Arrays

Question 1

What is an array?

Answer 1

• It is a data structure to store ordered collections
• Array elements are numbered starting with zero
• Arrays have many methods to manage the order of elements
• Can be created by a constructor or literal notation

Question 2

How do you create an array?

Answer 2

you can create a list of movies as follows.

const movies = [“The Wolf of Wall Street”, “Zootopia”, “Babysitting”];

Everything in the bracket makes up the array

Question 3

Can an array contain different types of values like string, number, booleans, objects?

Answer 3

Yes

Question 4

How do you access the array size?

Answer 4

The size of an array is the number of object in it and can be accessed as follows:

const arrayName = [ ]; // Create an empty array
console.log(arrayName.length); // 0

Question 5

How to access a particular element in an array?

Answer 5

You can access a particular element passing its index in squared braket

const movies = [“The Wolf of Wall Street”, “Zootopia”, “Babysitting”];

console. log(movies[0]); // “The Wolf of Wall Street”
console. log(movies[1]); // “Zootopia”
console. log(movies[2]); // “Babysitting”

Question 6

How do you update arrays content?

Answer 6

You add a new item to an array with the “push()” method. The new element to be added is passed as a parameter to the method. It is inserted at the end of the array.

const movies = [“The Wolf of Wall Street”, “Zootopia”, “Babysitting”];

movies. push(“Ghostbusters”);
console. log(movies[3]); // “Ghostbusters”

Question 7

How do you remove an element from an arrays?

Answer 7

You can use “pop()” or you can use “splice()”:

“pop()” : let you remove the last element

“splice()” : it need two parameters the first one is the index from which to begin removing, and the second one is the number of elements to remove.
E.g
const movies = [“The Wolf of Wall Street”, “Zootopia”, “Babysitting”];
movies.splice(0, 1);

Question 8

Main methods to add or remove items from arrays?

Answer 8

• arr.push(…items) – adds items to the end,
• arr.pop() – extracts an item from the end,
• arr.shift() – extracts an item from the beginning,
• arr.unshift(…items) – adds items to the beginning.

Question 9

What is the syntax for creating an array with

• Literal Notation
• Array constructor

Answer 9

Literal notation: let newArr = []
Array constructor: let newArr = Array()

Question 10

What are arrays populated with?

Answer 10

Elements

Question 11

newArr = [“Zebra” , , ture,21]

What is the element at index number 1 and 4

Answer 11

Undefined, because it is an empty space and nothing because the array stops at index 3

Question 12

How to get the last element from an array?

Answer 12

let lastElement = nameOfArray [nameOfArray - 1 ]

Question 13

Arrays iteration’

How do you iterate through an array passing the value and index for each elements?

Answer 13

Taking this example of array

let bestColors = [‘green’, ‘blue’, ‘yellow’, ‘black’]
bestColors.forEach((x,i)=> console.log(x,i,a))

x: will display the elements
i: will display the index
a: will display the whole array

Question 14

What is the method “.forEach” used for?

Answer 14

.forEach is a built in function that allows to to run a function for every element of the array.

Question 15

What is the syntax of the .forEach method?

Answer 15

arrayName.forEach(function(item, index, array) {
do something with item
});

Question 16

How can I assign a whole array to a different one?

Answer 16

let arrayOne = [ … ]

letArrayTwo = [ … ]

arrayOne = arrayTwo

Question 17

What methods are used to transform an array?

Answer 17

• map(func) – creates a new array from results of calling func for every element.
• sort(func) – sorts the array in-place, then returns it.
• reverse() – reverses the array in-place, then returns it.
• split/join – convert a string to array and back.
• reduce/reduceRight(func, initial) – calculate a single value over the array by calling func for each element and passing an intermediate result between the calls.

Question 18

What does the method ‘Filter’ do to an array?

arr.filter

Answer 18

‘Filter’ create an new array from an existing array and it run as many times as the elements in the array and if the element satisfies the function they are added to the new array

Question 19

How do you sort an array of numbers?

Answer 19

function functionName(arr){
    let sorted = arr.sort((a,b) => a-b)
}

Question 20

What can you use to move through arrays to use the content in them? (or iterates)

Answer 20

• For loop

- For.each loop

Question 21

how to sum all the number in an array?

Answer 21

Array.reduce( (acc,c) => acc + c, 0 )

Question 22

What does the method “.reduce” does?

Answer 22

It sets up a bucket for the accumulation value and it “reduce” down to a singular number.

Question 23

How does the method “.reduce” works?

Answer 23

This is going to run once for every in the array and it is going to grab 2 important value accumulator and the current value. Then it will add the current value to the accumulator. At the end of the function it will be then take a value that indicate the starting point.

Question 24

What is the syntax of “.reduce” method?

Answer 24

Eg.

Array.reduce( (acc,c) => acc + c, 0 )

Question 25

Array.reduce( (acc,c) => acc + c, 0 )

What the c stand for in this reduce method?

Answer 25

Current value

Question 26

What does the method ‘.map’ do?

Answer 26

Creates a new array populated with the results of calling a provided function on every element in the calling array

Question 27

How do you print the reverse of a given string?

Answer 27

let reverse = str => console.log( str.split(‘…’).reverse().join(‘…’) )

Question 28

what are the 2 ways for creating an array?

Answer 28

• Literal notation

- constructor

Question 29

What is an integer array?

Answer 29

It is an array of numbers

Question 30

How can the ‘reduce’ method be explained in simple terms?

Answer 30

reduce takes the value in one array and reduce them to one number. The ‘acc’ and ‘c’ value are defined in parenthesis. Then we add the current value from the array ‘c’ to acc. (‘c’ can also be squared cube divide by… etc ) at the end we insert comma 0 to establish where we start from.