Vectors

In this lecture we will learn about the main array like data structure in c++ - vectors

Vectors

Vectors are dynamic sized arrays; this means that we can resize them freely at runtime, and still access all elements in O(1). We can also add and remove elements from either side!

They are declared like this vector<data_type> name_of_the_vector(n, default_value)

• data_type and name_of_the_vector are self explanatory.
• n and default_value are optional, but if they are provided, the vector will start with n elements and each element will be default_value
• Note that if we include n, but don't include the value, the vector will be filled with zero like elements, depending on the data type they are 0, "", etc..

We can access the i-th element of the vector with a[i], where a is the name of our vector

Example:

int main(){

    int n = 3;

    vector<int> a(n); //we initialize the vector a with size n

    for(int i =0;i<n;i++){ //notice how we start counting from 0
    
        cin>>a[i]; //we input the i-th element
    }

    cout<<"The third element of the vector is "<<a[2]; //notice how the third element is indexed with a[2]

    return 0;
}

Input: 6 7 8
Output: The third element of the vector is 8

Whilst the vector does start from 0, there is nothing stopping us from ignoring a[0] and using a[1] as the first element.

We do need to make a few more changes so the code now looks like:

int main(){

    int n = 3;

    vector<int> a(n+1); //If we start counting from 1, we need n+1 elements

    for(int i =1;i<=n;i++){ //Notice how we use i<=n 
    
        cin>>a[i]; //we input the i-th element
    }

    cout<<"The third element of the vector is "<<a[3]; //Now the index of the third element is a[3]

    return 0;
}

Input: 6 7 8
Output: The third element of the vector is 8

Functions with vectors

There are a lot of useful built-in functions that vectors have.
For doing things like adding an element to the back of the vector, removing elements, and more

The most important ones are (assuming a is a vector<int>):

• a.push_back(x); - adds x to the back of the vector, so if we had [1,2,3] and we call the function with a.push_back(4), the vector will look like : [1,2,3,4]
• a.pop_back(); - We remove the last element of the vector, so [1,2,3] becomes [1,2]
• a.size() - Returns the length of the vector

Example:

#include <bits/stdc++.h>

using namespace std; 

void print_vector(vector<int> a){ // This is a custom function
	
	for(int i=0;i<a.size();i++){ //notice the a.size()
	
        cout<<a[i]<<" "; // this prints out the vector
    
    }
    cout<<'\n'; // this is a newline character, like pressing enter on a keyboard
}

int main(){

    int n = 3;

    vector<int> a(n);

    a[0] = 1;
    a[1] = 2;
    a[2] = 3;
	
	print_vector(a);// We call our custom function
	
    a.push_back(4);
    a.push_back(5);

    print_vector(a); // We call our custom function

    a.pop_back();

    print_vector(a); // We call our custom function

    return 0;
}

Output:
1 2 3
1 2 3 4 5
1 2 3 4