Array Cheatsheet

When attempting to secure your first job in programming, you’ll no doubt face a coding problem in the interview process, which will determine your performance. Whether the interview is in Java, C++ or Javascript, the basis remains the same. This is a test to see how strong we are in the foundations of programming logic. In this blog, we will use JavaScript.

In our interview questions, we commonly face 3 types of questions

1. Arrays: An array is the most fundamental data structure, which stores elements at a contiguous memory location. This is one of the most common interview topics e.g., reversing an array, sorting the array, or searching for elements in an array.
2. Strings: Another common interview question, however, is the skills learnt from working with arrays can be simply applied to strings.
3. Data Structures: Strings and Arrays are linear data structures, other linear data structures include Linked Lists, Stacks and Queues. However, with all the data in the world structured and unstructured there are other data structures such as Binary Tree, Graphs, Sets and Table coding questions. The former is the most common.

JavaScript Methods:

In this blog we will be looking at the Array Methods and how to solve a question using these methods:

Method	Description	Arguments	Returns	mutates
.Slice( )	Copies an array subset	Indices to copy	A new array	No
.Concat ( )	Creates an array with more values	New values/arrays to add	A new array	No
.Splice ( )	Inserts/deletes value	Index to mutate/delete values to insert	Items removed	Yes
. Join ( )	Stringifies array content	Strings used to separate items	A new string	No
.Push( )	Inserts new items at the end of the array	Items to insert	Array size after	Yes
.Pop ( )	Removes the last element of an array	None	Last array item	yes
.Unshift ( )	Inserts new items at the start of the array	Items to insert	Array size after	Yes
.Shift ( )	Removes first item from array	None	First array item	Yes
.Sort ( )	Sorts existing array	Callback to compare two items	Existing array reference	Yes
.Reverse ( )	Reverses the order of items in the array	None	Existing array reference	Yes
.Map ( )	New array by transforming values	Callback to transform one item	New array with callback return values	No
.Filter ( )	New array with values matching comparison	Callback to compare one item	New array with original items where callback returned true	No
.forEach ( )	executes a provided function once for each array element.	Callback to run logic for one item	Nothing	No
.Reduce ( )	Calculates one results value based on all items	Callback to accumulate running result	Final callback result value	No
.Includes ( )	Checks if exact value is in array	Value to search for	Boolean	No

The 3 most common methods would be:

The Reduce, Map and Filter Method.

Notes:

1. The fifth column in our table, shows whether the JavaScript Method mutates the original array. It is bad practice to mutate the original array in real world applications. Therefore when using methods that mutate an array, we should create a new array for them.
2. The .sort( ) method may surprise you at first use. On first attempt we would assume that the method will sort numbers however by default this method sorts a string variables alphabetically. We need to slightly manipulate it to take numbers into account. 

For Example, if we want to sort an array of ascending orders:

function getSecondLargest(nums) {
  return nums.sort();
}

console.log(getSecondLargest([5, 2, 9, 8, 24]));
// output: [2, 24, 5, 8, 9] 

We wouldn’t expect 24 to be outputted before 5, this is because the output we’re getting is a lexicographical sort. In other words, each integer is coerced into a string type and then the strings are compared character-by-character. Although 5 is numerically smaller than 24, lexicographically, it is larger therefore 24 appears more to the left of the array.

To rectify this, we add a function inside of the Sort method:

function getSecondLargest(nums) {
  return nums.sort((a, b) => a - b);
}

console.log(getSecondLargest([5, 2, 9, 8]));
// output: [2, 5, 8, 9, 24]

The array elements are sorted based on the relationship between each pair of elements “a” and “b” and the function’s return value. The three possible return numbers are: 

1. < 0 (less than 0) – Sort “a” to be a lower index than “b”.  (Ascending order)
2. Or 0 – “a” and “b” should be considered equal, and no sorting performed.
3. Or > 0 (greater than 0) – Sort “b” to be a lower index than “a”.  (Descending order)

This works the way it does because whenever “a” is less than “b”, a negative value is returned, which results in the smaller elements always appearing to the left of the larger ones, in other words, ascending.

Where to practice interview questions?

1. Codewars
2. Hackerrank
3. LeetCode (intermediate – advanced)

There are many other sites where you can practise these questions. The take-home message is being consistent, whether once a day, or whatever suits you will improve your programming logic. Good luck and all the best!

‘Kash works as a full-stack software developer at WayMaker Digital, helping clients to solve business and technology challenges.

WayMaker Digital provides technology consulting, product design and information products, that enable our clients to consistently thrive and innovate to meet user needs. The quality of our experiences and ideas set us apart when it comes to service delivery, customer experience design and product development. 

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