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Setting comparison operators

From: Foundations of Programming: Fundamentals

Video: Setting comparison operators

So when we're writing if statements and we want to check that one variable is equal to another variable, we use the double equals sign, the equality operator. In this case, if the contents of the variable a are equal to the contents of the variable b, we're going to execute whatever statements are inside the opening and closing curly braces. But there is a mistake that's very easy to make. In fact, pretty much every programmer in a C-based language will make this at sometime in their career, and that's to accidentally use assignment, single equals, instead of equality, the double equals sign or in JavaScript the triple equals sign.

Setting comparison operators

So when we're writing if statements and we want to check that one variable is equal to another variable, we use the double equals sign, the equality operator. In this case, if the contents of the variable a are equal to the contents of the variable b, we're going to execute whatever statements are inside the opening and closing curly braces. But there is a mistake that's very easy to make. In fact, pretty much every programmer in a C-based language will make this at sometime in their career, and that's to accidentally use assignment, single equals, instead of equality, the double equals sign or in JavaScript the triple equals sign.

What that means is let's say we create a couple of variables, variable a, set it equal to 5, variable b, set it equal to 10, using the single equals sign to assign to set the values of these variables. A little later on in the code, I might write an if statement, and I'm asking, is a = b? But I accidentally used the single equals sign instead of the double equals sign. Well, we have a problem now. What's going to happen is we're going to execute the if statement. It's going to hit that code and it will say, "Okay, single equals sign, that's assignment" and it will set a equal to the value b. This successful operation will then be regarded as true and the code inside the if statement block will always be executed, even when you think it shouldn't be.

So we have just made a = b. Now, this is perfectly legal syntax in JavaScript, but it can lead to code that just doesn't work the way you expect and it's difficult to debug because it's easy to miss. What we should have done here is use the double equals sign. So remember, when you see the single equals sign, that is the assignment operator, you are telling, this is a command. When you see the double equals sign, this is the equality operator. You're asking, is one thing equal to another thing? And in JavaScript and in a couple of other languages like PHP, there is another equality operator which is the three equals sign or triple equals sign.

Now even if you're an experienced programmer, you might not have come across this one before. Again, it's in JavaScript, it's in PHP, but it's not in most C-based languages. A three equals sign is what's called a strict equality operator. Now, what the devil does this mean? Well, let me show you an example. So in this simple code here, I'm creating a variable called a and I'm setting it to the number 123. Then I'm creating a variable called b and I'm setting it to the string 123.

Now remember that we talked about JavaScript being a weak typed language that any variable can hold any kind of value whether it's a number or a string or a Boolean. So what's happening here is I'm asking, is a = b? a is 123, b is 123, but it's a string. They are actually different types of data. Let me run this example. I'm going to run the HTML code that contains this, and it pops up the message Yes, they ARE equal.

And that's what happens if I use the double equals sign. Even though strictly speaking they are different kinds of data, JavaScript is being flexible here. It's kind of saying, "Yeah, I know what you mean, when you want to compare the two, you probably want to say this is true." However, if I used the triple equals sign here, and I'm just going to save that JavaScript, go back and reload the page, this would say, No, they're NOT equal, and this is the strict equality operator, the triple equals, which is going to make sure that not only do we have the values the same but the types better be the same too.

So variable a and variable b better both be numbers. One can't be a string. To make this equality check work, I'd have to remove the quotes here. If I then save that and refresh it again, it would say Yes, they ARE equal. They are equal, they are identical, they are both 123 and they're both numbers. When you're using JavaScript, some writers recommend using the triple equals sign all the time and never using double equals. I wouldn't go that far. When I see a double equals sign in some JavaScript, I don't think it's wrong, but it's certainly not a bad habit to get into, that when you're checking equality in JavaScript, you'd use the triple equals sign.

So to round those up, we have if (a == b), using the double equals sign, or not equal to, using the != sign. We have the strict equality, not just equal but identical. Similarly, we have the not strictly equal, the !==. All of this you are going to run into a lot. But we also have the other kinds of comparison. If a>b. Again, remember it doesn't really care what any of these values are.

Whatever is in the parentheses just needs to be true or false. a is either greater than b or it is not. a is either less than b or it is not. We also have the >= and not surprisingly, the <=. All of these operators, if there is more than one character in them, you can't put a space in between them. They are all considered one indivisible unit. What we often then have to do is take it a step further. We need to ask multiple things at the same time.

So let's say I've got four variables. What I want to ask is if the variable a is equal to the variable b and the variable c is equal to the variable d? Now, I could do this with two if statements but it's nice to combine them all on one condition. I can't use the word and here. There are some languages where you do. Things like the Basic based languages but not in the C-based languages. What we use instead is the double ampersand. Now what this means is both of these conditions, a has to be equal to b or strictly equal to b and c has to be strictly equal to d, for this whole thing to be regarded as true.

Again, the entire contents of the parentheses have to be true. And sometimes we don't want to go that far. We might ask if a is strictly equal to b or c is strictly equal to d, then we want that condition to be true. Well, instead of the two ampersands, we use the two vertical bars, pipe symbols depending on what you call them, are how we say an Or inside this condition. To make it a bit clearer to read, you often enclose the separate conditions inside their own set of parentheses.

You don't have to do this, but it can make it a bit more readable. So in this case, we're saying if a > b and c < d. And sometimes when you have these complex conditions with longer named variables, they can get pretty long so you can break them onto multiple lines. Although keep each unit together. I wouldn't typically take it this far, but because line breaks are insignificant, we could do it if we thought it made it more readable.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 96044 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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