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"if" and "unless" Statements

Name: "if" and "unless" Statements
Uploaded: 2017-10-11T11:16:27Z
Duration: 404 s
Description: We've learned about comparison operators, so we can test whether the user wants to order 50 or more widgets, or 100 or more. Now we just need to add code that applies a discounted price if those conditions are true.

6:44 with Jay McGavren

We've learned about comparison operators, so we can test whether the user wants to order 50 or more widgets, or 100 or more. Now we just need to add code that applies a discounted price if those conditions are true.

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Like most programming languages, Ruby has an if statement that executes some code only if a condition expression is true.
Starts with if keyword
followed by condition, which in these examples are the boolean values true and false
followed by one or more lines of code (like a method body, these are usually indented to make them easier to read, although it's not required)
followed by end keyword
Because condition for first if always true, code it contains will always be run.
Because condition for first if always false, code it contains will never be run.

if true # This will always be run
  puts "true"
  puts "additional code here"
end
if false # This will never be run
  puts "false"
  puts "additional code here"
end

The output shows that the code in the first if statement ran, but the code in the second if statement didn't:

true
additional code here

Actual programs will use expressions that evaluate to true or false, like comparison operators.

if 75 > 50
  puts "75 > 50"
end
if 75 > 100
  puts "75 > 100"
end
if 50 == 50
  puts "50 == 50"
end

Output:

75 > 50
50 == 50

Actual programs aren't likely to compare hard-coded numbers, either. Usually, they'll test the value of a variable.

number = 75
if number > 50
  puts "number > 50"
end
if number > 100
  puts "number > 100"
end
if number == 50
  puts "number == 50"
end

Output:

number > 50

if runs some code only if a condition is true. What if you need to run some code only if a condition is false?
Many languages require you to use some ugly code to make this work with an if statement
But Ruby provides a second keyword, unless, that's basically the opposite of if.
unless runs code only if its condition is false.

number = 75
unless number > 50
  puts "number > 50"
end
unless number > 100
  puts "number > 100"
end
unless number == 50
  puts "number == 50"
end

Output:

number > 100
number == 50

Let's try using if in our program:

# This is the same as before
def ask(question)
  print question + " "
  gets.chomp
end

# Updated to give discounts based on quantity
def price(quantity)
  if quantity >= 100
    price_per_unit = 8
  end
  if quantity >= 50
    price_per_unit = 9
  end
  if quantity < 50
    price_per_unit = 10
  end
  quantity * price_per_unit
end

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

This works fine if the quantity is 99 or less, but for quantities of 100 or more, price_per_unit gets set to 9 because 100 is also >= 50. We'll fix that next.

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We've learned about comparison operators. 0:00

So we can test whether the user wants to order 50 or more widgets or 100 or more. 0:01

Now we just need to add code that applies to the discounted 0:06

price if those conditions are true, like most programming languages, 0:09

Ruby has an If statement that executes some code only if a condition is true. 0:14

It starts with the if key word followed by a condition, 0:19

which in these examples are just the Boolean values true and false. 0:22

The if line is followed by one or more lines of code. 0:27

Like a method body these are usually indented to make them easier to 0:30

read although it's not required. 0:33

And then the end of the if statement is marked by the end keyword. 0:35

In this case, because the condition for 0:40

the first if statement is always true, the code it contains will always be run, so 0:41

you can see the output true and additional code here down at the bottom. 0:46

And because the condition for 0:50

the second if statement is always false the code it contains will never be run. 0:52

You can see these lines aren't included in the outputs in the bottom. 0:56

Just hard coding the values true and 1:00

false isn't going to be very useful in conditional statements, though. 1:02

Actual programs will use expressions that evaluate the true or 1:06

false, like comparison operators. 1:09

You can see here that we test whether 75 is greater than 50, 1:12

which results in the value true. 1:16

So this code runs, and you can see 75 is greater than 50 and 1:17

the output down at the bottom. 1:21

75 is not greater than 100, though, so this code is then included in the output. 1:23

Finally, we test whether the value 50 is equal to the value 50, 1:28

which of course is true so this code runs. 1:32

And you can see 50 equals 50 in the output down at the bottom. 1:34

Actual programs aren't likely to compare pairs of hard coded numbers either. 1:39

Usually, they'll test the value of a variable. 1:43

So here we assign the value 75 to the variable number, and 1:46

then we test whether a number is greater than 50. 1:49

Which of course it is, so we can see number greater than 50 down in the output. 1:52

However, 75 is not greater than 100, so we don't see this in the output. 1:58

And 75 is also not equal to 50, so we don't see that in the output either. 2:02

If statements run some code only if a condition is true. 2:07

But what if you need to run some code only if the condition is false? 2:11

Many languages require you to use some ugly code to make this work with an if 2:14

statement, but Ruby provides a second keyword. 2:18

Unless, that's basically the opposite of if. 2:22

Unless runs code only if its condition is false. 2:26

So let's try changing all these if statements to unless 2:29

statements, And try running this. 2:32

Now we get the opposite of the previous results. 2:41

The value in number 75 which is greater than 50, so that returns true, 2:44

but because this is an unless statement it only runs if the result is false. 2:49

So this code doesn't run and doesn't appear on the output below. 2:53

75 is not greater than 100, so this returns false but 2:58

because this is an unless statement it does run because the condition is false. 3:02

So you see number greater than 100 in the output below. 3:07

And likewise 75 is not equal to 50, so that returns a false value. 3:13

But because this is an unless statement, it runs because the condition was false. 3:17

So you see number == 50 in the output below. 3:23

Let's try updating the price method in our widget shopping program to use if 3:27

statements to determine which discount is should apply. 3:31

We'll get rid of the existing code and we'll say, if the quantity that 3:36

the user has requested is greater than or equal to 100. 3:41

We'll add an end keyword for the if statement early. 3:50

And in the body of that if statement, we'll say that the price_per_unit, 3:54

we'll create a variable to hold that, should be set to 8. 3:59

Because if the user is ordering 100 widgets or more, 4:04

they should only be charged $8 per widget. 4:07

If the quantity, Is greater than or 4:11

equal to 50, Then we're going to set the price per unit, 4:16

Variable to equal 9 instead of 8 because we don't wanna give them quite as big 4:25

a discount if they're only ordering 50 units. 4:30

And if the quantity is < 50, 4:35

Then we're going to want to charge full price. 4:44

So we'll set a price_per_unit of 10. 4:46

Down here at the bottom we'll take the quantity that they entered and 4:53

we'll multiply that by the price per unit. 4:58

So that will be the quantity will be multiplied by 10 if they're ordering less 5:04

than 50 units. 5:09

It'll be multiplied by 9 if they're ordering more than 50 units, and 5:09

it'll be multiplied by 8 if they ordered more than 100 units. 5:13

So let's save this and try it out. 5:18

Ruby widgets.rb. 5:23

Okay, so we'll start with an order of less than 50, let's order 10. 5:27

And it's multiplied by 10 for a total of $100. 5:31

That part seems to be working correctly. 5:35

Now let's try an order of 50. 5:38

That time, the price per unit was multiplied by 9 to get $450. 5:43

We'll get the same results if we order 51 units. 5:49

We're still in the $9 per widget price tier, so 5:53

adding one widget adds $9 onto the total. 5:56

Now let's try ordering 100 widgets or more. 6:00

For 100 widgets here, the total is $900. 6:07

It looks like 100 was multiplied by 9 again, not by 8. 6:10

It's supposed to be multiplied by 8. 6:14

The reason for this is that when the user enters 100 or more, 6:16

this condition here does result in true and the price per unit gets set to 8. 6:20

But a quantity of 100 or more is also going to be greater than or 6:24

equal to 50, so this condition is also true. 6:29

And this code here runs as well. 6:33

The price per unit get set to eight first, then it gets set to nine. 6:35

So, we've got a bug here that we need to fix. 6:39

We'll see how to do that in the next video. 6:42

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"if" and "unless" Statements

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