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.

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    // A C++ program to demonstrate that push_back()
    // happens at the end of resized vector.
    #include<bits/stdc++.h>
    using namespace std;
      
    int main()
    {
        vector<int> vect;
        for (int i = 0; i < 5; i++)
            vect.push_back(i);
      
        // Resizing vector to size 10
        vect.resize(10);
      
        // Prints 0 1 2 3 4 0 0 0 0 0
        for (int i = 0; i < vect.size(); i++)
            cout << vect[i] << " ";
        cout << "\n";
      
        vect.push_back(50);
      
        // Prints 0 1 2 3 4 0 0 0 0 0 50
        for (int i = 0; i < vect.size(); i++)
            cout << vect[i] << " ";
      
        return 0;
    }

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    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.

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    // C++ program to demonstrate that when vectors
    // are passed to functions without &, a copy is
    // created.
    #include<bits/stdc++.h>
    using namespace std;
      
    // The vect here is a copy of vect in main()
    void func(vector<int> vect)
    {
       vect.push_back(30);
    }
      
    int main()
    {
        vector<int> vect;
        vect.push_back(10);
        vect.push_back(20);
      
        func(vect);
      
        // vect remains unchanged after function
        // call
        for (int i=0; i<vect.size(); i++)
           cout << vect[i] << " ";
      
        return 0;
    }

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    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)
    {
    
    }
    

This article is contributed by Supiya Shrivatsa. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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