C++ check if element exists in vector

Question

I'm working with a vector and need to check if there exists an element at a specific spot in the vector, like myvec[7]

I need to work outside of a loop and be able to perform this check for any given point in the vector. What would be the most efficient way to do this with a vector?

Boris Strandjev · Accepted Answer · 2012-04-07 17:08:03Z

8

This is the check you are looking for: (myvec.size() >= 8). In C++ there are no blank elements in a vector - i.e. the vector elements are with consecutive indices.

answered Apr 7, 2012 at 17:08

Boris Strandjev

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2 Comments

EduardoMaia Over a year ago

Hi Boris. I'm new in c++. You are saying that something like: std::vector<int> test; test.push_back(0); test[2] = 2; std::cout << test[0] << ", " << test[2]; Is not possible? I thought that it was possible to have a "blank" element on a vector, like in the example I just mentioned. How can we identify those blank elements?

Boris Strandjev Over a year ago

@EduardoMaia definitely not possible. This is not Ruby or Python. This will throw an exception in C++

kilotaras · Accepted Answer · 2012-04-07 17:29:47Z

3

There are two ways of doing this.

Next code examples will assume we want to do something with element v[n] in vector v.

Way 1:

if(n<v.size()) 
    //do something with v[n]
else
    //do something else

Way 2:

//do something using v.at(n) instead of v[n]

This will raise an exception if you try to access element that isn't in vector.

Which one to use? Depends on the case.
Can you work if element isn't in the vector? Use first method.
Having this element is crucial for your code. Use second method, let STL check its presence for you and don't forget to catch an exception.

answered Apr 7, 2012 at 17:29

kilotaras

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