Have in C++ analog ContainsKey (TKey) from C# - what is best practics?

Question

Have in C++ analog IDictionary.ContainsKey (TKey) or List.Contains (T) from C# ?

For example I have array of elements and need to know have this array some value or not ? What is best way or best practics - without "foreach" for each element ! It will be good if it's will from std library for example.

UPD 1: In std lib have many containers, but I want to find a best way - faster, little code, less complicated and so on ...

Lookind that better desigion is std::unordered_set if going on this logic

    #include <unordered_set>

    std::unordered_set<std::string> NamesOfValues = { 
    "one", 
    "two", 
    "Date",
    "Time" 
    };

  // and now check is value exists in set
  if(NamesOfValues.count(value))
    {
        // value exists
    }

Could take a look at std::map (in particular - find() function): http://www.cplusplus.com/reference/map/map/find/ — Alfie J. Palmer, Aug 03 '17 at 13:43
May be, I have take a look - but may be have better desigion - for example std::map::count look even better... — Aleksandr Zolotov, Aug 03 '17 at 13:47
Possible duplicate of [Determine if map contains a value for a key?](https://stackoverflow.com/questions/3136520/determine-if-map-contains-a-value-for-a-key) — underscore_d, Aug 03 '17 at 13:50
@underscore_d- I know that for example std lib have many containers for values like map, set and so on - I ask about better desigion and best practics for do it ! — Aleksandr Zolotov, Aug 03 '17 at 14:01
May be you can find answer [here](https://stackoverflow.com/questions/3136520/determine-if-map-contains-a-value-for-a-key) — , May 30 '18 at 11:43

Krupip · Answer 1 · 2017-08-03T13:51:05.433

2

Your are looking for std::find. Find looks for an arbitrary type input to an arbitrary iterable and returns an iterator to that element.

For example, to find an element in a dictionary you can do the following:

std::unordered_map<char,int> my_map = { {'a', 1},{'b', 2}};

auto found_element = std::find(my_map.begin(), my_map.end(), 'a');
if( found_element == my_map.end() ){
   //nothing was found
}
else{
   // do something
}

For standard map you also have map.find(T) for O(1) access instead of O(n).

if( my_map.find('a') != my_map.end() ){
   //something was found!
}
else{
   //nothing was found
}

This is more clear than my_map.count()... you would only use that if you were actually trying to figure out how many elements you have and if you were using non unique keys.

edited Aug 03 '17 at 13:51

answered Aug 03 '17 at 13:42

Krupip

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@Alexander what do you mean by "big code", also i included an example now. There's no other standard way to do this as far as I know – Krupip Aug 03 '17 at 13:46
@Alexander you could wrap code like this in a boolean function and then just pass the parameters through. That way you could have something like .containsKey(...) return true or false – Alfie J. Palmer Aug 03 '17 at 13:48
@Alexander That doesn't make much sense if you are using unordered maps... it also would take the same amount of space, I'll edit my post to show it. – Krupip Aug 03 '17 at 13:53

underscore_d · Accepted Answer · 2017-08-03T14:03:41.000

2

It seems most concise to use count, and this should work for any container.

if ( my_map.count(key) ) { // shorthand for `count(key) != 0`
    // It exists
} else {
    // It does not
}

If we're talking about [unordered_]map and [unordered_]set, which are closest to your original dictionary type, then these containers enforce unique keys, so the returned .count() can only be 0 or 1, and there's no need to worry about the code pointlessly iterating the rest of the container once it finds a match (as would occur for containers supporting duplicates)

Either way, simply using implicit conversion to bool leads to the most concise code. And if you end up having a design that might allow/need different counts per key, then you could compare against a specific value.

edited Aug 03 '17 at 14:03

answered Aug 03 '17 at 13:49

underscore_d

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May be better if (my_map.count(key) > 0) ? – Aleksandr Zolotov Aug 03 '17 at 13:50
1

I don't see why. Neither `map` nor `set` can contain more than 1 element of a given key. So the result will either be 0 or 1. Why complicate things? But if you prefer the semantics of writing `> 0`, then sure, feel free. I guess it might make the code more easily transferable to another type of container later. – underscore_d Aug 03 '17 at 13:52
1

@Alexander I updated it simply to implicitly convert the returned count to `bool`, which looks even more concise. – underscore_d Aug 03 '17 at 13:56
Excuse me - what is your opinion - what is beter choise in map, set , unordered_map and unordered_set ? If I don't worry about order inside array ? – Aleksandr Zolotov Aug 03 '17 at 14:14
2

Isn't it simple? If you don't care about order, then use the unordered version, so you won't be paying for forced ordering that you don't care about. If you only need a key, then don't use a map, which also has a value that you don't care about. More generally, consult [In which scenario do I use a particular STL container?](https://stackoverflow.com/questions/471432/in-which-scenario-do-i-use-a-particular-stl-container) – underscore_d Aug 03 '17 at 14:15
1

You're welcome! Enjoy C++ and the many options in the Standard Library. :D – underscore_d Aug 03 '17 at 14:16

Have in C++ analog ContainsKey (TKey) from C# - what is best practics?

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