# 4.1: Intro to Logic ## Learning Objectives By the end of this lesson, you should be able to do the following: * Know how logic is used to make programs more complex * Understand and use the `if` statement. * Understand and use the equality operator: `==` ## Introduction {% embed url="" %} So far our apps have always performed the *same operations*, no matter the input. The next level of complexity is to create programs that perform *different* operations, depending on the input. ### What is Logic? From a programming perspective, logic is the ability of the computer to **make decisions** based on input data. To begin with logic we'll be using the most basic JavaScript logic syntax, the `if` condition. That means that some code will run or not depending on values we test. To start off with, those values will be what the user is typing in- so depending on what the user types, some different things will be displayed in the grey box. ## If Statement An "**if statement**" is a control-flow "**code block**" that runs if a condition is `true`. A code block is a section of code surrounded by curly braces. We'll talk more about what `true` means when we introduce the boolean data type in [Module 4.3: Boolean And, Not](/4-conditional-logic/4.3-boolean-and-not.md). ### Simple Conditional Example: Secret Phrase Let's write a program that changes the output value of `"hello world"` if we type in a particular phrase. ```javascript var main = function (input) { // Set a default value for myOutputValue var myOutputValue = 'hello world'; // If input is our secret phrase, change the value of myOutputValue if (input == 'palatable papaya') { myOutputValue = 'you wrote the secret phrase!'; } // return myOutputValue as output return myOutputValue; }; ``` Our if statement is on line 5. The conditional inside it tests if `input` is equal to `'palatable papaya'`, our secret phrase. If `input` is equal to `'palatable papaya'`, the code runs between the curly braces on lines 5 and 7, i.e. the "if block". If `input` is not equal to our phrase, the if block does not run. Code blocks may or may not run depending on "[**control flow**](https://en.wikipedia.org/wiki/Control_flow)", i.e. the logic of our app. The 1st way we learned to use code blocks was with functions. If statements are a 2nd way. We'll learn a 3rd code block syntax later in [6.2: Loops](/6-arrays-and-iteration/6.2-loops.md). Try inputting secret and non-secret phrases into the program. Enter the secret phrase and click the button to see the different output. Enter anything else and click the button to see the default output. {% hint style="info" %} Note the distinction in our code example between variable ***declaration*** and ***assignment***. On line 2 we "declare" the variable `myOutputValue`. This creates the named container that we can store values inside. On line 5 we "assign" a new value to the `myOutputValue` container. The old value is overwritten and non-retrievable. Notice that declaration with the `var` keyword is only done *once* per variable. Please do not use `var` when assigning new values to existing variables in conditionals like if statements. {% endhint %} ### Equality We're using the "[**comparison operator**](https://www.w3schools.com/js/js_comparisons.asp)" `==` to test if `input` is equal to `'palatable papaya'`. {% hint style="info" %} There are two comparison operators in JavaScript to check for equality, `==`, known as the abstract equality operator, and `===`, the strict equality operator. For the purpose of this course, `==` will suffice, but you are free to explore and experiment. You can read more about the in-depth differences between them in [this ](https://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons)discussion. {% endhint %} ## Comments As our apps get more complicated, we can and should leave notes to ourselves and others to clarify what our code does. "**Comments**" let us write notes in our code files that are ignored on program execution. In JavaScript, comments are denoted by 2 slashes (`//`) at the start of the comment. Every programming language has commenting functionality, though commenting syntax varies by language. ```javascript // This is an example comment. It won't actually "run". ``` ## Knowledge Application: Dice Game Let's recap and build onto [3.1: Functions ](/3-structuring-and-debugging-code/3.1-functions.md)by building a **function** that generates random dice numbers. We will be using this 'Dice Rolling function' as a base to explore Logic and Control Flow for the rest of this Module. ### Random Number Generation To simulate dice, we first need random number generation. JavaScript can produce random numbers using a built-in "**library**" called [`Math`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math) (*case-sensitive*). `Math` contains functions that perform common and helpful math operations. The function we need is `Math.random()`. **N*****ote the function call using ()*** ```javascript var myRandomValue = Math.random(); ``` Calling `Math.random()` returns a random decimal number between 0 and 1, inclusive of 0 and exclusive of 1. Note that the `Math.random()` function does not take in an input. Since we wish to simulate dice with numbers between 1 to 6 inclusive, we have to manipulate the randomly-generated number to get what we want. To convert our random number to a valid dice roll value, we'll use another `Math` function: `Math.floor()`. We will follow the random integer generation example [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) to use `Math.floor()` to convert decimals to integers. With `Math.random()` and `Math.floor()`, we can make a function that produces any random integer from 0 to a provided `max` number: ```javascript var getRandomInteger = function (max) { // Generate a decimal from 0 through max + 1. // This decimal will be inclusive of 0 and exclusive of max + 1. var randomDecimal = Math.random() * (max + 1); // Remove the decimal with the floor operation. // The resulting integer will be from 0 through max, inclusive of 0 and max. var resultInteger = Math.floor(randomDecimal); return resultInteger; }; ``` ### Dice Roll Program Logic Since we wish to build a program that generates random dice numbers. We use the logic from the above `getRandomInteger` to always return an integer from 1 to 6. We call the new function `rollDice`. ```javascript var rollDice = function () { // Generate a decimal from 0 through 6, inclusive of 0 and exclusive of 6. var randomDecimal = Math.random() * 6; // Remove the decimal with the floor operation. // This will be an integer from 0 to 5 inclusive. var randomInteger = Math.floor(randomDecimal); // Add 1 to get valid dice rolls of 1 through 6 inclusive. var diceNumber = randomInteger + 1; return diceNumber; }; ``` ### Dice Game Logic Using our new knowledge of '**logic**' in this chapter, let's implement a simple rule to create a "dice game": if the user enters the same number as the dice roll, they win. ```javascript var main = function (input) { // Generate a random dice number var randomDiceNumber = rollDice(); // Default output value is 'you lose'. var myOutputValue = 'you lose'; // If input matches randomDiceNumber, update output. if (input == randomDiceNumber) { myOutputValue = 'you win'; } // Return output. return myOutputValue; }; ``` ## Exercises ### **Follow Along** Duplicate and run the code above. ### **Twice the Guess** Update our dice game logic such that the user wins if the dice roll is 2 times the guess, e.g. a guess of 1 and roll of 2, a guess of 2 and roll of 4, etc. To win this updated game for a 6-sided dice, the user should only guess numbers between 1 and 3, but the game does not restrict what the user can guess. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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