Iterating through map keys in C++ 17

Question

I am looking for a way to iterate through the keys of a map in C++ 17. The way that I have in mind right now is based on the answer of this question, the way is presented below.

for (auto const& [i, val] : myMap)
...

However, I do not need to use the value val, I just need the value i. Therefore, the piece of code ... does not contain any call to the value val. Consequently, whenever I compile the code, the following warning message appears:

warning: unused variable ‘val’ [-Wunused-variable]
         for (auto const& [i, val] : myMap){
                                  ^

What I want is to find a way to iterate through (only) the keys of the map, ignoring the values. Does anyone have any idea how to do it?

einpoklum · Accepted Answer · 2020-01-10T18:24:42.587

Two options:

A ranged-for version of @πάνταῥεῖ 's answer:

for (auto const& pair : myMap) {
    auto key = pair.first;
    // etc. etc.
}

Use the ranges-v3 library (or std::ranges in C++20) to adapt the range myMap.begin() and myMap.end() by projecting it onto its first coordinate. Then you'd write something like:
```
for (auto key : keys_of(myMap) ) {
    // etc. etc.
}
```
you can do this without ranges if keys_of() materializes all of the keys, but that could be expensive for a larger map.

_{(If you have weird, heavy keys, then const auto& key instead of auto key.)}

score 2 · Answer 2 · answered Dec 25 '19 at 19:53

2

Does anyone have any idea how to do it?

Sure! You can just use a traditional for loop with an iterator:

for(auto it = myMap.begin(); it != myMap.end(); ++it) {
    auto i = it->first;
    // ...
}

answered Dec 25 '19 at 19:53

πάντα ῥεῖ

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Thank you. But I was thinking about something like: `for (auto key : myMap.keys())` similarly to Java. – Matheus Diógenes Andrade Dec 25 '19 at 20:36
@MatheusDiógenesAndrade There's no such equivalent in c++. – πάντα ῥεῖ Dec 25 '19 at 20:37

score 1 · Answer 3 · answered Jan 13 '20 at 21:44

1

How about using Go-like unused variable declaration with _:

for(auto const& [i, _] : myMap)
...

answered Jan 13 '20 at 21:44

1

how is that supposed to help? `warning: unused variable '_' ` – Spongman Jun 14 '21 at 19:20

Ayxan Haqverdili · Answer 4 · 2020-01-12T09:00:21.393

0

You can avoid the unused warnings by casting to void:

for (auto const& [key, val] : m) {
  (void)val;  // To avoid unused warnings

  // use key
}

edited Jan 12 '20 at 09:00

answered Jan 08 '20 at 20:13

Ayxan Haqverdili

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5
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Thank you, very interesting. Can you explain to me why it works? Because I don' know exactly what this "(void) val" means. – Matheus Diógenes Andrade Jan 10 '20 at 17:29
@MatheusDiógenesAndrade [What does casting to `void` really do?](https://stackoverflow.com/q/34288844/10147399) – Ayxan Haqverdili Jan 10 '20 at 17:33

score 0 · Answer 5 · answered Jan 13 '20 at 20:43

0

for(auto const & pair : myMap) since map internally stored element in std::pair format, hence you can access the element by writing, pair.first; for key or pair.second; for value.

answered Jan 13 '20 at 20:43

Owl

123
10

That was the solution proposed by @einpoklum. But thank you for the response. – Matheus Diógenes Andrade Jan 14 '20 at 20:42

score -2 · Answer 6 · edited Jan 11 '20 at 21:18

-2

for (auto& [key, std::ignore] : m) {
  // use key
}

edited Jan 11 '20 at 21:18

Ayxan Haqverdili

23,309
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answered Jan 11 '20 at 13:42

user12694894

1

7

Welcome to SO. Please edit your answer and add some description about the code. Code only answers are discouraged. – Pedram Jan 11 '20 at 14:54
2

Structured bindings must be identifiers; this is a *declaration*, not an assignment or reference binding. – Davis Herring Jan 11 '20 at 19:40
3

At least make sure your code compiles before dumping it. ([It doesn't](https://coliru.stacked-crooked.com/a/f96fc728ed856200).) – Lightness Races in Orbit Jan 11 '20 at 21:21
[std::ignore with structured bindings?](https://stackoverflow.com/q/40673080/10147399) – Ayxan Haqverdili Jan 11 '20 at 21:27

Iterating through map keys in C++ 17

6 Answers6