How can I print out C++ map values?

Question

I have a map like this:

map<string, pair<string,string> > myMap;

And I've inserted some data into my map using:

myMap.insert(make_pair(first_name, make_pair(middle_name, last_name)));

How can I now print out all the data in my map?

See [this question](http://stackoverflow.com/q/4850473/485561). — Mankarse, Dec 28 '12 at 14:26
You loop through the map using an iterator, take the key, the members of the pair, and apply `cout::operator< — , Dec 28 '12 at 14:28
http://stackoverflow.com/questions/1063453/how-to-display-map-contents This question and answer should do it. — Benjamin Trent, Dec 28 '12 at 14:28
possible duplicate of [How to loop through a c++ map](http://stackoverflow.com/questions/4844886/how-to-loop-through-a-c-map) — Hogan, Dec 28 '12 at 16:02

Armen Tsirunyan · Accepted Answer · 2021-07-26T10:59:23.373

109

for(map<string, pair<string,string> >::const_iterator it = myMap.begin();
    it != myMap.end(); ++it)
{
    std::cout << it->first << " " << it->second.first << " " << it->second.second << "\n";
}

In C++11, you don't need to spell out map<string, pair<string,string> >::const_iterator. You can use auto

for(auto it = myMap.cbegin(); it != myMap.cend(); ++it)
{
    std::cout << it->first << " " << it->second.first << " " << it->second.second << "\n";
}

Note the use of cbegin() and cend() functions.

Easier still, you can use the range-based for loop:

for(const auto& elem : myMap)
{
   std::cout << elem.first << " " << elem.second.first << " " << elem.second.second << "\n";
}

edited Jul 26 '21 at 10:59

answered Dec 28 '12 at 14:27

Armen Tsirunyan

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isn't the last one slower? – Paul Jul 26 '21 at 06:05
2

@Paul: I edited it to to `const auto&` to avoid the copying of the elements. Hope that is what you meant. Other than this, no, the last one is not slower – Armen Tsirunyan Jul 26 '21 at 11:00

score 29 · Answer 2 · edited Jun 28 '17 at 00:46

29

If your compiler supports (at least part of) C++11 you could do something like:

for (auto& t : myMap)
    std::cout << t.first << " " 
              << t.second.first << " " 
              << t.second.second << "\n";

For C++03 I'd use std::copy with an insertion operator instead:

typedef std::pair<string, std::pair<string, string> > T;

std::ostream &operator<<(std::ostream &os, T const &t) { 
    return os << t.first << " " << t.second.first << " " << t.second.second;
}

// ...
std:copy(myMap.begin(), myMap.end(), std::ostream_iterator<T>(std::cout, "\n"));

edited Jun 28 '17 at 00:46

mk12

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answered Dec 28 '12 at 14:40

Jerry Coffin

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1,067

Why is the & required in the above snipped? `for (auto& t : myMap) std::cout << t.first << " " << t.second.first << " " << t.second.second << "\n";` – Manjunath Oct 13 '17 at 18:49
5

Using a reference `&` avoids making a copy during the loop iteration. – CyprUS Mar 08 '18 at 23:20

honk · Answer 3 · 2019-03-21T10:59:41.557

25

Since C++17 you can use range-based for loops together with structured bindings for iterating over your map. This improves readability, as you reduce the amount of needed first and second members in your code:

std::map<std::string, std::pair<std::string, std::string>> myMap;
myMap["x"] = { "a", "b" };
myMap["y"] = { "c", "d" };

for (const auto &[k, v] : myMap)
    std::cout << "m[" << k << "] = (" << v.first << ", " << v.second << ") " << std::endl;

Output:

m[x] = (a, b)
m[y] = (c, d)

Code on Coliru

edited Mar 21 '19 at 10:59

answered Mar 21 '19 at 10:48

honk

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Good update! You might want to take your parameters as const reference in the for loop. – andreee Mar 21 '19 at 10:50
When I try doing the for loop, I get "identifier "k" is undefined." – AlphaFoxtrot Aug 20 '20 at 17:24
@AlphaFoxtrot: Did you really compile it for C++17? As you can see from the link to Coliru, the code is actually working. – honk Aug 20 '20 at 18:05
Ah I see. I recently downloaded Visual Studio and I thought it would put me on the latest. Turns out I was using C++98. – AlphaFoxtrot Aug 20 '20 at 18:26

score 0 · Answer 4 · answered Apr 24 '22 at 17:29

The easiest way is to declare an iterator first as
map<string ,string> :: iterator it;

and after that print it out by looping over the map using iterator from myMap.begin() to myMap.end() and print out key and value pairs in the map with it->first for key and it->second for value.

  map<string ,string> :: iterator it;
    for(it=myMap.begin();it !=myMap.end();++it)
      {
       std::cout << it->first << ' ' <<it->second;
      }

How can I print out C++ map values?

4 Answers4

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