Introduction to Arrays – Data Structure and Algorithm Tutorials

What is an Array?

An array is a collection of items of the same variable type stored that are stored at contiguous memory locations. It’s one of the most popular and simple data structures and is often used to implement other data structures. Each item in an array is indexed starting with 0.

The dream of every programmer is to become not just a good, but also a great programmer. We all want to achieve our goals and to achieve our goals, we must have a great plan with us. In this context, we have decided to provide a complete guide for Arrays interview preparation, which will help you to tackle the problems that are mostly asked in the interview, such as What is an Array, What is Array in C language, How do you initialize an Array in C, How to sort an Array, etc. We have also covered the topics such as Top Theoretical interview questions and Top interview coding questions in this complete guide for Array interview preparation.

Complete guide for Arrays interview preparation

We can directly access an array element by using its index value.

Basic terminologies of array

• Array Index: In an array, elements are identified by their indexes. Array index starts from 0.
• Array element: Elements are items stored in an array and can be accessed by their index.
• Array Length: The length of an array is determined by the number of elements it can contain. 

Representation of Array

The representation of an array can be defined by its declaration. A declaration means allocating memory for an array of a given size.

Array

Arrays can be declared in various ways in different languages. For better illustration, below are some language-specific array declarations.

C++

int arr[5];        // This array will store integer type element
char arr[10];    // This array will store char type element
float arr[20];  // This array will store float type element

C

int arr[5];        // This array will store integer type element
char arr[10];    // This array will store char type element
float arr[20];  // This array will store float type element

Java

/* Static Arrays are arrays that are declared before runtime
and are assigned values while writing the code.
*/

// The syntax of declaring a static array is:
<data type><variable name>[]
    = {<data1>, <data2>,…..<dataN> };

// Example:
int arr[] = { 2, 5, 6, 9, 7, 4, 3 };

C#

int[] arr = new int[5]; // This array will store integer type element

Javascript

// JS code
let arr=[10,20,30];  // This array will store integer
let arr2 = ['c', 'd', 'e'] // This array will store characters
let arr3 = [28.5, 36.5, 40.2] // This array will store floating elements

Python

# Python code
arr = [10, 20, 30]  # This array will store integer
arr2 = ['c', 'd', 'e']  # This array will store characters
arr3 = [28.5, 36.5, 40.2]  # This array will store floating elements

Array declaration

However, the above declaration is static or compile-time memory allocation, which means that the array element’s memory is allocated when a program is compiled. Here only a fixed size (i,e. the size that is mentioned in square brackets []) of memory will be allocated for storage, but don’t you think it will not be the same situation as we know the size of the array every time, there might be a case where we don’t know the size of the array. If we declare a larger size and store a lesser number of elements will result in a wastage of memory or either be a case where we declare a lesser size then we won’t get enough memory to store the rest of the elements. In such cases, static memory allocation is not preferred.

Is it possible to create dynamic array? 

The answer is Yes. It is possible to allocate memory dynamically. So, dynamic memory allocation is the process of assigning the memory space during the execution time or the run time.

Below are the languages that support dynamic memory allocation:

C++

int *array = new int[5];

Java

// The syntax of declaring a dynamic array is:

// <data type> <variable name> [ <Total Number of elements to be stored in array>];
// Example:
int arr[10]; // Store integer elements
String arr[5]; // Store String type of elements

Python3

# list of integers
my_list = [1, 2, 3, 4]

# Empty list
my_list = []

# list of mixed data types
my_list = ["Hello", 1, 5.5]

C#

int[] numArray = new int[] {};

Javascript

// JavaScript has dynamic arrays: their size is not predetermined, nor the type of data.

 // Create array using literal
var x = ["a", "b", "c"];

// Create array using constructor
var x = new Array();

// Create array using constructor with items
var y = new Array("a", "b", "c");

PHP

$arr = array("a","b","c");

Why Array Data Structures is needed?

Assume there is a class of five students and if we have to keep records of their marks in examination then, we can do this by declaring five variables individual and keeping track of records but what if the number of students becomes very large, it would be challenging to manipulate and maintain the data.

What it means is that, we can use normal variables (v1, v2, v3, ..) when we have a small number of objects. But if we want to store a large number of instances, it becomes difficult to manage them with normal variables. The idea of an array is to represent many instances in one variable..

Need for Array

Types of arrays: 

There are majorly two types of arrays:

• One-dimensional array (1-D arrays): You can imagine a 1d array as a row, where elements are stored one after another.
  

1D array

• Two-dimensional array: 2-D Multidimensional arrays can be considered as an array of arrays or as a matrix consisting of rows and columns.
  

2D array

• Three-dimensional array: A 3-D Multidimensional array contains three dimensions, so it can be considered an array of two-dimensional arrays.
  

3D array

Types of Array operations:

• Traversal: Traverse through the elements of an array.
• Insertion: Inserting a new element in an array.
• Deletion: Deleting element from the array.
• Searching:  Search for an element in the array.
• Sorting: Maintaining the order of elements in the array.

Advantages of using Arrays:

• Arrays allow random access to elements. This makes accessing elements by position faster.
• Arrays have better cache locality which makes a pretty big difference in performance.
• Arrays represent multiple data items of the same type using a single name.
• Arrays store multiple data of similar types with the same name.
• Array data structures are used to implement the other data structures like linked lists, stacks, queues, trees, graphs, etc.

Disadvantages of Array:

• As arrays have a fixed size, once the memory is allocated to them, it cannot be increased or decreased, making it impossible to store extra data if required. An array of fixed size is referred to as a static array. 
• Allocating less memory than required to an array leads to loss of data.
  An array is homogeneous in nature so, a single array cannot store values of different data types. 
• Arrays store data in contiguous memory locations, which makes deletion and insertion very difficult to implement. This problem is overcome by implementing linked lists, which allow elements to be accessed sequentially.  

Application of Array:

• They are used in the implementation of other data structures such as array lists, heaps, hash tables, vectors, and matrices.
• Database records are usually implemented as arrays.
• It is used in lookup tables by computer.
• It is used for different sorting algorithms such as bubble sort insertion sort, merge sort, and quick sort.

Top theoretical interview questions

S.no	Question	Answer
1	What will happen if you do not initialize an Array?	View
2	Why is the complexity of fetching a value from an array be O(1)	View
3	When should you use an Array over a List?	View
4	What is a circularly sorted array?	View
5	Comparing two arrays using hashmap?	View
6	“What are the advantages of a linked list over an array? In which scenarios do  we use LinkedList and when Array?”	View
7	How do I iterate rows and columns of a multidimensional array?	View
8	What is meant by Sparse Array?	View
9	What are the benefits of Heap over Sorted Arrays?	View
10	Is there any difference between int[] a and int a[]?	View
11	Can we declare array size as a negative number?	View
12	We know that Arrays are objects so why cannot we write strArray.length()?	View
13	What are the advantages of Sorted Arrays?	View
14	What defines the dimensionality of an Array?	View
15	How to check array contains a value or not?	View
16	How to create an array/list inside another array/list?	View
17	How to get the largest and smallest number in an array?	View
18	How can I return coordinates/indexes of a string in a multidimensional array?	View
19	How do I remove objects from an array in Java?	View
20	How does C allocate data items in a multidimensional array?	View
21	Get adjacent elements in a two-dimensional array?	View
22	C++ How to use and pass a 3-dimensional char array?	View
23	Anonymous Array in Java	View
24	What is the default value of Array in Java?	View
25	How to copy an array into another array?	View
26	How to iterate an array in java?	View
27	How to merge two sorted Arrays into a Sorted Array?	View
28	Can we make the array volatile in Java?	View
29	What is the logic to reverse the array?	View
30	How to get the index of an array element?	View
31	Can we extend an array after initialization?	View
32	How to fill elements (initialize at once) in an array?	View
33	Difference between Array and String in Java	View
34	Print all subarrays with 0 sum	View
35	Equilibrium index of an array	View
36	How to check array contains a value or not?	View
37	How to get the top two numbers from an array?	View
38	How to implement 3 Stacks with one Array?	View

Top 50 interview coding question

Easy Problems on Arrays

S.no	Question	Article	Practice
1	Write a program to reverse the array	View	Solve
2	Find the minimum and maximum element in an array	View	Solve
3	Peak Element	View	Solve
4	Write a program to sort the given array	View	Solve
5	Find the Kth largest and Kth smallest number in an array	View	Solve
6	Find the occurrence of an integer in the array	View	Solve
7	Sort the array of 0s, 1s, and 2s	View	Solve
8	Subarray with given Sum	View	Solve
9	Move all the negative elements to one side of the array	View	Solve
10	Find the Union and Intersection of the two sorted arrays	View	Solve

Medium Problems on Arrays

S.no	Question	Article	Practice
1	Write a program to cyclically rotate an array by one	View	Solve
2	Find the missing integer	View	Solve
3	Count Pairs with given sum	View	Solve
4	Find duplicates in an array	View	Solve
5	Sort an Array using the Quicksort algorithm	View	Solve
6	Find common elements in three sorted arrays	View	Solve
7	Find the first repeating element in an array of integers	View	Solve
8	Find the first non-repeating element in a given array of integers	View	Solve
9	Subarrays with equal 1s and 0s	View	Solve
10	Rearrange the array in alternating positive and negative items	View	Solve
11	Find if there is any subarray with a sum equal to zero	View	Solve
12	Find the Largest sum contiguous Subarray	View	Solve
13	Find the factorial of a large number	View	Solve
14	Find Maximum Product Subarray	View	Solve
15	Find the longest consecutive subsequence	View	Solve
16	Find the minimum element in a rotated and sorted array	View	Solve
17	Max sum in the configuration	View	Solve
18	Minimum Platforms	View	Solve
19	Minimize the maximum difference between the heights	View	Solve
20	Minimum number of jumps to reach the end	View	Solve
21	Stock Span problem	View	Solve
23	Find a triplet that sums to a given value	View	Solve
23	Smallest positive missing number	View	Solve
24	Find the row with a maximum number of 1’s	View	Solve
25	Print the matrix in a Spiral manner	View	Solve
26	Find whether an array is a subset of another array	View	Solve
27	Implement two Stacks in an array	View	Solve
28	Majority Element	View	Solve
29	Wave Array	View	Solve
30	Trapping Rainwater	View	Solve

Hard Problems

S.no	Question	Article	Practice
1	Maximum Index	View	Solve
2	Max sum path in two arrays	View	Solve
3	Find Missing And Repeating	View	Solve
4	Stock buy and sell Problem	View	Solve
5	Pair with the given sum in a sorted array	View	Solve
6	Chocolate Distribution Problem	View	Solve
7	Partition Equal Subset Sum	View	Solve
8	Smallest Positive integer that can’t be represented as a sum	View	Solve
9	Coin Change Problem	View	Solve
10	Longest Alternating subsequence	View	Solve

Frequently asked questions (FAQs) about arrays

1. What is an array in data structure with example?

An array is a collection of items of the same data type stored at contiguous memory locations. Ex. int arr[5] = {1,2,3,4,5};

2. Why array is a data structure?

Arrays store elements of the same type, they are classified as homogeneous data structures. They can store numbers, strings, characters, boolean values (true and false), objects, and so on.

3. What data structure is an array?

 An array is a linear data structure that stores similar elements in contiguous memory locations.

4. What are the types of arrays?

There are majorly two types of arrays:

• One dimensional array
• Multidimensional array

5. How is data stored in an array?

An array is a collection of items of the same data type stored at contiguous memory locations or says the elements are stored one after another in memory. An array uses an index system starting at 0 and going to (n-1), where n is its size.

6. Difference between array and structure?

The structure can contain variables of different types but an array only contains variables of the same type. 

7. What are the limitations of an array?

An array is a collection of items of the same data type, That means, in an integer array only integer values can be stored, while in a float array only floating values and character array can have only characters. Thus, no array can have values of two data types.

8. What are the advantages of an array?

There are multiple advantages of array data structure and some of them are:

• Arrays allow random access to elements. This makes accessing elements by position faster.
• Arrays store multiple data of similar types with the same name.
• Array data structures are used to implement the other data structures like linked lists, stacks, queues, trees, graphs, etc.

9. What is the purpose of using arrays?

An array is used when several variables of the same type need to be used, and it can be defined as a sequence of objects of the same type.  

10. What is a multidimensional array?

A multi-dimensional array can be termed as an array of arrays that stores homogeneous data in tabular form. Data in Multidimensional Arrays are stored in row-major order. 

Conclusion

After the discussion, we concluded that arrays are a simple method of accessing elements of the same type by grouping them and we can find the elements efficiently by their indexes and can perform different operations using them. Thus, they are more efficient when it comes to memory allocation and should be used in all modern programming languages. So, this becomes a favorite topic for the perspective of the interview and most of the companies generally asked about the problems on the array. For all these reasons, we must have a good knowledge of it.