Plain Old Basics

Introduction

What would you do if you were asked to write a program to take input of marks of Ayush, Rohan and Vinay and then print the average marks?

I'd probably create 3 variables, take input, and print the average.

#include<bits/stdc++.h>
using namespace std;

int main()
{
  int m1, m2, m3; cin >> m1 >> m2 >> m3;
  double avg = (m1 + m2 + m3)/3.0;
  cout << avg << '\n';
  return 0;
}

What if the number of students increased from 3 to the whole class? I don't know about you, but my college batch had ~75 students. There is no way my lazy ass creates 75 different variables 😛

Fortunately, this is exactly the problem that Arrays solve.

An array is like a list of items, but each has a specific place. It's similar to lining up a bunch of boxes in a row. You can put things in these boxes (items) and easily find a specific box by knowing its position in the row (index). This helps you organize and work with lots of items efficiently.

There are a few languages (like Python) where the items can be of any type, but specifically in C++, an array can contain items of the same data type only.

Creating an Array

Declaring

The following code explains one of the ways to declare an array in C++:

// The skeleton:
// data-type array-name[array-size];

// Examples:
int marks[5];
char grade[5];
string name[20], address[20];

// Example for variable sized array:
int n; cin >> n;
int arr[n];

When we declare an array inside a function and don't initialise it with anything, the values at different indices are random for many data types (e.g. int, double, char etc.) and empty for a few others (e.g. string). The random values are termed garbage values.

Note: In C++, it's not feasible to change the size of the array, once created. Sure, you can create a new array of desired size and copy the elements, but resizing the existing array isn't possible.

Initialising

In a normal way, the following is the way to do it where you enclose the values in {}. Eg.

int arr[5] = {1, 2, 3, 4, 5};

What if you enter fewer values than the array_size in {}?

int arr[5] = {1, 2, 3};

int arr[5] = {};

In the cases above, the elements aren't assigned garbage values; the values are 0.

FAQs

So, the above is all about initialisation. Following are a few questions you may have:

Accessing the array elements

If we create an array of size N, then the positions of the elements in the array are 0, 1, 2, and so on ... N-1. The positions are called "indices" (singular being "index"). In other words:

1. The 1st element is present at index 0.
2. The 2nd element is present at index 1.
3. ... The pattern goes on ...
4. The (N-1)^th element at index N-2.
5. Finally, the N^th element at index N-1.

How to access the value at index i?

The way to access the ith element of an array having the name arr is arr[i] (where [] is called the subscript operator). Let me explain this using different use cases.

Imagine we have an integer array arr of size 10.

for(int i = 0; i < 10; ++i)
  cin >> arr[i];

// Let's say we want to divide the elements by 2 until they're even,
// and then print the odd leftover.
for(int i = 0; i < 10; ++i) {
  while(!(arr[i]%2))
    arr[i] /= 2;
  cout << arr[i] << ' ';
}

FAQs