How can I modify values in a map using range based for loop?

c++, dictionary, for-loop

Solution

You can turn `auto` into `auto&` if you want to mutate/modify the container, for instance:

#include <map>
#include <iostream>

int main()
{
  std::map<int, int> foobar({{1,1}, {2,2}, {3,3}});
  for(auto& p : foobar) {
    ++p.second;
    std::cout << '{' << p.first << ", " << p.second << "} ";
  }
  std::cout << std::endl;
}

compiles ands outputs

{1, 2} {2, 3} {3, 4}

live example

Problem

I have a range based for loop to iterate over elements in `foobar` as follows: ``` #include <map> #include <iostream> int main() { std::map<int, int> foobar({{1,1}, {2,2}, {3,3}}); for(auto p : foobar) { ++p.second; std::cout << "{" << p.first << ", " << p.second << "} "; } std::cout << std::endl; for(auto q : foobar) { std::cout << "{" << q.first << ", " << q.second << "} "; } std::cout << std::endl; } ``` This code produces the following output: ``` {1, 2} {2, 3} {3, 4} {1, 1} {2, 2} {3, 3} ``` The first line is modified and printed inside a for loop and the second line supposedly prints the same modified values. Why don't the outputs match? Are changes to `std::map` only effective in the scope of the loop? Is there a way I can not only access but modify these values? A running version of this code can be found on cpp.sh. EDIT: The example given here was modified to match the accepted answer for clarity.

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