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If, Else and Elif
11:12Let's explore conditional branching
Now that we can do some boolean reasoning,
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we should look at how to use it
to our advantage in scripts.
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We make choices all the time.
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If it's raining outside,
I'm gonna get an umbrella.
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If I'm hungry, I'm gonna eat some food.
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If it's late and I'm tired, I go to bed.
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Well, unless I'm stuck on a coding
problem that I just have to fix.
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I should just go to bed at that point.
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The answer usually comes in my sleep.
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All those scenarios
start with the word if.
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And that is the first way that we are
gonna control the flow of our application.
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If you want to learn more,
keep watching this video.
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In our script here, we ask for
a name, we say hello,
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and then we say the person
is learning Python.
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That's not always necessarily true, is it?
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So let's do this to fix it.
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If the name inputted is actually your
name, then we'll print that message.
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So in order to do that,
we're going to need to check and
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see if two values are actually equal.
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You can compare two objects
using the double equal sign.
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Let's take a look real quick in the shell.
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So here's an example.
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We'll say fruit = "apples".
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And so we've assigned apples to
fruit with a single equal sign.
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And then we'll check the quality
by using the double equal signs.
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We'll say fruit does that equal apples?
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True.
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And then if we compare apples to oranges
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which you're not supposed to do,
you get back False.
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And that's because fruit is apples and
apples do not equal oranges.
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So, note how the single
equal sign is used for
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assignment and the double equal
sign is used for comparison.
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Accidental assigning by using just
a single equals is a super common problem.
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And that's probably because when we say,
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fruit equals oranges in English,
not fruit equals equals oranges.
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So when comparing make sure that
you double check for double equals.
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So in our code we want to only show
the learning Python bit if it's us.
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So I'm gonna add a new line here.
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And start it with a new keyword that you
haven't seen yet and it's called if.
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So if,
then what follows next is an expression.
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So let's see.
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We have first name already,
so if first name is equal to,
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note the double equals there,
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Craig, Double-check
the double equals always.
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And then, we add a colon,
which is a new character for us.
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What happens is the colon opens
up the body of our if statement.
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Now, the body is what will run
if this expression is true.
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The way that you group the code together
that will run when the expression is true
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is by indenting each line.
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Now, you can indent your code
with a tab or a series of spaces.
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The only requirement is that all
indentation in your file is exactly
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the same.
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As you can imagine,
that might cause some problems.
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So in order to standardize
that decision for you,
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there is a very strong movement to use
four spaces to indent your Python code.
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So as you can see down here in the editor,
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this says we're gonna use
spaces instead of tabs.
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So we're gonna use spaces and then this
2 here is saying the number of spaces.
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And I just said that we wanna use four.
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So I'm gonna change the default to be 4.
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Four spaces is the default in
most text editors for Python.
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And now we've just changed ours.
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More in the teacher's notes.
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So you'll notice that when I press
Enter after this if statement, you'll
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see that the cursor is automatically
indented to where the code should go.
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Here you'll see that we
are on line 4 which is true.
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Line 4 and we're in column
5 which is 4 spaces, right?
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There's 4 line spaces and
then we start writing, awesome.
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But actually what we really
want is this line here, right?
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We want this line here up there.
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So what I'm gonna do is I'm gonna
move to the front of this and
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I'm gonna press Backspace and
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you'll see that it automatically
indented into our if block there.
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You can finish this grouping or
block of code as we call it, block.
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You can end that by going back to
the original indentation level.
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So if we come to the end here and
we press Enter.
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It's waiting for us there.
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I can press Backspace back here.
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So let's just add a new closing line so
that we can see it.
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So we'll say print,
we'll say ("Have a great day.
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Let's use our format string.
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We'll use the name there one more time.
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And we'll say format, and
we'll pass in first_name.
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Do you see how the indentation
should stops right,
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it shows that this is clearly
not part of the if statement.
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So let's run it and see if it's working.
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I'm gonna drop out of here,
I'm gonna make sure that this is saved,
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there's no red mark up there,
we'll say clear here.
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Let's go ahead and say python hello.py.
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We've got an error.
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Let's see what's happening.
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Did you spot it?
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Unexpected end of file while parsing.
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So, here we go, look what I forgot.
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I forgot the final paren there.
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So there we go.
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So it was just waiting for us to finish.
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That's awesome,
that's exactly the error I showed.
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See, it happens.
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Here we go.
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What is your first name.
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My name is Craig.
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And so there we see,
Craig is learning Python,
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because it went in to the if block and
checked it.
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And now let's test the other side of it.
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Let's run it again without the name
Craig or without your name.
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So how about my buddy Kenneth?
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He's a great teacher and Pythonista and
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you'll meet him in
a future course I'm sure.
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Hello Kenneth.
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Have a great day Kenneth.
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So there we go.
We don't have the output Kenneth
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is learning Python because
our if condition here, right?
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This condition first name equals
Craig Kenneth does not equal Craig, so
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that was false.
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You know what though,
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I'm kinda feeling like everybody should
probably start learning Python, don't you?
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So how about everybody that isn't you
gets a message about learning Python.
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And this way you could run it for
your family or friends, and
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give them a little motivation,
a little wink wink, nudge nudge.
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So you can very easily do this,
and the key word is Else.
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So at the same indentation level as
your if, use the keyword else and
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then a colon because
elses also have bodies.
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So the next body of code and
see how this indented automatically there?
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We'll say print("You should
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totally learn Python, {}!.
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Go ahead and close that string,
format, push in our first name and
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then this time I'm gonna remember
to close my print statement.
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There we go.
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So what happens is, we come in here,
we check the first_name,
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if this is true, this runs,
otherwise, this block runs.
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Do you see how the indentation really
just makes the code kinda jut out?
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It's kinda like branches of a tree,
this is why it's called branching.
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Now, this spacing that you see is
also referred to as white space.
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One thing to do before we verify that this
is working is to talk about our final
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branching keyword.
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It's one of the only Python keywords
that isn't actually a word in English,
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it's called elif, and it's
a combination of the words else and if.
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And what this allows you to do is chain
different conditions, so let's do this.
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Now I told you before that you're
not alone in your journey.
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Your fellow students are taking
this course right now.
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So let's go to the community and
get some names.
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So here I am on the Treehouse site,
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it's most likely gonna look different
cuz we're constantly improving things.
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But up here in the header,
there is a link to the Community link.
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And this by default is all of the topics,
but we can narrow that.
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So let's narrow that down here to Python.
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Let's take a look at this first one here.
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Is there a reason we are not setting
the condition in the while statement?
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We're not quite there yet, but
we're going to see that here in a second.
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So let's go here.
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Let's take a look at this,
and this is Maximilliane.
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Let's grab her name here.
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Let's say Maximilliane Quel,
if I can, Maximiliane.
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So I'm gonna copy that.
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And you'll see here,
she's gone and she's asked and
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she's put some questions in here.
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She got an answer back.
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And there's an answer
there from Steven Parker.
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And she was told good job on
what it was that she was asking.
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We'll get to this here just in a bit.
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So I'm gonna pop back over and we're gonna
use Maximiliane in our example here.
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So if it's me,
we wanna see this is learning Python.
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But if it's specifically Maximiliane,
cuz we know she's in there,
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we'll say if first_name =, and I hope I'm
saying her name right, "Maximiliane".
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Then we'll come in here and
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will say print (first_name,
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"is learning with fellow
students in the Community!
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Me too!").
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And there we go, we added another branch.
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So let's go ahead and
let's run it for Maximiliane.
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It's gonna be, First
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name is M-A-X-I-M-I, Maximiliane.
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Maximiliane is learning with
fellow students in the community.
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Me too.
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Cool, that's awesome that
you're gonna hang out there.
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Helping others is a wonderful way
to make sure your learning sticks.
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I'm so proud of you.
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So looking at the code,
we see that it checks this if statement,
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is Maximiliane equal to Craig?
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That is not true.
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Elsif Maximiliane is equal
to Maximiliane print this.
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And because that happened,
we're not gonna hit this otherwise block.
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So let's run one that does
hit the otherwise block.
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Let's do that real quick.
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We'll say Python, and let's use one of
our JavaScript teachers here, Treasure.
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And there it says, hello, Treasure,
you should totally learn Python, Treasure!
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I think she's learning Python actually,
she's kind of a polyglot,
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she talks a whole bunch of these
different programming languages.
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So that equality comparison operative,
the double equals,
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is just one of many ways that
we can compare our objects.
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So if you would like to learn more
about other comparison operators
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then watch the upcoming video.
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Elsif you are feeling like you want to
check out the community forum then you
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totally should, else you should take
a break and then come back refreshed and
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ready to compare things.
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