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Comparison Operators

Name: Comparison Operators
Uploaded: 2017-10-11T11:16:27Z
Duration: 479 s
Description: We need to figure out which discount we should apply to widget store orders. Is the user ordering 100 widgets or more? 50 to 99? Less than 50? To do this, we're going to need a way to compare numbers.

7:59 with Jay McGavren

We need to figure out which discount we should apply to widget store orders. Is the user ordering 100 widgets or more? 50 to 99? Less than 50? To do this, we're going to need a way to compare numbers.

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We need to check the number of widgets the user is ordering, and then apply an appropriate discount.
This code will be a little too complicated to embed within a string. So first, let's move the part that calculates a price out to a method.

def ask(question)
  print question + " "
  gets.chomp # Add ".chomp" here
end

# New method here
def price(quantity)
  quantity * 10
end

puts "Welcome to the widget store!"
answer = ask("How many widgets are you ordering?")
number = answer.to_i
total = price(number) # Call new method
puts "For #{number} widgets, your total is: $#{total}"

In order to know which discount to apply, we need to be able to tell if the order quantity is over 100, if it's between 50 and 100, or if it's below 50.

Ruby uses comparison operators to tell whether one value is equal to, greater than, or less than another.

== # equal
!= # not equal
<  # less than
>  # greater than
<= # less than or equal to
>= # greater than or equal to

Comparison operators return a boolean value. Boolean values are either true or false. They're represented in Ruby code by the words true and false. Note that these are not strings, so they're not surrounded by quotes.

true
false

Comparison operators

$ irb
2.3.0 :001 > quantity = 75
 => 75
2.3.0 :002 > quantity > 50
 => true
2.3.0 :003 > quantity < 50
 => false
2.3.0 :004 > quantity == 75
 => true

Don't confuse == with =! If you do, you could accidentally change the value a variable holds!

2.3.0 :005 > quantity == 99 # Tests equality.
 => false
2.3.0 :006 > quantity # Value unchanged.
 => 75
2.3.0 :007 > quantity = 99 # Assigns a new value!
 => 99
2.3.0 :008 > quantity # Value changed!
 => 99

>= returns true if the first value is greater than or equal to the second.
<= returns true if the first value is less than or equal to the second.

2.3.0 :009 > quantity = 99
 => 99
2.3.0 :010 > quantity >= 50
 => true
2.3.0 :011 > quantity <= 50
 => false
2.3.0 :012 > quantity = 50
 => 50
2.3.0 :013 > quantity >= 50
 => true
2.3.0 :014 > quantity <= 50
 => true

!=
- The exclamation point can be read aloud as "not".
- So != can be read aloud as "not equal".
- It's the opposite of ==.

2.3.0 :015 > quantity = 99
 => 99
2.3.0 :016 > quantity != 50
 => true
2.3.0 :017 > quantity == 50
 => false

All the comparison operators we just showed you work with strings as well.

2.3.0 :018 > string = "zebra"
 => "zebra"
2.3.0 :019 > string < "ant"
 => false
2.3.0 :020 > string > "ant"
 => true
2.3.0 :021 > string == "ant"
 => false
2.3.0 :022 > string == "zebra"
 => true
2.3.0 :023 > string == "ZEBRA"
 => false
2.3.0 :024 > string != "ant"
 => true

Don't try to compare values of different classes, it won't work.

"3" == 3 # == will always result in false
"4" > 3  # > and < will give an error

You should only do a comparison if you can convert one class to match the other.

"3".to_i == 3
"4".to_i > 3

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[MUSIC] 0:00

Our program always charges users $10 per widget. 0:04

It takes the number of widgets the user wants to order and 0:08

multiplies it by ten to get the total cost. 0:11

Our last requirement is to provide a discount for large orders of widgets. 0:14

Specifically, if the order is for 100 widgets or 0:19

more, the price should be $8 per widget. 0:21

If it's for 50 widgets or more, the price should be $9 per widget. 0:24

And all other orders should be the full price, $10 per widget. 0:28

Our first problem here is to figure out which discount we should apply. 0:32

Are they ordering 100 widgets or more? 0:36

50 to 99, less than 50? 0:38

To do this, we're going to need a way to compare numbers. 0:41

This code will be a little too complicated to embed within a string, so first, 0:46

let's move the part that calculates a price out to a method. 0:51

We'll define a method named price. 0:55

And we'll take the quantity that we are calculating a price for as a parameter. 0:59

Type the end keyword to end the method. 1:11

And here in the method body, we'll simply return the value quantity * 10. 1:14

We're going to assume that the quantity's already been converted to an integer 1:21

before the method is called. 1:24

So we'll simply return the quantity times 10. 1:26

And now we need to change our main code down here to call that method. 1:29

So we'll say total =, we'll call price, and 1:32

we'll pass it the number that our user entered. 1:36

Then here, instead of saying number * 10, 1:40

we'll just put the results of calling the price method. 1:43

Let's try running this, and we should get the same result when there's 7 widgets, 1:47

and for 7 widgets your total is $70. 1:52

But our calculation is being routed through the price method this time, 1:55

instead of directly interpolated into the string. 1:59

In order to know which discount to apply, we need to be able to tell if the order 2:02

quantity is over 100, if it's between 50 and 100, or if it's below 50. 2:07

Ruby uses comparison operators to tell whether one value is equal to, 2:12

greater than, or less than another. 2:17

Comparison operators return a boolean value. 2:20

Boolean values are either true or false. 2:23

They're represented in Ruby code by the words true and false. 2:27

Note that these are not strings, so they're not surrounded by quotes. 2:31

Let's try out some of these comparison operators in IRB. 2:35

So first we're going to create a variable, we'll call it quantity. 2:40

And we'll assign the variable 75 to it. 2:44

Now, let's test the value and quantity to see if it's greater than 50. 2:46

quantity > 50. 2:50

We get the value true because, of course, 75 is greater than 50. 2:53

Let's test whether it's less than 50. 2:57

quantity < 50. 3:00

That, of course, is false. 3:03

Now, let's try the equality comparison operator. 3:05

We'll test whether quantity is equal to 75. 3:08

We get true because, of course, it is. 3:13

Note that it's important not to confuse the double equals comparison operator with 3:16

the single equals assignment operator. 3:21

For example, if we try typing quantity == 99, we get false. 3:24

Because it compared the value that quantity already held with the number 99, 3:31

and of course those two are not equal. 3:35

If we take a look at the quantity variable, 3:37

we can see that it was unaffected by that comparison. 3:39

But if we were to accidentally type quantity = 99, 3:42

the result would be the number 99, which Ruby treats as true. 3:48

So not only do you mistakenly get a true value for your comparison, but 3:54

you've overwritten the value that's in the quantity variable. 3:59

So be very sure not to confuse the double equals comparison operator with the single 4:04

equals assignment operator. 4:09

Now let's try out the greater than or equal to and the less than or 4:11

equal to operators. 4:14

Now remember that the quantity variable currently contains the value 99. 4:17

Let's see whether it's greater than or equal to the value 50. 4:21

That, of course, is true. 4:29

Let's see if it's less than or equal to 50. 4:31

That's false. 4:38

If we assign the value 50 to quantity, 4:39

then both of those comparisons will return true. 4:42

Is quantity greater than or equal to 50? 4:49

Yes, that's true. 4:52

Is quantity less than or equal to 50? 4:54

That's also true because quantity is equal to 50. 4:56

There's one more comparison operator you should know, and 5:01

that's the not equals operator. 5:05

Quantity != 50. 5:07

The exclamation point can be read aloud as not, so 5:09

exclamation point equals sign can be read aloud as not equal. 5:13

Since quantity is currently set to the value 50, 5:17

quantity not equals 50 is going to return false. 5:20

The not equal comparison operator can be thought of as the opposite 5:24

of the double equals comparison operator. 5:27

Which in this case, of course, return true. 5:33

All the comparison operators we just showed you work with strings as well. 5:36

So let's try creating a variable named string, we'll assign it the string zebra. 5:40

And now let's try comparing that value to the string ant. 5:46

So we'll say string, is it less than ant? 5:49

We get the value false because z comes after a in the alphabet and 5:53

is therefore treated as a greater value. 5:58

So let's try a greater than comparison, is string greater than ant? 6:02

That returns true. 6:07

Is string equal to ant? 6:08

Well, no, that's not what it holds, so that returns false. 6:13

Is string equal to zebra? 6:16

That returns true. 6:21

Is string equal to ZEBRA in all caps? 6:23

Well, no, one is capitalized and the other is lowercase. 6:30

So they're not equal. 6:32

Is strong not equal to ant? 6:34

Well yes, the two strings are different and therefore they're not equal. 6:39

And it's not just strings. 6:43

Although you use them most often with strings and numbers, 6:44

comparison operators work with almost any Ruby class. 6:48

Let's create a variable named time and 6:51

assign it the results of the Time.now method. 6:54

and now let's test whether time is less than another call to Time.now. 6:59

That's true because the value in the variable time is a few seconds in the past 7:04

and therefore it's less than Time.now. 7:09

Now let's test whether time is greater than Time.now. 7:11

That of course is false. 7:17

Make sure you don't try to compare values that are of two different classes, 7:19

because that won't work. 7:23

Let's take the string 3, and test whether it's equal to the number 3. 7:24

The equality comparison operator will always return false if you're comparing 7:28

values of two different classes. 7:33

If you try to use a greater than or less than comparison, 7:35

that will return an error. 7:38

Let's see see whether the string 4 is greater than the number 3. 7:40

We get an actual error for that. 7:45

So if you're going to compare values, make sure they're of the same class. 7:47

By the way, all the comparison operators we just showed you work in pretty much 7:51

every programming language out there. 7:54

So remember them because you'll be using them a lot. 7:56

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Ruby Basics

Comparison Operators

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