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[MUSIC]

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Hi, I'm John.
Welcome to my first video for this course.

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So, we're going to be looking at a bunch
of examples.

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First on errors.

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When you get an error message, how do

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you understand what it means and fix the
problem.

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Second we're going to look at a few
probing tips

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that should help you when you're writing
your own code.

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So here's our first example.
Let's run it and see what happens.

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It starts to print out this message about
the volume of a cube,

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but then has a name error saying that
volume is not defined, kay.

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It highlights the problem line here, where
in fact we

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are using the function named volume to do
our computation.

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But where did I define volume?

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Well, I didn't.
I defined volume cube, kay.

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here I should be calling volume cube, so
let's fix that, kay.

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So, now let's run it again and make sure.
Oh, whoops!

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We have another name error.

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This time, I used the name Sidde here in

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this line instead of the variable named
side, side.

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Kay, so a typo here.

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Kay, so running it finally it works.

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So, notice that we get a name error when
we misspell either a

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function name or a variable name here and
don't have a definition of that name.

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Alright, let's see what happens with this
next example.

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We'll run it and well, once again we get

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a name error here, name random is not
defined.

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So well, we're trying to use
random.randrange to roll a die.

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Hmm, what's the problem here?

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well, here let's go to the documentation,
kay.

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And we can scroll down here to the random

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module, and it tells us well, yes
random.randrange is right.

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But whenever we need to use it well, yeah,
import

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random, we forgot that and it tells us up
here.

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You know, whenever we're using anything
from this module, import random.

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We forgot to do that in our code here so
let's add that.

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Yeah, we didn't provide that

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definition of random that it's a module
here, so

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let's try this again, running it and it
works fine.

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It, it, rolls our dice and each time we
get a different value for our pair of die.

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In this example we want to compute the
volume of the sphere.

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Let's see what happens when we run it.

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We get a attribute error.
Module object has no attribute pi.

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Hmm, well we're using math.pi and I'm
importing math correctly.

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So what's the problem?

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I thought pi was defined.

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Let's re-, let's go to our docs, and we've
got a little delay on load right now.

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let's search for pi and there we go.

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There's math.pi, but it's lower case,
okay.

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All of our names are case-sensitive, so I
misspelled this, and any time

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you misspell the part after the period,

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that's going to give you an attribute
error.

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So changing that and rerunning, it now
works.

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In this example we want to calculate the
area of a triangle so, well.

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We get a type error, area triangles takes
exactly two arguments but one was given.

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So let's look at our function call which
is here, area triangle b.

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Well, in fact, I did give it one argument.

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instead of the two that it's expecting I
forgot

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to pass an h, so let's add that.
Okay, and re-run.

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[SOUND].

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I get another type error.
Oh, hmm?

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But it, at least it's different.
we fixed that one problem.

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Type error can't multiply sequence by non
ends of type number, huh?

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Alright, let's see what it's complaining
about.

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Okay, so I am doing a multiplication like
it's talked about.

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As we need to in this formula.

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But, what's going on?

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So, let's look up at, at what's actually
being printed out, and it says,

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the area of the triangle with base five
and height two plus two is.

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Height should be four.

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Why isn't it adding two plus two there to
get four?

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Well, the problem is we have strings here
not numeric expressions.

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This is

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simply the string two space plu- two space
plus space two.

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It's not a mathematical expression
resulting in four.

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So that's why this is resulting in
multiplying a

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sequence because, well, it's trying to
multiply a sequence base,

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which is a string and a string is a

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kind of a sequence by something that's not
an integer.

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Well, height here is also not an integer.

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Turns out you can actually multiply
strings from an

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integer, but that's a whole separate
thing, separate operation.

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Kay, so let's just get rid of the double
quotes here.

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Turn these into mathematical expressions
and rerun and we get what we want.

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So, next here we have a little function

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that's supposed to simply check whether
we're giving

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it Mary, and if so it gives us a message,
that yes it has Mary, otherwise no.

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Okay, so let's try running this, 'kay.

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Syntax error, bad input on line one.

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[SOUND].

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Quote, you know, and then quote down here
on the next line.

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Kay, so it tells us there's a syntax error
where it is roughly, on what line.

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Okay, and the fact that, well, it's trying
to tell us here that it's

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it's at the end of the line, where, where
it's encountering a new line.

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Kay, and so here it's just kind of giving
us a new line there.

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Kay, so that is hinting that

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it must be at the end of the line here.

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And in fact we forgot to put the colon on
here.

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Remember, every function header begins or,
ends with a colon.

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Just like a every if here ends with a
colon, else ends with a

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colon, anytime that you're going to start
a block of code, kay, so, fix that.

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Let's see it run now.

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Oh, another syntax error.

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Bad input on line two now.
Okay, we have if x is equal to Mary

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there's something wrong with the equals.
What's wrong with the equals, hmm?

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Let's go back to the documentation here.
And we could, let's try comparing things.

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Let's see, where would I find something
about comparing?

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So, compare, oh, comparison operators
there.

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let's see.

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Oh, the comparison here is double equals
not single

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equals kay, very common mistake.
Single equals is assigning a variable,

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double equals is comparing two things.
Okay, so

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let's try this again here.
Oh, name error, name Mary is not defined.

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Oh, kay, yeah, down here I gave it.

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Well, Mary is not defined, I, I meant to
pass in just a string.

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Okay?

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Here, note that note the color, okay?
Well this is kind of useful here.

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So, in this code here where we have

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some strings, note the strings are
color-coded red.

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And this is not color-coded red.

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It's just black like any other name.

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And well, notice over here we actually
have purple stuff too, for keywords.

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Kay.
Notice when you're typing,

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whether things are turning up the color
that you wanted.

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Kay, and so here, basically just another
kind of syntax error.

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I meant to give a, meant to give it a, a
string instead of a variable name.

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So let's see if I'm good now.

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Oh, good.

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You know, it found Mary and then Fred,
well, is clearly not Mary.

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So, fixed it.

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So let's turn to the issues of code style
and readability now.

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So, just trust me and that this code
actually works.

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So, we have some code, what does it do?

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Def area a, b, c and some math here.

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Not really clear, you might not know this
formula.

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You know, if somebody else is looking at
this

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code after you've written it, they don't
know what you meant probably.

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In fact if you're looking at this a month

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after you wrote it, you might not remember
either.

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So, what should we have done?

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We need to provide more information to
actually explain what's going on here.

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The first thing is what is this supposed
to be doing?

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Kay, let's make that clear.

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So, this is actually supposed to calculate
the area of a triangle.

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Kay, so let's give it a more useful name.
Area of a triangle and I'm going to

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call it area triangle_sss to be sure that
it's taking three sides, okay?

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And so well, no matter how long a name
should, can be you don't want to make it

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too long, it gets just kind of unwieldy.
You can provide

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more information in what Python calls a
documentation string, or a doc string.

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And in Python, that traditionally goes
right here, right

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after the function header, and it's put in
a string.

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Kay?

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Even though this is just a string, we

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traditionally put it in three quotes,
three double quotes.

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what the three double quotes allows is
that, it allows a string

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to go over multiple lines.
Kay?

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Now, once you go into a document string,
you should talk

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about what are the inputs what's the
output and how those relate.

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So, here we could say for example,

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that this returns the area of a triangle

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given the lengths of its three sides.
Kay?

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Now, the documentation string tells you
what it does, it

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does not tell you how it actually does
that, kay?

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So we've got some weird math here.

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You might recognize it, you might not,
kay?

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It's actually a standard formula known as
Heron's formula.

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Kay.
So, I'm going to write that down.

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You know, somebody can look up Heron's
formula and

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say aha, you actually typed in Heron's
formula correctly.

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Kay?
Well, I'm going to put this in a comment.

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A comment starts with one of those hash
lines or pound signs.

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Kay.

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And note that I'm not putting this

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in the documentation string, kay, this is
different.

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This is telling somebody, how I'm
calculating

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this, as opposed to what I'm actually
calculating.

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Kay, so that's already better.
Somebody could understand,

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given the English and the better name,
what's going on.

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But still let's, let's improve this here.

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A, b, c, you know, those are kind of poor
names.

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What are those supposed to be?

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They're supposed to be sides, but let's
make this clear.

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Let's rename them side one.

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And I'm going to add in some space that is
for readability, too.

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Side two and side three and be

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sure to do that consistently.

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[SOUND].

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Alright, and similarly S, well that,

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that's kind of a weird name.
What is that?

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Well if you add up the three sides of

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a triangle and divide it by two, what is
that?

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Kay, adding up the three sides, that's the
perimeter.

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Divide by two, that's half of a perimeter.

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In other words

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it's the semi-perimeter.

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Kay, so that's why it's traditionally
called s.

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Let's actually give it a longer name
called semi_perin.

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Let, you know, more descriptive.
Add a couple more spaces

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in here for readability and rename
consistently here, kay.

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Be sure of the consistency.

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[SOUND].

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Now one thing you can see that's not

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very readable is now I have a really

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long piece of code, a really long

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line.
Well you can break that up,

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kay.
Notice the indentation.

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So, I have the one formula within the
square

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root that's broken up on four lines so
it's

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more readable and, and in fact they stack
on

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top of each other, four things that I'm
multiplying.

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00:13:51,790 --> 00:13:55,500
So, this is definitely a much more
readable version

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and somebody could read this and
understand what's going on.

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So, this little program here is supposed
to tell us,

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you know, what is the favorite thing of
our various instructors.

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So, Joe loves games, that's this course.

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00:14:11,760 --> 00:14:14,880
Scott loves ties, as you should be seeing
from his videos.

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And I love the outdoors but, well, I'm
here in front of a computer all the time.

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So, well, let's run it.

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00:14:22,090 --> 00:14:24,410
And, well, it reports that I love the
outdoors.

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But, hm, Jeannie, my wife,

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who's not an instructor and none.
Where's that none coming from?

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00:14:32,300 --> 00:14:37,110
Well, first of all you know, note that
because Jeannie is not

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one of these options, it doesn't meet one
of these branches here.

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It just falls through and at the end of
the function there's nothing left to do,

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so it just returns a special value called
none which signifies that there's

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no value there.
Kay?

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00:14:55,630 --> 00:14:57,074
Now that seems kind of strange, and in

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00:14:57,074 --> 00:14:59,380
particular in a larger program, if this
was just

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a piece of some larger thing you'd
probably have

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a hard time tracking down where the
problem was.

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Kay, so we'd like to have some way of
saying, hey something's wrong

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00:15:10,170 --> 00:15:16,030
here, you gave me a valid, an invalid
input that I wasn't expecting.

233
00:15:16,030 --> 00:15:18,370
and so here is one really simple

234
00:15:18,370 --> 00:15:23,700
way of doing it.
Just make sure that each of your ifs has

235
00:15:23,700 --> 00:15:28,855
an else case, kay?
Else, kay, some other inner

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00:15:28,855 --> 00:15:33,623
value of instructors here let's

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00:15:33,623 --> 00:15:37,944
print out a message you know,

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00:15:37,944 --> 00:15:44,050
favorites, saw invalid instructor.

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00:15:44,050 --> 00:15:47,780
and here's our value Jeannie.
Whoops, instructor.

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00:15:50,950 --> 00:15:56,440
And so we can run that, and it gives us
this message saying, hey, this function

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00:15:56,440 --> 00:16:02,490
saw an invalid input, you know, now I know
where to look, and where the problem was.