How can I iterate in reverse over a map in C++?

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春和景丽 2021-02-06 23:14

I\'m having trouble iterating in reverse over a map in GCC C++. When I use a reverse iterator, it seems I can\'t assign anything to it - the compiler complains. I\'m working aro

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孤城傲影 (楼主)

2021-02-06 23:22

Here's an example of iterating backward through a std::map:

#include 
#include 
#include 

int main() {
    std::map m;
    m["a"] = "1";
    m["b"] = "2";
    m["c"] = "3";

    for (auto iter = m.rbegin(); iter != m.rend(); ++iter) {
        std::cout << iter->first << ": " << iter->second << std::endl;
    }
}

If you are pre-C++11, you'll just need to spell out auto, which is:

std::map::reverse_iterator

Note that if you're using boost, you can use a range-based for loop with a reverse adapter:

#include 

for (auto& iter : boost::adaptors::reverse(m)) {
    std::cout << iter.first << ": " << iter.second << std::endl;
}

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