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.

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]

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.

// 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;
}

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

clear() function It makes the vector to have zero elements, i.e- no elements and not making the elements to all 0s.
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++.

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.

// 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;
}

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)
{

}

Common Subtleties in Vector STLs

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