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Using the switch statement

From: Foundations of Programming: Fundamentals

Video: Using the switch statement

You will end up writing if statements in pretty much every program you ever write, but sometimes there's a specific situation where it's not the only option for checking a condition. Let's say you're checking a variable for a selection of very specific values. Here is what I mean by that. We create a variable in this case called grade, and somewhere in the program I want to check is it Regular or is it Premium or is it Diesel? I've got a few very specific values I'm asking for. We can do this as a series of separate self-contained if statements like I have here or we could even start using the else condition to actually start combining them altogether, but this can get a little clunky, particularly if you don't just have three or four but you have 10 or 12.

Using the switch statement

You will end up writing if statements in pretty much every program you ever write, but sometimes there's a specific situation where it's not the only option for checking a condition. Let's say you're checking a variable for a selection of very specific values. Here is what I mean by that. We create a variable in this case called grade, and somewhere in the program I want to check is it Regular or is it Premium or is it Diesel? I've got a few very specific values I'm asking for. We can do this as a series of separate self-contained if statements like I have here or we could even start using the else condition to actually start combining them altogether, but this can get a little clunky, particularly if you don't just have three or four but you have 10 or 12.

Well, in this kind of situation, JavaScript and most other languages have something called the switch statement instead of the if statement. Now in some other languages, it's called the select or select case statement, but it's the ability to list in one place several situations or cases that you're looking for in a very readable format. So if I know I'm checking here for this variable being either Regular or Premium or Diesel, I'm going to delete the if version of it and create a switch statement.

Now, it has a very similar format to the if statement. We've seen that the if uses this general format here. I'm not going to fill it out. But we have the word if, we have the opening and closing parentheses for our condition, and then we have the opening and closing curly braces for our code block. Well, all I'm going to do here is replace the word if with the word switch, all lowercase. Of course, all these words in JavaScript are lowercase. And switch needs to know what are we switching on? What is the important variable that we're looking at? And it is of course grade.

So all I need to do is put grade inside the parentheses. It's not grade is greater than something, grade is less than something. We're just saying we're looking at the variable called grade. Then what we do is inside the code block inside the body of the switch statement, we describe the different situations, the different cases that we're looking for, and the way that I do it is this. I use the word case and we describe the first option. And in the case, the grade is equal to Regular.

I don't use the equals sign or the double equals sign because we already know what variable I'm looking at. I'm looking at grade. So case "Regular" and then I use a colon, and I'm going to describe what I'm going to do. Let me just bake this out a little more and that will start to make sense and fall into place here. Now, because I'm checking a variable that contains a string, I am using the double-quotes when I am checking these string literals. If I was checking a variable that was a number, I could just say case 1, case 2, case 3 without putting them in double-quotes.

Now, this is the first time we've actually used the colon here. It looks a little unusual, because we're used to the semicolon ending a line. That's not what we're doing here. We're actually saying in the case that it's Regular, we're going to do something. I just haven't described what that is yet. Well, this is how I start to fill it out. I'm going to put in the alert statement that I had a moment ago. Now, as I start to do this, hopefully it becomes apparent that this is a very readable format. It's very easy to scan this and figure out what we're actually looking at and what we're going to do.

One of the benefits of using the switch statement is we can actually finish off this. We can also describe what happens if it isn't any of these. And instead of using the word case, I can finish this off with just the word default. This is not in double-quotes. We're not checking that variable for the value default. We're just saying in any other situation I'm going to do something else. In this case, alert("That's not a valid grade"). now, there is one more thing I have to add to this for this to work properly.

You see when we come down into the switch statement, let's imagine we're going through this line by line. We create a variable called grade, set it equal to Premium. We come into the switch statement, and it asks, what are we looking at? We're looking at the variable grade. Is it Regular? No, it isn't. So we jump down to the next case. Is it Premium? Yes, in this case, it is. We're going to run the alert, but because of something that exists in switch statement called fall-through, we will immediately then just jump down and we'll find the next statement we can execute, which is this alert statement, and then we'll do this alert statement.

That's not what we want. We need to prevent fall-through in our cases here. The way that we do that is basically to add the word break. We can think of it as saying, now we're done with this case. So right at the end before the next one, we say break. And I'm going to put in three of those here. What break is going to do is jump us out of this switch statement. It will actually jump us right to the end of the closing curly brace and continue on executing any code that may come afterwards.

Technically, we could put a break keyword in after the default one. We don't really need to because this one is actually just going to jump out anyway because we're right at the end of the switch. So I'm going to save this and we're going to run it and what should happen is we'll start this code, we'll create a variable called grade and set it equal to Premium. We'll come into the switch, we'll check, is it Regular? No, it isn't. Is it Premium? Yes, it is. We should pop up the alert saying it's $3.35. We should then break and then jump outside of the switch statement.

Let's see if that's what's going to happen. I'll double-click the webpage that's pointing to this JavaScript file, and there we go! It's $3.35. No extra messages, no fall-through going on. This is the basics of a switch statement. Very useful in the situations where you have a range of set values that you need to check against. And you'll find that between your if and else statements and your switch statements, you have all your conditional code covered.

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This video is part of

Image for Foundations of Programming: Fundamentals
Foundations of Programming: Fundamentals

61 video lessons · 92372 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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