checking whether a particular element exists or not in a c++ STL vector

Question

I wanted to check whether an element exist at a particular vector location, say i, before accessing it like v[i]. Could you let me know how can I do that?

Thank you.

Answer 1

if (0 <= i  &&  i < v.size()) {
  // OK
  std::cout << v[i]; // Example
} else {
  // Wrong
}

Answer 2

Use v.size().

Answer 3

If you want to know if an element exists in a vector, the quickest method is to sort the array then use a search method such as binary search.

If this action is performed many times, perhaps changing the data structure will yield better performance. An std::map is good for this, and if your compiler has one, use a hash table or map.

Otherwise the only way to determine if a value exists in an vector without accessing the vector is to use a second data structure to remember the value and position.

Answer 4

An element is guaranteed to exist at every position i where i >= 0 and i < v.size() as vectors are contiguous sequences of elements and "holes" are not possible.