Javascript - Arrays

Learning Goals

  • Creating an array

    • index

    • .length

  • Array methods

    • push

    • indexOf

  • Iterating through an array

    • forEach

    • for of

  • Using conditional logic on each data element from an array

Flipped classroom videos

Teacher instruction

  • Nye grupper idag. Folk der før har programmeret og folk der ikke har

  • Kom med til Copenhagen JS d. 19/10. Der er mad og drikke! Link her

  • Studiekort? https://education.github.com/pack

  • Lav opgaver, men hop over opgave 5, 6 og 7. Dem laver vi i klassen sammen

  • Vi mødes 15:30 hvor jeg vil løse en af opgaverne og vise den "rigtige" løsning

Peer instruction

Question 1

How many times does this loop print "Hey everyone"?

for (let i = 0; i < 5; i++) {
    console.log("Hey everyone")
}

  1. 4

  2. 5

  3. 6

  4. Syntax error

Question 2

How many times does this loop print "Hey everyone"?

const friends = 5;

for (let i = 0; i < 5; i++) {
    if(friends >= 5){
        console.log("Hey everyone");
    }
}

  1. 0

  2. 4

  3. 5

  4. Syntax error

Creating an array

If you ever find yourself writing code like this...

const mentor1 = "Daniel";
const mentor2 = "Irina";
const mentor3 = "Rares";

...then it's probably time to use an array!

Arrays are data structures that hold a list of values. We call these values the elements of the array.

const mentors = ["Daniel", "Irina", "Rares"];

Arrays can hold any type of value (although almost always you only have one data type per array).

const testScores = [16, 49, 85];
const grades = ["F", "D", "A"];
const greetings = ["Hello, how are you?", "Hi! Nice to meet you!"];

Index

An array is based on an index. The first element is saved at index 0

image-20220914094110716

Accessing elements using index

You can access elements in an array using the index of an element with bracket notation. If you want the first element you use index 0 which can cause some confusion 😱

const trainees = ["Ahmed", "Maria", "Atanas", "Nahidul", "Jack"];
trainees[0]; // "Ahmed"
trainees[3]; // "Nahidul"

🔔 Remember: All arrays start at position 0! To access the first element in an array, you need to access index 0, the second element at 1. There are some very intense reasons for this, but most people just accept it and move on.

Assigning elements

You can also assign new values to parts of an array:

const trainees = ["Ahmed", "Maria", "Atanas", "Nahidul", "Jack"];

trainees[2] = "Bianca";

console.log(trainees); // ["Ahmed", "Maria", "Bianca", "Nahidul", "Jack"]

.length

To get the number of elements in an array you can use the .length property

const trainees = ["Ahmed", "Maria", "Atanas", "Nahidul", "Jack"];

console.log(trainees.length); // 5

📝 Exercise 1 - level 1

Create an array that contains a list of your interests. Get the number of interests you have added to the list. Give the variable a good name

Log out the following string I have 10 interests

📝 Exercise 2 - level 1

Using the following array

const fruits = ['banana', 'apple', 'strawberry', 'kiwi', 'fig', 'orange'];

Now, using the correct indexes, print the following values from the array:

  • strawberry

  • kiwi

  • orange

  • banana

taken from https://syllabus.codeyourfuture.io/js-core-1/week-2/lesson#arrays

📝 Exercise 3 - level 2

Write some code that can get the first and last elements in an array.

const names = ['Peter', 'Susan', 'Charlotte'];
// Use the names array
const firstNameInArray = YOU WRITE CODE HERE;
const lastNameInArray = YOU WRITE CODE HERE;

console.log(firstNameInArray); // Peter
console.log(lastNameInArray); // Charlotte

const fruits = ['pear', 'banana', 'apple', 'grape'];
const firstFruitInArray = YOU WRITE CODE HERE;
const lastFruitInArray = YOU WRITE CODE HERE;

console.log(firstFruitInArray); // pear
console.log(lastFruitInArray); // grape

Array methods

There are lots of different array methods. Today we will focus the two most important ones

.push

Elements can be "pushed" into an array. The pushed element will figure as the latest element.

const trainees = ["Ahmed", "Maria", "Atanas", "Nahidul", "Jack"];

trainees.push("Nicklas");

console.log(trainees); // ["Ahmed", "Maria", "Bianca", "Nahidul", "Jack", "Nicklas"]

.indexOf

If you know the value and want the index number, you can use indexOf

const trainees = ["Ahmed", "Maria", "Atanas", "Nahidul", "Jack"];

const indexOfMaria = trainees.indexOf("Maria");

console.log(indexOfMaria); // 1

More methods documented here

📝 Exercise 4 - Favorite Songs - level 2

  1. Create an array called favoriteSongs and add five of your favorite song titles.

  2. Use the .push method to add another song to the end of the array.

  3. Use the indexOf method to find the position of a song in the array.

Iterating an array

Iterating an array is about going through each element in an array. Here is an example

const names = ["Benjamin", "Mie", "Klara"];
// Instead of doing this 👇 (what if we had 1000 elements in the array 😱)
console.log(names[0]); // Benjamin
console.log(names[1]); // Mie
console.log(names[2]); // Klara

// Do this:
for (const name of names) {
  // This log is called 3 times because there are 3 elements in the array
  console.log(name);
}
// this will log Benjamin then Mie then Klara

For...of

const cars = ['fiat', 'volvo', 'peugeot'];

for (const car of cars) {
  console.log(car);
}

// Output: "fiat"
// Output: "volvo"
// Output: "peugeot"

ForEach

Does the same, but we can get the index aswell!

const cars = ['fiat', 'volvo', 'peugeot'];

cars.forEach((car, index) =>  {
  console.log(index, car);
});

// Output: "0 fiat"
// Output: "1 volvo"
// Output: "2 peugeot"

📝 Exercise 5 - teachers and students

Duration: 20 min

In the following exercise one group will randomly be selected to be teachers and the other group will be students

In groups of two people prepare a small 5 minute lecture. The lecture should explain the topic of forEach or for...of any way you like. That might be with a small slideshow or it might be with code, thats up to you.

  • As teachers present the 5 minute lecture

  • As students ask good interesting questions

📝 Exercise 6 - In class let's analyze this code

let arr = [34, -5, 3];
let sm = arr[0];

for (i = 1; i <= arr.length; i++) {
    if (arr[i] < sm) {
        sm = arr[i];
    }
}

console.log("The smallest number is: " + sm);

📝 Exercise 7 - Analyze this code

Ask the following questions

  • What does this code do?

  • How would you improve it?

  • Does it work?

  • Do you like the variable names?

  • Is the code easy to understand?

let givenArray = [1, 2, 3, 4, 5];
let revarr = [];

givenArray.forEach((element, index) => {
    revarr[index] = givenArray[givenArray.length + index];
});

console.log(revarr);

📝 Exercise 8 - level 2

Given the following array:

const words = ["apple", "banana", "cherry", "date", "elderberry"];

Write a program that prints out each word and its length.

Here is the expected output:

apple: 5
banana: 6
cherry: 6
date: 4
elderberry: 10

📝 Exercise 9 - level 2

Given the following array:

const animals = ['cat', 'dog', 'bird', 'fish', 'elephant'];

Write code that finds the index of 'bird' in the array and then replaces it with 'parrot'. Print the updated array.

Expected output: ['cat', 'dog', 'parrot', 'fish', 'elephant']

📝 Exercise 10 - level 3

Given the following array:

const grades = [85, 90, 78, 88, 76, 95, 89];

Write code that

  1. Calculates the total grades from the array

  2. Calculates the average grades in the the array

Expected output: Average grade is 86

📝 Exercise 11 - level 3

Using the following array:

const words = ['apple', 'banana', 'cherry', 'date', 'fig'];

Write code that prints the words that have more than 5 letters.

Expected output: banana cherry

📝 Exercise 12 - level 3: Data Filtering

You are given an array of user input data. Each item in the array is a string that a user has entered into a search bar on a website.

const searchData = ['apple', 'banana  ', '  Cherry', 'apple ', 'APPLE', '  Banana'];

  1. Clean the data: Remove any leading or trailing whitespace from each item.

  2. Standardize the data: Convert all items to lowercase so that the search is case-insensitive.

  3. Remove any duplicate items from the array.

Print the cleaned and standardized array

Expected output: ['apple', 'banana', 'cherry']

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