C++11 Check if at least one element in vector is not in another vector

Question

I wrote the following code in order to check if at least one element in vector is not in another vector.

There are no duplicates in the vectors. Only unique elements

Is there a more elegant way to do it by using the stl?

// Online C++ compiler to run C++ program online #include <iostream> #include <vector> #include <algorithm> using namespace std; bool areVectorsDifferent(vector<int> &a, vector<int> &b){ if(a.size() != b.size()){ return true; } std::sort(a.begin(),a.end()); std::sort(b.begin(),b.end()); for(int i = 0; i < a.size();i++){ if(a[i] != b[i]) { return true; } } return false; } int main() { bool isDifferent = false; vector<int> a = {1,2,3,5}; vector<int> b = {4,3,2,1}; std::cout << areVectorsDifferent(a,b) << std::endl; return 0; }

You're missing a return value if they are not different, and return a != b; is a bit nicer than your loop. — molbdnilo
– molbdnilo, Commented Dec 17, 2021 at 14:39
I don't think this would work for something like {1,2,3,5,5}; and {1,1,2,3,5}; for example. — 001
– 001, Commented Dec 17, 2021 at 14:41

Botje · Accepted Answer · 2021-12-17 14:53:59Z

4

It depends on your definition of "different", but:

bool areVectorsDifferent(const vector<int> &a, const vector<int> &b){ return a.size() != b.size() || std::set<int>{a.cbegin(), a.cend()} != std::set<int>{b.cbegin(), b.cend()}; }

edited Dec 17, 2021 at 14:53

answered Dec 17, 2021 at 14:42

Botje

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12 Comments

Hani Gotc Over a year ago

sorry it should be IsAtLeastOneElementDifferent

Botje Over a year ago

So checking if the sizes are equal first should fix it, or do you have other constraints?

Hani Gotc Over a year ago

that's basically it

Vlad from Moscow Over a year ago

@Botje I do not know who upvoted this answer but it is a wrong answer. For example the two different vectors { 1, 1, 2 } and { 1, 2, 2 } will be identical according to the approach.

Botje Over a year ago

@VladfromMoscow Normally, yes. But OP stated there were no duplicates

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Ayman Al-Qadhi · Accepted Answer · 2021-12-17 14:59:53Z

I hope this solves your problem:

 using namespace std; template <typename T> auto are_different(const vector<T>& lhs, const vector<T>& rhs) -> bool { return lhs.size() != rhs.size() || any_of(cbegin(lhs), cend(lhs), [&rhs](const auto& item) { return find(cbegin(rhs), cend(rhs), item) == cend(rhs); }); }

Now imagine if there were 100000 elements in each vector. This will iterate 10000000000 times.

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