Check if element is in the list (contains)

Question

I've got a list of elements, say, integers and I want to check if my variable (another integer) is one of the elements from the list. In python I'd do:

my_list = [1,2,3,4] # elements
my_var = 3 # my variable
my_var in my_list # returns boolean

How to do that in C++? I thought of using std::list, but I can find no find method in it. I can see such method in std::set structure.

More deeply, the problem is that my program is given some unique ids (a list, a set, whatever) and I iterate over a long list of input data (ids) and check if they are included in the list (boolean value returned for each iteration step). And I'm not sure how should I do that in C++.

std::vector. In c++ use std::vector unless you have special reason not to. — user1937198, Jun 10 '14 at 11:07
Why use a vector if you're looking up values? Surely you'd want a `std::set` or `std::multiset` to give better performance than a linear search unless your list is tiny or you're using a sparse vector and keying on index. — Component 10, Jun 10 '14 at 11:10
my list/set would be rather tiny (several elements, less than 10). — ducin, Jun 10 '14 at 11:14

score 111 · Accepted Answer · edited Sep 09 '17 at 08:48

111

You can use std::find

bool found = (std::find(my_list.begin(), my_list.end(), my_var) != my_list.end());

You need to include <algorithm>. It should work on standard containers, vectors lists, etc...

edited Sep 09 '17 at 08:48

Rakete1111

47,013
16
123
162

answered Jun 10 '14 at 11:08

Matzi

13,770
4
33
50

141

..concise as always with C++. Who needs a trivial method like `contains()`?! – Jay Jun 13 '16 at 10:23
@Alex: Fixed the naming. – Matzi Jun 17 '17 at 22:49
9

I feel like adding support for C#-style extension methods would easily fix every complaint I ever made about C++. – BTownTKD Nov 09 '17 at 18:44
@Jay STL must cover all use cases but be as easy as possible to port between the 70-something platforms C++ runs on. `contains` would be one more method to support that doesn't add any functionality. You're looking for something like Boost. – c z Nov 09 '20 at 10:28
4

c++ solution as usual catastrophic. – bmi Jan 03 '21 at 11:27

score 31 · Answer 2 · edited Jul 13 '20 at 18:12

31

std::list does not provide a search method. You can iterate over the list and check if the element exists or use std::find. But I think for your situation std::set is more preferable. The former will take O(n) time but later will take O(lg(n)) time to search.

You can simply use:

int my_var = 3;
std::set<int> mySet {1, 2, 3, 4};
if(mySet.find(myVar) != mySet.end()){
      //do whatever
}

edited Jul 13 '20 at 18:12

Markus Dutschke

9,341
4
63
58

answered Jun 10 '14 at 11:09

Rakib

7,435
7
29
45

5

`myset.count(myVar) != 0` might be preferable – Tom Tanner Nov 23 '15 at 17:33
std::vector is preferable then - as it will in most cases be quite a bit faster. Don't get fooled just by the Big-O notation, it tells you nothing about the actual speed. Ifthe vector is sorted it is in general quite a bit faster than set.... and if you are even considering using a set then that also means you can just sort your vector anyways. – ABaumstumpf Jan 29 '21 at 09:44

score 9 · Answer 3 · answered Jun 10 '14 at 11:07

9

you must #include <algorithm>, then you can use std::find

answered Jun 10 '14 at 11:07

Radu Chivu

1,057
7
17

score 9 · Answer 4 · answered Jun 11 '18 at 16:23

They really should add a wrapper. Like this:

namespace std
{
    template<class _container,
        class _Ty> inline
        bool contains(_container _C, const _Ty& _Val)
        {return std::find(_C.begin(), _C.end(), _Val) != _C.end(); }
};
...
    if( std::contains(my_container, what_to_find) )
    {

    }

score 5 · Answer 5 · answered Jul 13 '20 at 18:20

A one-liner solution, similar to python, would be (std::set<int> {1, 2, 3, 4}).count(my_var) > 0.

Minimal working example

int my_var = 3;
bool myVarIn = (std::set<int> {1, 2, 3, 4}).count(my_var) > 0;
std::cout << std::boolalpha << myVarIn << std::endl;

prints true or false dependent of the value of my_var.

score 1 · Answer 6 · answered Jun 10 '14 at 11:10

1

Use std::find, something like:

if (std::find(std::begin(my_list), std::end(my_list), my_var) != std::end(my_list))
    // my_list has my_var

answered Jun 10 '14 at 11:10

Paul Evans

27,315
3
37
54

score 1 · Answer 7 · answered Feb 23 '19 at 08:31

Declare additional helper function like this:

template <class T, class I >
bool vectorContains(const vector<T>& v, I& t)
{
    bool found = (std::find(v.begin(), v.end(), t) != v.end());
    return found;
}

And use it like this:

void Project::AddPlatform(const char* platform)
{
    if (!vectorContains(platforms, platform))
        platforms.push_back(platform);
}

Snapshot of example can be found here:

https://github.com/tapika/cppscriptcore/blob/b7f3d62747494a52a440482e841ffb016a3fc56e/SolutionProjectModel/Project.cpp#L13

score 0 · Answer 8 · answered Mar 13 '22 at 12:41

0

Since C++20 you can use l.contains(my_var)

https://www.modernescpp.com/index.php/more-convenience-functions-for-containers-with-c-20

answered Mar 13 '22 at 12:41

ZyReS

9
1

1

You should mention that this only works for associative containers, like `set`, `map`, etc. It won't work for other containers, such as `vector`. – cigien Mar 13 '22 at 14:56
1

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - [From Review](/review/late-answers/31277856) – Abhishek Dutt Mar 16 '22 at 02:24

Check if element is in the list (contains)

8 Answers8

Linked