Skip to main content

Booleans, Conditionals & Loops

Setup​

  1. Navigate to your student_examples folder for today.
  2. Create a file called loops_and_conditionals.js for practicing your work.
  3. Open this file in your text editor. Add a console.log in your file run the code in your terminal node loops_and_conditionals.js.

Lesson Objectives​

After this lesson, students will be able to:

  • Use conditionals and booleans and loops

Loops​

Let's make a new file called loops.js

Loops allow us to do something repeatedly. We'll use for loops to build out some more control flow examples

for (let step = 0; step < 5; step++) {
  // Runs 5 times, with values of step 0 through 4.
  console.log("Walking east one step", step);
}

The Problem​

Find the sum of all the even integers between 0 and 10

Pseudo Code​

Let's start out by pseudo-coding:

tip

We should write down what we think we need to do or what the questions is asking us to do in human language - not computer language.

Activity​

Take ~ 3 - 5 minutes to write some pseudocode of how we can get the sum

  • Post your pseudocode in slack

Let's Work Through it Together​

Note: You may have worked through this in different steps. That's ok! There are many different approaches. The key is to break it down into small steps and test each one.

Start Small​

First we have to 'get' the numbers 1 to 10. We could console.log each one separately:

console.log(1);
console.log(2);
//etc.

What's another way?

Loop​

for (let i = 0; i < 10; i++){
  console.log(i);
}

Now we have a way to 'get' the numbers 1 to 10

Control Flow​

We have to figure out how to determine if one of the numbers is even. This is where Control Flow comes in:

We can choose to do something depending on the case.

For this problem:

  • If it's even, let's do something with it...
  • If it's odd, let's ignore it.

With control flow, you can decide what to do in different instances.

The Power of If Statements in Control Flow​

This is where if statements come into play! We can tell our program to do something if it meets certain criteria.

for (let i = 0; i < 10; i++){
  if (i === 1) {
    console.log("the number is 1")
  }
}

Note: You don't always need an else statement. If your else statement would just be a comment like:

if (true) {
  console.log("awesome");
} else {
  // do nothing
}

Then you can omit the else statement.

We don't just need the number 1, we need all the even numbers. How can we check if a number is even?

for (let i = 0; i < 10; i++){
  if (i % 2 === 0) {
    console.log("the number is even", i)
  }
}

Now we need a variable to hold our sum

let evenSum = 0;

for (let i = 0; i < 10; i++) {
  if (i % 2 === 0 ) {
    evenSum += i;
  }
}

Experiment: what happens if you set evenSum to undefined or empty string instead of 0?

Why? What is going on?