Data and Variables

THE BASIS OF STORING INFORMATION

Table of Content

• Data Types
• The Variable
• The Constant
• What's in a Name?
• Evaluating Expressions
• Operators
• Getting User Input

Data Types

String

• Words, letters, digits, special characters
• Must be in single or double quotes
  • "Use double quotes"
  • 'Or single quotes!'
• Quotes within quotes (as long as they're not the same kind)
• Can be very short or very long
• Can be empty (but still a string!)

Example Code

let emptyString = ""; 
let space = " "; 
let zeeOrZed = "z"; 
let farmBoy = "Kojo"; 
let manInBlack = "Dread Pirate Roberts"; 
let memorableQuote = 'Inigo said, "Hello. My name is Inigo Montoya. You killed my father. Prepare to die."'; 
let rottenTomatoesScore = "97"; 

Number

• Integers
  • Positive numbers
    • 55
  • Negative Numbers
    • -43
• Real Numbers
  • Floats (floating points)
    • 66.888 (of size 4 byte)
  • Doubles
    • 6.657888899900098 (of size 8 byte)
• In JavaScript world, they're all the same type!
• NO quotes - otherwise it's a string
• NO commas

Example Code

let numberOfOutlaws = 3; 
let mostlyDead = 0.985; 
let energyLevelAfterRevival = -86; 
let canPriceBeNegative = -77.66

A Variable

• Stores information
• Can store many different data types or even collections of data
• The word "variable" means "changeable"
• Declared with the keyword let
• Value is assigned with =
• In older examples of code you might see the keyword var used instead - that's outmoded!
• "Declaring" vs "initializing" and "Declare and initialize

Delaring

let school;

Initializing

school = "University of Ghana;

Delare and Initialize

let age = 90;

A Constant

• Stores information
• Value is protected and cannot be changed
• Declared with the keyword const
• Sometimes ALLCAPS is used instead of camelCase.
• Useful in situations where you want to ensure the value is never altered

Delare and Initialize

const PI = 3.14;

Comment

• They are readable guidelines for programmers which are not overlooked by the compiler or interpreter
• Single line comment

// This is how we do Single Line Commenting 

• Multi line comment

/*
This is how
we do multi line
commenting
*/

Semicolon (;)

• It's like a full stop in ending instructions.
• Thus it tell the Javascript interpreter the end of every instructions

let msg = "We go na be alright";

The Importance of Good Variable Naming

• Be descriptive!
• Readability is more important than length.
• Avoid naming things in a way that one
• variable could be confused for another.
• It should be obvious from the name not only what it is but what data type it is.
• Use camelCase for JavaScript

Example Code

/** WHAT'S IN A NAME? **/

// These are not great choices for variable names. 
let book = 'The Princess Bride: S. Morgenstern\'s Classic Tale of True Love and High Adventure, The "Good Parts" Version';
let name = "William Goldman";
let type = "Adventure";
let kind = "hardcover";
let books = 10;

// Declare new variables for the values above
// They should be descriptive, specific and obvious about data type
let bookTitle = 'The Princess Bride: S. Morgenstern\'s Classic Tale of True Love and High Adventure, The "Good Parts" Version';
let bookAuthor = "William Goldman";
let bookGenre = "Adventure";
let bookFormat = "hardcover";
let numCopiesAvailable = 10;

Output Stream

Printing on the Console for end Users

• What is an output device ?
• What is a function? Remember from maths: f(x) = y, where x is input and y is output and f is the function
• Use console.log() to print the value in the console

console.log("You don't own me")

Evaluating Expressions

What Is an Expression?

• Combines values, variables, operators, and more
• Can be as simple as a variable or it can be complex with multiple operations
• The expression is computed and returns a value, which is often stored in a new variable
• You can use multiple expressions to compute new values "behind the scenes"

Operators

Basic Arithmetic

• (+) Addition
• (-) Mubtraction
• (*) Multiplication
• (/) Division

Example Code

/** EVALUATING EXPRESSIONS WITH OPERATORS **/

/* Arithmetic */

let a = 7;
let b = 2;
// Print the difference between a and b using the arithmetic operator for subtraction
console.log(a - b);

let m = 7

// In this case, create a variable to store the result, then print the variable name
let mToTheFourthPower = m**4
console.log(mToTheFourthPower);

Increment and Decrement

• (++) Increment
  • Pre and Post
• (--) Decrement
  • Pre and Post

Example Code

/* INCREMENT & DECREMENT */

// Incrementing
let x = 5;

// Print x++, then print x, then print ++x
// Try to guess the values that will result with each console.log()
console.log(x++); // returns existing value, then increments
console.log(x); // new value
console.log(++x); // increments, then returns new value

// Increment x one more time without printing it, then print x on the next line
x++;
console.log(x);

// Decrementing
let y = 19;

// Print y--, then print y, then print y--
// Try to guess the values that will result with each console.log()
console.log(y--); // returns existing value, then decrements
console.log(y); // new value
console.log(--y); // decrements, then returns new value

// Decrement y one more time without printing it, then print y on the next line
y--;
console.log(y);

Exponentiation

• (**) "to the power of"
• (**2) "squared"
• (**3) "cubed"

Example Code

/* ORDER OF OPERATIONS */

console.log(8 * (2 + 2 ** 2) - 36 / (14 - 2 ** 3));

Modulo

• (%) Modulo
• Produces the remainder after dividing
• • 34 % 6 = 4
  • (%) is the modulo
  • 6 is the modulus

Example Code

/* MODULO */

// Print the remainder from 100 with a modulus of 7
// 7 * 14 = 98, so we expect the value to be 2
console.log(100 % 7);

// Print the remainder from 6 with a modulus of 2
// 2 * 3 = 6, so we expect the value to be 0
console.log(6 % 2);

String

• (+) Combine two strings
• "Concatenation"

Example Code

/* STRING BUILDING */

let titleAfterMawage = "Princess";
let originalName = "Buttercup";

// Print the title and name together
console.log(titleAfterMawage + " " + originalName);

// What happens if you have a mix of strings and numbers? Try it out.
let numberOfShips = 4;
console.log("You write " + numberOfShips + " copies of a letter. I'll send my " + numberOfShips + " fastest ships, one in each direction."); 

Compound Assignment

• (+=) Add and update value
• (-=) Subtract and update value
• (*=) Multiply and update value
• (/=) Divide and update value

Example Code

/* COMPOUND ASSIGNMENT */

let numberOfBoos = 3;

// After Buttercup asks the Ancient Woman why she is booing her, she booed 5 more times. Increase the value of the variable, then print it.
numberOfBoos += 5;
console.log(numberOfBoos);

Input stream

Interacting with a User in the Console

• What is an Input device ?
• Use the readline-sync library (only have to do this once at the top)
• input.question() will print your question to the console
• Usually you need to store their response in a variable
• You can then use that variable however you'd like
• Importing library
  • Incase you don't have access to this library yet

npm install --global yarn
yarn --version
yarn add readline-sync

Example Code

/** GETTING USER INPUT **/

// Import the readline-sync library as the constant input
// Normally this would go at the top of the file
const input = require('readline-sync');

// Print a greeting to the user
console.log("\nWelcome to the Princess Bride Fan Club!");

// Ask the user for a name and store it in a variable
let favoriteCharacter = input.question("\nWho is your favorite character?\n\t");

// Print a response to the user that includes their input
console.log("\n" + favoriteCharacter + "? That's my favorite character too!");

// Ask the user for another name and store it in a variable
let sixFingeredMan = input.question("\nPop quiz! Who is the Six-fingered Man?\n\t");

// Print a response to the user that includes their input
console.log("\nThat's correct! " + sixFingeredMan + " was the man Inigo was looking for to avenge his father's death.");