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- [Narrator] In this video,
we're going to talk a

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little bit more about the if statement

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before we begin looking at
its alternative versions.

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Now, back in the preceding
lesson we demonstrated

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the basic functionality
of the if statement.

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Let's do that again here by
first declaring a variable

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called "grade" and we'll
give it the value of 85,

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and let's write an if
statement that's going to check

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whether the grade value is
greater than or equal to 60,

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which when you're taking a
course is normally an indication

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that you've passed the course,
and in the body or the suite

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of that if statement, let's go ahead

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and simply display the string passed,

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if in fact the grade is
greater than or equal to 60.

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Now, based on what we define
the grade to be up above,

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this condition will be true
and as I execute this statement

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it's going to show me the word "passed."

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Now, before I continue, I
just want to point out here

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that when you press enter
on an indented suite line

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inside of a control statement
like the if statement,

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it assumes that you'd like
to continue defining the body

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or the suite of that statement.

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So in order to actually execute the code,

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I now have to press enter once again,

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and at this point I can see

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the string "passed" gets displayed.

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Now, we started to briefly mention

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in the preceding lesson this
issue of indentation in Python.

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Unlike most C-based languages,

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which typically use curly braces,

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like the left curly brace
and the right curly brace

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that I just typed here,

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to delimit the body of a control statement

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like the if statement,
in Python, the suite

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of an if statement, and similarly
other control statements,

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is defined by indentation only.

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The default indentation
level is four spaces,

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and that's what I got here automatically

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when I simply pressed
enter while typing the code

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in the context of the
interactive I-Python interpreter.

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If you're working in an integrated
development environment,

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then you'll typically have
four spaces by default,

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although that is configurable,

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and there's nothing that says
you have to use four spaces.

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What Python does require, however,

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is that whatever indentation
level you choose,

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and whether you choose
to use spaces or tabs,

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you have to do that uniformly

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for everything in a given suite.

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So, if, for example, I
recalled the statement

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that I just did, and, let me
go back up to the print line

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and get rid of the indentation.

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So when you're defining an if statement,

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it expects its suite to be indented.

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This is actually an error in Python,

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and to demonstrate that, let
me go ahead and press enter,

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and now you can see that I
have an indentation error

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and it's telling me in this statement

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that was not indented properly.

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Now, similarly, going back
into my if statement here,

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let me put the four spaces back in,

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if the indentation is not consistent,

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that also is a problem.

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So, for example, in this
first suite statement,

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I have four spaces before the print.

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On the next line, I've
removed two of those spaces,

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and let's create another
print statement that says,

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"Good job!" and if I now go ahead

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and try to execute this
statement, once again,

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I get an indentation
error, this time pointing

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to the "Good job!" line and the reason

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is there are only two
spaces in front of print

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in this case as opposed
to four spaces up above.

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So, not only do you have
to maintain the consistency

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of your spacing, the
interpreter is going to check

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that spacing for you, and it turns out,

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there's also static code
analysis tools that can scan

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larger code files for
things like this, as well.

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So, that's the basics of the if statement.

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Now, another thing I want to point out

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about the if statement is the fact

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that the condition can be any expression

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that evaluates to true or false,

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and it turns out that in
Python everything has a value

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of true or false associated with it.

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So for example, if I go ahead and say,

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"If one," any value
that's nonzero is going

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to be treated as true, and
zero is treated as false.

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So, if I say, "If one," and I want

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to do a print statement
here that simply says,

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"Nonzero values are true,
so this will print."

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So we got a little if statement there.

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I expect to see the
print happen in this case

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because one is considered to be true,

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so when I execute that,
sure enough it does execute.

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On the other hand, if I use zero,

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which is false by default in Python,

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and I do a little print statement and say,

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"Zero is false, so this will not print."

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You can see that indeed
nothing is displayed

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as a result of that condition.

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Now as we move forward
through subsequent lessons,

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we're going to see that
more complex objects

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also can be evaluated as true or false.

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So for example, a string
that contains characters

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would be considered
true, and an empty string

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that contains no characters
would be considered false,

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and similarly, when we get into

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the various Python data structures,

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which we'll start to
do here in this lesson,

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they, too, can be evaluated that way.

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So, a collection of items
that has elements in it,

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has items in it, like a
collection of random integers,

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that would be considered true,
whereas an empty collection

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would generally be considered false.

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Now, there's one other thing

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that I want to point out about conditions

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specifically with the
double equals operator.

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If we go and write an expression
like grade is assigned 85,

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which is what we did up above,

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that is going to assign the
value 85 to the variable grade.

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Now, before I execute that
let me get rid of that.

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Let's assume grade already exists,

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which it does up above we defined it.

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So if I go and write by accident,

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"Grade double equals 85," you
can see that I get a result

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of true because if I scroll back up here,

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you can see that indeed, grade was 85.

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So, if I mistakenly accidentally
typed two equal signs,

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then I would get a boolean value.

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Similarly, if I mistakenly
was trying to assign a value,

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and let's say we made it 58.

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In this case, I get back false,

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when what I really wanted to do

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was change the value of grade,

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so of course I need a single equal sign

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in order to be able to do that,

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and now grade has a new
value associated with it.

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Another potential logic error, would be,

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let's say I want to compare "X" to 85,

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and I have not defined "X" yet,

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in that case as we
demonstrated once before,

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you get a name error because the name "X"

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does not yet exist in
this interactive session.