auto &entry: boost::adaptors::reverse(map)) {

std::cout << "{" << entry.first << ", " << entry.second << "}" << std::endl;

}

return 0;

}

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Output:

{two, 2}
{three, 3}
{one, 1}

This is equivalent to the following code using reverse iterators:

#include <iostream>

#include <map>

#include <boost/range/adaptor/reversed.hpp>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

for (auto it = map.rbegin(); it != map.rend(); it++) {

std::cout << "{" << it->first << ", " << it->second << "}" << std::endl;

}

return 0;

}

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Output:

{two, 2}
{three, 3}
{one, 1}

That’s all about iterating over a map in C++.

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