Common Subtleties in  Vector STLs

Prerequisite - Vector Basics Following are some important points that can save time on little things in an interview or an important coding contest.

1. vector <int> vect(10) vs vector<int> vect[10]

   // Creates a vector vect[] of size 10
   vector <int> vect(10) 
   
   // creates an array of vectors vect[] of size 
   // 10 where each vector has int members
   vector<int> vect[10]
   

2. resize() and push_back(): After the resize() function has been used on a vector, if push_back() is used on the same vector, the elements being pushed back get added at the end of the resized vector, and not into it.

   Output:
   0 1 2 3 4 0 0 0 0 0
   0 1 2 3 4 0 0 0 0 0 50

3. clear() function It makes the vector to have zero elements, i.e- no elements and not making the elements to all 0s.
4. Creating a two dimensional vector

   // This doesn't work
   vector<vector<int>> vect;
   
   // This works fine
   vector< vector <int> > vect; 

   The difference between these two statements is that the first statement has a space between the angular brackets ( > >). Writing without the space doesn't work because >> is an operator in C++.
5. Passing vectors to functions: When a vector is simply passed to a function, a copy of the vector is created. This might take a lot of time in cases of large vectors.

Output :

10 20

In situations where we don’t actually need to have a copy of the vector, the declaration should be made as follows:

// It is recommended to pass vectors by reference
// wherever possible.
int func(vector<int>& vect)
{

}