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- [Instructor] We can
use Boolean expressions

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to ask questions in our programs,

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but how can we branch control
flow based on the answer?

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To do that, we need conditionals.

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Conditionals form the basis of selection.

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They allow the computer to decide

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which code to run depending
on whether the answer

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to a specific question or
condition is true or false.

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The first piece of a
conditional is the IF statement.

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We start an IF statement
with the keyword If.

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Then we add our condition,

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which can be any boolean expression.

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We always end this line with a colon

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because an IF statement
comes in two parts.

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This first part, which is the condition

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and the second part, which
is the instructions to run

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if that condition evaluates to true.

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In Python, we indicate those
instructions using indentation.

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Any lines of code immediately
following that condition

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that are indented one level in

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are considered inside that IF statement,

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if the condition evaluates to true,

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the computer will go on to
execute all of the instructions

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indented inside the IF statement in order.

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When it's done, it'll go on to execute

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the rest of the program as normal.

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If the condition evaluates to false,

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the computer will skip
all the lines of code

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that are indented inside
of that IF statement.

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Instead, it'll jump directly
to the next line of code

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that's outside of the IF statement.

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That means the first line of code

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that is indented at the same level

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as that initial if.

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Let's try an example.

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Here we have the Boolean expression,

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num_orders equals equals
10, which asks the question,

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is this the customer's 10th order?

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We can then use this Boolean
value in our IF statement.

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We'll indent one print function call

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inside of the IF
statement and one outside.

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The one inside will only
execute if it is the 10th order,

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that is if the variable is 10th order

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contains the Boolean value True.

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We can also write this
without the variable

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where we just inline that
whole Boolean expression

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in the IF statement.

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Let's trace how the
computer executes this.

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It starts with the first line

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and assigns a value 10 to
the variable num_orders.

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Then it hits this IF statement,

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so first it evaluates
the Boolean expression.

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10 is equal to 10, so
this evaluates to true.

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Because it's true,

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the computer will
execute the lines of code

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indented inside of the IF statement.

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First, it'll print, "Your order is free!"

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And then it'll assign the value zero

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to the variable num_orders.

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Then it just keeps moving through

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the rest of the program in order,

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so it'll print the value of
num_orders, which is now zero.

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Now let's say instead num_orders

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was originally set to three.

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When the computer executes this program,

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it'll assign the value
three to num_orders,

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evaluate the Boolean expression.

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Three is not equal to
10, so this is false,

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and then because it's false,

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we'll skip the lines of code indented

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inside the IF statement.

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It'll jump to the next
line of code outside,

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which is print num_orders,
so it'll print three.

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Note that if we didn't
indent the line of code

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num_orders equal zero,
it would be considered

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outside of the IF statement,
so in the false case,

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num_orders would be set to zero,

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and then it would print zero.

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In the true case, we would
print, "Your order is free."

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Then we would set num_orders to zero,

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and then we would print zero.

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You can write IF statements
with any Boolean expressions

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as conditions, and you can put any

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and as many instructions
as you want inside of them.

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With this conditional, I could make sure

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to only ask the follow-up question,

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chicken or tofu, if the
user ordered pad Thai,

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because if they order the papaya salad,

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that question doesn't make sense.

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Note that in most IDEs we
indent using the tab key.

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Our standard lines of code

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should line up directly
with the left margin.

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That is, there should be no
spaces or indents before them.

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Lines of code indented
inside of an IF statement

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should be one tab key over.

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Getting the indentation right is crucial

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because this is the only
way we can tell the computer

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which lines of code are
inside the IF statement

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and which are outside.

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We need to be careful here though

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because now our control flow branches.

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If the user orders pad Thai,
this program works fine,

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but if the user orders something
else, we get a name error.

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The variable protein is only defined

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inside the IF statement.

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If the condition is false,

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this assignment statement doesn't execute,

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so the computer doesn't
know a variable protein.

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To fix this, we either need to make sure

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we're only accessing the variable protein

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inside the IF statement where
we're guaranteed it exists,

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or we need to initialize
the variable protein

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before the IF statement,

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which guarantees that it's always defined.

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It's common in situations like this

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to initialize the variable

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to an empty or placeholder value

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like the empty string or no protein.

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Now there's no error on
either possible path.

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If the order is equal to
pad Thai or if it's not.

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Now that our control flow
branches, when we run a program,

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we're only testing one

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of perhaps many possible passive
execution through the code.

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Just because it works for one case

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doesn't mean it works for the other,

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and it's up to us to test for that.

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Otherwise, it'll be our users who suffer

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when they're the first
ones to find the bug.