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)
   {
   
   }
   

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