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We've learned about comparison operators.

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

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Now we just need to add code
that applies to the discounted

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price if those conditions are true,
like most programming languages,

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

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

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

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

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

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read although it's not required.

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

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In this case, because the condition for

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

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

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And because the condition for

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

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

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Just hard coding the values true and

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

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Actual programs will use expressions
that evaluate the true or

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false, like comparison operators.

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

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which results in the value true.

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

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the output down at the bottom.

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

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Finally, we test whether the value
50 is equal to the value 50,

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which of course is true so this code runs.

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

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

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

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

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then we test whether
a number is greater than 50.

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

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

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

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

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

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

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statement, but
Ruby provides a second keyword.

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Unless, that's basically
the opposite of if.

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Unless runs code only if
its condition is false.

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

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statements, And try running this.

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Now we get the opposite
of the previous results.

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

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

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

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

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

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

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

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

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So you see number == 50
in the output below.

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

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statements to determine which
discount is should apply.

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

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

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

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

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

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Because if the user is
ordering 100 widgets or more,

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they should only be charged $8 per widget.

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If the quantity, Is greater than or

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

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

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a discount if they're
only ordering 50 units.

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And if the quantity is < 50,

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Then we're going to want
to charge full price.

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So we'll set a price_per_unit of 10.

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

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we'll multiply that by the price per unit.

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

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than 50 units.

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

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

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So let's save this and try it out.

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Ruby widgets.rb.

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

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

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That part seems to be working correctly.

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Now let's try an order of 50.

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

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

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

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adding one widget adds $9 onto the total.

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Now let's try ordering 100 widgets or
more.

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For 100 widgets here, the total is $900.

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

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It's supposed to be multiplied by 8.

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

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

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

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equal to 50, so
this condition is also true.

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And this code here runs as well.

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

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

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