Python for Informatics: Chapter 2 - Expressions

This is the second chapter of Python for Informatics. We cover constants, variables, reserved words, assignment statements, operators, types, type conversion functions, input using raw_input(), and the basic structure of a sequential program. Material provided by www.pythonlearn.com.
All Lectures: http://www.youtube.com/playlist?list=PLlRFEj9H3Oj4JXIwMwN1_ss1Tk8wZShEJ

Hello, and welcome to Chapter Two.
Hope you enjoyed Chapter One.

It was one of the longer lectures.
Trying to motivate you a little bit.

And now we're going to kind of go back to
the basics, to the, chapter

Chapter One covered sort of the first four
to five chapters of the book.

So as always, this this video,
these slides are

copyright Creative Common Attribution, as
well as the audio.

And so,

now we're going to talk about
sort of the really

low-level things that make up the Python
language.

Constants. So I'm going to summarize this
terminology just so I

can like say the word "constant" and you
won't freak out.

A constant is as contrasted with something
that changes, a variable.

We talk about variables in the next slide.
But for now, constants.

Constants are in things that are sort

of natural and instinctive.
Things like numbers.

A hundred and twenty-three.

98.6, or Hello world.

And so in, in, what, what I'm doing here
is we're, we're using

a Python interpreter, and that, that's how
you can tell, the chevron prompt.

And I'm saying print 123, and then Python
responds with 123,

print 98.6, Python responds with 98.6,
and print 'Hello world'.

So the constants are the 123, 98.6, and
'Hello world'.

So these are things.

We can use either single quotes or double
quotes to make strings.

And so programs kind of work with numbers
and work with

strings and we have these non-varying
values that we call constants.

So the other side of the picture is a
variable.

And the way I like to characterize a
variable

is it's a place in the memory of the
computer.

We give it a name as a programmer.

We pick the variable name.

In this, I'm saying x equals 12.2 and y
equals 14.

I am choosing the name and I'm choosing
what to put in there.

This is a statement called an assignment
statement, and the way to

think of the assignment statement is that
it sort of has a direction.

We're saying, dear Python, go find some
memory.

I will use label x later to, to refer to
that

memory, and take the number 12.2 and stick
it into x.

Then this is sequential code.

Then the next thing I want you to do is
I'd like you to go find some

more memory, call it y, I will call it y
later, and stick 14 in there, okay?

And so that ends up sort of with two
little areas

of memory.

You know, the one labeled x, and
here's a

little cell in which we, like a drawer, or
something.

And one labeled y. And we put 12.2.
After these

lines run, we have 12.2 in one and
14 in the other.

Then, for example, if there's another line
that's down here, so there's this

third line after this has happened, after
this has happened, x equals 100.

Remember, this has kind of got an, a
direction to it, see?

Oh, remember that x that I had, you know,
I would like now to put 100 in that.

So as I'm thinking this through, I think
of that as sort of

removing the 12.2 or overwriting the 12.2
and putting 100 in its place.

And so at the end here, x is left with 100
and y is left with 1 4 with 14.

So these variables can kind of have one
value in them and

what we can look at them and we can

reuse them and put different values in if
we want.

There are some rules for naming your
variables.

Again, you get to pick the variable names.

Often we pick variables that make some
sense.

We'll talk about that in a second.

In Python variables can start with an
underscore.

We tend not to, as normal programmers, use
those.

We let libraries use those.

It has to have letters, numbers, and
underscores.

And, and start

with start with a letter or an underscore.

Case matters, so spam is good, eggs is

good, spam23 is good because
the number is not

the first character, _speed, that's also
perfectly fine

because it start with an
underscore or a letter.

[COUGH] 23Spam starts with a letter,
starts with a number, so that's bad.

This starts with something

other than a letter or an underscore.

And you can't use a dot in
the variable name.

It turns out the dot has meaning to Python
that would confuse it.

That would confuse it and wouldn't
understand [COUGH] what we

really mean there, and so that would be a
syntax error.

That would be a syntax error.

Because case is sensitive, that means that
things like all lowercase

spam is different than a upper case S and
all uppercase.

These are three distinct variables that
are unique.

Most people don't use, choose variables
that might be so confusing.

So that's to

you as you write it and as to anybody that

might read it would find three variables
named this very confusing.

So it's a bad idea.

Don't do it, but I'm just showing you as

an example that case can make a variable
name distinct.

And again, this variable is a place in
memory

that we are going to store and retrieve
information.

Whether that be numbers or strings or
whatever.

These are things that we control.

Now Python also has a set of reserved
words.

What it really means is you can't use
these for variables.

These words have very special meaning.
And, for, is, raise, if.

So you can't make a variable named i-f.
It would be like, oh no, that is "if".

I know what "if" is.

So these are words that Python has as its
core vocabulary.

And forbids you to use them

for other purposes, like
variable names or later function names.

So that's kind of the vocabulary.
Constants, variables, and reserved words.

Now, we take these and we start assembling
them

into sort of sentences, statements, Python
statements that do something.

So we've already talked about an
assignment statement.

It has kind of an arrow here.

It says, hey Python, go find me a place
called x.

Take the number 2 and stick it in there
for later, then continue on.

Now, because there's an arrow,
the right side of this is done first.

And so it said, so this right side, you
can kind of ignore for the moment the

left-hand side and it calculates the
right-hand

side by looking at the current
value for x.

Which happens to be 2, and adds these
two things together, and then gets 4.

And then, at the point where it knows
4, that this

number is 4, it will then store that
back into X.

And so then, later, we print x and we will
get the 4. And so again, this is

a sequence of steps and the, the
variable x

changes as these steps continue.
And when we're saying print x,

that really means print the
current value for x.

So, we can do a number of different
operators and assignment statements.

We calculate this right-hand side.

This is sort of all calculated, whatever
this is, based on

the current value for x does this
calculation, and then when

it knows what the answer is, it assigns
that into the

variable that's on the left-hand side of
the assignment statement.

Again, calculate the right-hand

side completely and then move it to the
left-hand side.

Some early languages actually didn't use

the equals sign for the assignment
operator.

This assignment operator in, in a way it
kind of [INAUDIBLE]

Some languages

An early language actually
used an arrow.

Arrows aren't really on people's
keyboards.

Another language used colon equals as
this assignment operator.

But we use equals.

Now, if you're familiar with math this can
be a little confusing, like x equals 1

and then X equals 2. That as mathematics
would be bad math

because in a proof or a problem, x can only
have one value.

But in programming if this was two
statements, that means

just x had a value, and then the value
for x changed later.

Okay. So just kind of go through this
because it's

working from the right-hand side to the
left-hand side on assignment statements.

It is pulling out these x values, so x may
have 0.6.

It pulls the values out before, sort of
ignoring this part

right here, and it's just going to try to
resolve this expression.

And it has multiplication and parentheses
and things like that.

So it basically pulls the 0.6 into the
calculation,

does the 1 minus x, which gives you 0.4.

Then it multiplies these three things
together, giving 0.93.

And then when it is all done with all of
that, it takes that.

Oops.

It takes that 0.93, and then puts it back
into x.

And so this is just sort of emphasizing
how the right-hand side is computed to

produce a value, then it is moved into the
variable, and that is why you

can have sort of x on both sides.

Because this is like the old, and this is
the new.

This is the old x participates in the
calculation, and

then when the calculation is done, it
becomes the new x.

Hope that makes sense.

So, this, on the right-hand side here is
a numeric expression.

So we have a number of different
operators.

Some of them are instinctive,
intuitive.

The plus and the minus.

The reason some of these are so weird is
in the really old days, we

didn't have too many things on the
keyboard,

and a lot of programs were very
mathematical.

And so they figured out what was on

the keyboard of the computer equipment of
the day.

And then they had to fake certain things.

So, it turns out that plus and minus
were on the keyboard,

and so plus and minus are
addition and subtraction, respectively.

There was no kind of times

operator for multiplication, and dot was
used for decimal points.

So they used asterisk for multiplication.
So on computers' languages, nearly

all of them, they basically use a mult
times for multiplication.

Slash is used for division.

So we say like, 8/2, which is 8 divided
by 2.

Raising something to the power like 4
squared,

that is double asterisk.
And then remainder is if you

do a division that gives you the remainder
rather than divisor.

So 8 over 2 is 4 remainder 0. So

the remainder is what you get with this
particular operator.

There's a few cool things that we can do

with remainder that we won't talk about
right away.

But it's there.

And so here's just a couple of sample
expressions.

That's giving me green.

Okay.

So, so again, I'm using a Python
Interpreter.

So you can kind of, this is just the
prompt.

These chevrons are the prompt.

Create the variable xx, and
assign it to 2.

Retrieve the old value and an addition.

Then print it out and put it back into xx
so xx

has 4.
yy, this is a multiplication, 440 times 12.

It is 5,280. yy over 1,000.
Now this is a little counter-intuitive

Because yy is an integer,
it then does it in a truncated division.

And so, 5,280 divided by 1000 is 5.
Now if, and,

and so that's an integer division.
We'll see in a second

about floating point division.

Now we take the variable jj and we set it
to 23.

And now we're going to use the modular or
modulo or remainder operator.

Say what is jj, what is the remainder when
divide this jj by 5.

And so if you think about this, we take
old long division,

23 divided by 5, you end up with 4 and
then remainder 3.

The modulo operator, or the percent of

the remainder operator, gives us back this
number.

And so that's why kk is 3.

It is the remainder of 23 when
divided by 5,

or the remainder of the division
of 5 into 23.

And the raising to the power, 4 cubed.
That's not so nice.

4 cubed is 4 star, star 3.

And so that ends up being 64.

So that's just operations.
Now, just like in algebra and mathematics

we have rules about when to which, which
operations happen first.

In general, things like the power happens
before the

multiplication and division, and then the
addition and subtraction happen.

And so there are some rules that, when

you're looking at an expression and trying
to calculate

what its value is, if you don't have
parentheses, it follows these rules.

And so the, the most, the rule
that sort of

trumps all the rules is that parentheses
are always respected.

So a lot of us just write these with

parentheses in place, even sometimes
though you don't need it.

Then after parentheses have been handled,
then it does exponentiation.

Then it does multiplication,

division, and remainder.
And then it does addition and subtraction.

And then, when all else being equal, it
just works left to right.

So let's, let's look through an example.
So here is a

calculation that is, you know, 1, 1 plus 2
times 3 divided 4 over 5.

And the question is, what order does this
happen, okay?

And so let's sort of take a look at this.

And so, we start with are there any
parentheses?

And the answer is no, there are no
parentheses.

So let's go next.
Power.

And so the, the power says okay, let's
look across and find those things that

are raised to a power. And 2 cubed or 2 to
the third power is the, the power.

So we're going to do that one.
Okay.

And then we can, the way I do it when

I'm sort of doing these slowly is I
rewrite it.

So the 2 to the third power becomes 8, so
it's 1 plus 8 over 4 times 5.

And then now we can say oh power, that's
taken care of.

Now we're going to do multiplication and
division and we go across.

Now we have both a division and
multiplication.

Okay? Multiplication and division are done
at the same

time, so that means we do left to right,

which means we do the first one we
encounter first.

And so that will be 8 over 4 because of
the left-to-right rule.

And so we find that one, and that's the

one that gets computed next, and that
turns into 2.

And again, I like to rewrite these
expressions

just to keep my brain really, really
clear.

After a while you just do it in your head,
but I rewrite them.

When I was first learning it,

at least, I rewrote it all the time.

And and so next looking at this, there's a
multiplication.

We're not done with multiplication yet.
So the 2 over 5 is the next thing.

And then we do that calculation, and that
becomes 10, and again we rewrite it.

And now we've done the multiplication, and
we're going to do addition next.

And that's just 1 over 10, and that
becomes 11.

And so basically, this big long thing,

through a series of successive steps,
becomes 11.

And indeed, when we print it out, that's
what we get.

Okay?

So, there's the rules that are
parentheses,

power, multiplication, addition, and then,
left to right.

But smart people usually just put
parentheses in, you know?

So here's this, here's an exam.
Oop, go back, go back.

Here's an exam question.

Now, I wouldn't write this code, right, I
wouldn't write this code this way.

I would put a parentheses here.

And a parentheses there.

It's the same thing because that's exactly
the 2 times 3 is going to happen and

4 over 5 is going to happen and then the

plus and the minus will happen
left to right.

But why not make it easier on your readers

and just put the parentheses in. Because
they're redundant.

They're not necessary, but away you go.

Now, if you don't want it to happen in
that order, of

course then you have to put parentheses if
you want the addition

to happen before the multiplication, then
you

have to put parentheses in, which you can.

But we tend to recommend that you use more
parentheses rather than less parentheses.

Now, Python integer division
in Python 2,

which we're using Python 2
for this class.

There's a new Python 3 that the world is
slowly transitioning

to and a lot of people are
using it in teaching.

But it's not as common, sort of, in the
real world with libraries and utilities.

And so we'll stick with
Python 2 for a few

more years until Python 3 really kind of
turns the corner.

It's nice to have it there, but there's so
much Python and it's so

popular, Python 2, that it's just kind of
hard to get everybody up to Python 3.

So in Python 2, integer division truncates
and you saw that before where

I did the 5280 by 1000 and I got 5 as and,
and, but we

can look at a couple of examples that make
this really very quite, quite clear.

So, 10 divided by 2 is 5 as you would
expect.

9 Divided by 2 is 4.

Not exactly what you'd expect.

You kind of expect that to be 4.5,
instead of 4.

But in Python 3, it will be 4.5, but for
now, in Python 2,

9 over, 9 over 2 is 4.
And 99 over 100 is 0.

Now that seems rather counter-intuitive,
but it is a truncating

division, it's not a rounding division,
it's a truncating division.

Now, interestingly, if you make either of
these numbers have a decimal, make them

what we call floating point numbers, then
the division is done in floating point.

So, 10.0 over 2.0

is 5.0.
Now, these are different.

This is an integer number, and this is a
floating point number.

It's 5.0.

And then 99.0 over 100.0 is exactly as you

would expect, and it's a floating point
number, so.

Now you can also mix integers and floating
point numbers as you go.

So here we have 99 over 100.

Those are both integers.
Integer, integer.

And, or, and that comes out with 0 because
it's truncating.

Now if we have an integer and
a floating point

number, 99 over 100.0, then that comes out
as 0.99.

And either one, if we have 99 over 100,
that's a floating point, and

that's an integer.

We still end up with a floating point, so
this is a floating point, floating point.

And even in complex expressions, as it
evaluates when

it sees an integer, so the first thing
when

you evaluate is this would become a 6, so
it would be 1 plus 6 over 4.0 minus 5.

Then it would be doing the 6 over 4.0 and
that would be 1.5, 1 plus 1.5

minus 5. And so this is an integer
and that's

a floating point and the result becomes a
floating point.

And then the rest of the calculation is
done floating point

to the point where the ultimate is a
floating point negative 2.5.

So you can throw a floating point into a
calculation and as soon as the

calculation touches the floating point,
the remainder

of the calculation is done in
floating point.

It kind of converts at the floating point
but it doesn't

want to convert it back because it
considers floating

point sort of the more general of the
representations.

So, here we are, talking about integers
and floating points.

These are a concept in programming
languages and in Python called type.

Variables and constants have a type.

We can see that if you say 1, versus 1.0,

they have different, they, it works
different, it functions differently.

And so Python keeps track of both
variables and literals/constants, and

having them have a type.
And we've seen this, right?

Now, the interesting thing is, is Python
is very aware of

the type and can use the same syntax to
accomplish different things.

So if we look at this line here, where we
say dd equals 1 plus 4.

Well it looks at the 1 and looks at the 4
and it says,

oh those are two integers. I will add those
together and give you a 5.

So it gives you an integer, an integer, and an
integer comes back,

Okay?

And then ee equals 'hello ' plus 'there'.
Well these are two strings,

'hello ' and 'there'. And it says hmm, this must
be a concatenation.

Alright? So I'm going to concatenate those
together because

those are strings and I know how to
concatenate strings.

And that's kind of like string addition,
right?

And so we see a "hello there" as a result.
Now the interesting thing is, where

did this space come from?
Let me change colors here.

Oops.

Where did that space come from?
Well, the plus does not add the space.

Here's a space right there, and that's the
space.

So I can concatenate it, hello space plus
there, and that's how I got hello there.

But, the key thing is, is this plus

operator, clear, this plus operator looks
to either side

and says oh,

they're strings.
I think you mean concatenation.

Here it looks either side and says oh,

those are integers, I think you mean
addition.

So Python is very aware of type and type
informs Python what you really mean.

So, it looks like those are kind

of the same, but they're quite different
operations.

So the type can get you into trouble.
Remember Python is looking at the type.

So here we have a little problem, our

first traceback, first of many
tracebacks.

So here we have ee which is hello there
which is

exactly what we did. This is a string and
this is a string.

So ee should be a string. And then we try
to add 1 to it.

And again, Python is saying oh, I see

a plus sign here, so I'm going to look
over here, yeah,

that's a string, and look over here, and
that's an integer.

And it's like, aaah! And this is a traceback.

Now, here's a good time to talk about
tracebacks.

Tracebacks, I color them red.

Because you might think that Python
dislikes you or

thinks that you're, you know, unworthy of
its brilliance.

And certainly the way these things are
worded it sounds like,

you know, the, you're being scolded.
It's like, hey, type error.

You can, cannot concatenate str and int
objects, right?

That's, I'm, I'm scolding you, you bad,
bad programmer.

And it does feel a bit like you're
scolded.

But, if you go back to lecture one, this
is also

the moment where, really, we shouldn't
think of this as like scolding.

We should think of this as Python sort of
asking for help.

It's like,

wow, you gave me this line, and I, Python,
have no idea.

In all your greatness, could you give me
some possible

clue as to what you really
mean for me to do?

Because I'm so lost.

And given that I'm Python and I'm lost and
you are the only

purpose for my existence, I must stop
until you give me better guidance.

So, don't look at tracebacks as scolding.

They sound like scolding.
I'll stop coloring them red after a while.

So, if Python is so obsessed with the type
of things, you

should be able to ask Python what the type
of something is.

So there's a built-in function called
type.

This is part of the Python language.

Type (), and you can put a variable in
here.

What's the type of the variable ee?

And it says, oh yeah, I know what that is,
that would be a string.

And then you can also put a constant in
here.

And say what's the type of quote, hello,
quote, and that's a string too.

And what's the type of the number 1?

Well that would be an integer.

So it's picky about the type, but it will

also share with you what it believes the
type is.

And there's several types of numbers.

As I've already mentioned, there are
integers, which are the whole numbers.

They can be positive and negative and
zero.

And then there are the decimal numbers,

the floating point numbers,
like 98.6 or negative 2.5 or 14.0.

Python knows these as well because it does
division different if it's presented with

two integers, or an integer and a float,
or a float and a float.

And so here we have x is 1, and we'll say
what is it?

It's an integer.

And we say it's 98.6, and we'll say, well,
what's that?

It's a float.

And you can ask for both variables and
constants.

So what's the type of 1? It's an integer.

And what's type of up 1.0?
And it's a float.

You can also convert types.

It has a bunch of type conversion
functions built into it.

So, there's implicit conversion going on

when you're sort of saying, you know,
divide an integer by a floating point.

It says okay I see, I look to
the sides and

I will make the, I will make the
conversion for you.

But you can also be explicit.

So in this case we're going to say,
take this

99 and convert to a floating point
version of itself.

Which is 99.0.
And then do the division.

So Python looks out here and goes oh,
after that, that's

a float, and that's an integer if I look
over here.

And then that means that the
result is a float.

And the division is done as a float.

So we are force converting the 99 integer
into a 99.0 float.

And we can even do this like and just
stick it in the variable.

So we can just put 42 in i and that is an
integer.

Then we can say, hey, convert float that i

into a float and stick it into the
variable f.

And so we can print it.
And now it's 42.0 instead of 42.

Right? They're not the same.

They're both kind of 42, but one is a

floating point 42 and the other is an
integer 42.

And we can ask, and that is a float.

And you can also do the same thing in the
middle of

a calculation, where you have 1 plus 2
times a float of 3.

This float is done quickly.
So the first thing that happens

this is 1 plus 2 times 3.0 over 4
minus 5.

So

the first thing that happens is these
floats

are done because they are parentheses so
they matter.

So this is a built-in function called
float that takes, as its

argument, a non-floating point number and
gives you back a floating point number.

We'll talk more about functions
in Chapter Four.

You can also convert between strings and
numbers, and if you

recall, I, we did the example where we
took a string.

In this case, I'm being a little
confusing, because

I'm making a string with the
characters 1, 2, 3.

Now, this is not the same as 123.

This is a three-character string
with 1, 2, 3 in it.

And I can ask what kind of thing is in
there, and it says,

oh, there's a string in there.
I know about that.

And then I can try to add 1 to it, and

it seems intuitive that quote 123 plus 1
would be somehow 124.

But it's not.

Python takes a look at the plus and says,
oh there's

a string on that side, and an integer on
that side.

I am going to freak out and tell you

that you cannot concatenate
a string and an integer.

Okay?
But there is an int function

that converts various things, including
strings, to an integer.

So we can give as its parameter, its input,
the string value, then it

converts it to an integer, and then we'll
put the result in the variable ival.

We can ask what the type of that is, it's
an i, it's a integer.

And now we can use it in an expression,
print ival plus 1, and

so now Python looks to both sides, sees an
integer, sees an integer, and

gets 124.
Voila.

Now, if I make a new variable and I stick
hello Bob in it, and I

say hey, let's convert hello Bob to an
integer, as you might expect, it blows up.

And it says, invalid literal
for int.

These, these tracebacks again, once you
kind of get used to the kind of harsh

wording of them, because they're not
saying, sorry, comma,

they're trying to tell you what's
going on.

So, cannot concatenate string and integer,
and invalid literal for int.

It's trying to be as helpful as it
possibly can

be to give you a clue as to what to fix.

So, again, not scolded.

Okay, so that's variables and types and
type conversion.

Now we'll talk a little bit about
user input.

And there's a function that's built into
Python called raw_input.

And what happens when raw_input runs is
it, it has as one of

its parameters, a prompt, which is
something that shows up on the screen.

Who are you?

And then,

it waits, tik, tik, tik, tik, tik.
Sits and waits, says, what next?

And then, you type a string,
dot, dot, dot, dot, dot,

and then you hit the Enter key.

The Enter key.

And then whatever you typed here goes
into a variable.

And it is a string.
And, then you,

then you can use it.

So I'm going to print the string Welcome,

comma. So that means I'm printing two
things now.

The comma adds a space between Welcome and
then nam, and so

Welcome is a literal, and then Chuck is
coming from this nam variable.

So this is a two-line program.

I think this is one of your
assignments, actually,

to well, it's one of the exercises
in the book.

To a prompt for a user's name, and

then welcome them, okay?

So raw_input is a function that issues a
prompt, waits, and then takes whatever

string that's entered, and then returns it
and then puts it into that variable.

So, now we're going to create kind of the
first useful program.

It's not a powerful program.

It is a, an interesting problem of the
fact that for some reason there

is a difference in the numbering scheme

of United States elevators and
European elevators.

European elevators, the floor that you
walk out on is the

zero floor.

The floor above that is the
one floor and the

floor below that, the basement,
is the minus one floor.

And so you walk in and you can go either
up the elevator or down the elevator.

Of course, in the United States, the floor
that you walk in is the

one and then there's the two floor above
that and then there's like, the basement.

So this is the
imagination that the Americans

have as to how to number floors, right?

The Europeans go zero, one, minus one.

So, children who go to hotels learn
instantly the notion of zero and the

notion of positive and negative
numbers and

the symmetry between positive and
negative numbers.

I mean, I just wish the United States
hotels would switch to this

to teach young people zero immediately and

negative numbers.

So we somehow think that numbers all in
the United States start at 1

and then there are no
negative numbers, there's the

basement.

I wonder why that is, but whatever.

For people who travel a lot, they may be
confused by this.

They need a way to convert back and

forth between the US and European
numbering system.

So this is a simple program that
demonstrates

a real classic pattern of input processing
and output.

It's just three lines, but it has the

essential things that all programs that
are useful.

They generally read some data,
do some work with

the data, and then produce some
kind of results.

And so, so the first line is a raw_input

that effectively, that puts out a prompt
and then it waits.

It says, please enter your Europe floor.
It sits there.

We type a zero,

then zero goes into inp, but it is a
string.

It's not a number.

It's a string.

So we can't add to it. But we can take

and convert it to an integer with
the int function.

Int of inp, thats a string being
converted to an integer

so now its a real numeric zero.

And we can add 1 to that and we
sum that together.

And we put it into the

variable usf and then we print US floor,
comma, and then

whatever the variable for usf is. And out
comes US floor 1.

So we've written a very simple elevator
floor conversion

from a European floor to a
United States floor.

Don't ask about negative numbers, it's not
really good at that.

But from zero and positive numbers it
works great.

So another thing to think about in any
programming language is comments.

Comments are like commentary, you know,
and basically it's a way for us to

add notations for ourselves and for other
humans interspersed in the code.

And so in Python anything after a pound
sign is ignored.

You can have a pound sign at the beginning

of a line and then the
whole line is ignored.

There are two or three reasons why you
could do this.

One is sort of like paragraph headings,
where you can

say what's going to happen in English or,
or your language.

And you can write documentation says this

code was written by Charles Severence,
December 2010.

And you can also just hide
a line of code to

test and, and turn it on and off without
actually deleting

the line of code.
It's a real common thing in debugging.

So for example, here is a, here is a, the
program that we've been playing with.

This is our word counting program that

we've been talking about
from the beginning.

And here is an example of four comments,
one, two, three, four.

Four comments that basically tell us what
these paragraphs are going to do.

Now, they don't have any effect on the
program whatsoever.

But this one says get the name of the file
and open it.

Kind of helpful, right?

Count the word frequency.

That's what this little bit does.
Find the most common word.

That's what this little bit does.

And all done, print this out.

So it's really can be very helpful just to
add a little tiny bit of stuff.

And you don't want to overuse comments.

You don't want to say like x equals 12,
take 12 and put it into x.

Sometimes people teach

you and try to say, oh you need a one
comment every three lines.

I don't believe in any of those rules.

I basically say if its useful to describe
it, then describe it.

So that's comments.
So some operators apply to strings.

We've already talked about plus.

It's kind of silly, although
useful in places.

You can actually multiply strings.
The asterisk looks and

says I've got a string and an integer, and
it prints out the string five times.

Not a lot of use for that.

Now, let's talk a little bit about
choosing variable names.

This is something that is
really confusing.

So I said like, x equals 1, x equals x
plus 1, what does x mean?

And the answer is, it doesn't
mean anything.

I chose it.
I wanted to make a variable,

and so I picked x.

We pick x a lot, probably because
we learned

in algebra in sixth grade that
x was a variable.

So, and it's short, and so,
why not call it x?

But as your programs get larger this gets
kind of frustrating

to have all your variables like x
and y and z.

And so the notion of mnemonic, it means
memory aid.

We choose our variable names wisely, so
they remind us of what the variable's

going to do internally.
And so, as I go through this lecture

in the beginning if I choose a variable
that's too clever

you're going to think that it's
part of the language.

And so I sort of switch back and forth
between well-chosen variable names

and stupid variable names to kind of
reemphasize the notion that I can choose.

Mnemonic is a good practice, okay?

So here we go.
Let's take a look at a bit of code.

So the question is, what is
this code doing?

What will it even print out?

Is it syntactically correct?

Now you could probably cut and paste this
into your brow, into Python and figure

out that it is syntactically correct.
There are three variables.

This one here and this one here match.

This one here and that one there match.
And these two match.

So it's taking these two numbers and

multiplying together, and then printing
out the product

of the two numbers, if you're real careful
and like look at every, every character.

Now, this would be called
non-mnemonic variables.

They're really messy.

Now Python, it's happy, because all it
wants is to say, oh.

Here is then name that

I, the programmer, decided I
wanted to call this

piece of memory and I'll refer to
it down here, okay?

And so Python is happy.

Now if you hand this to another human
being they are going to be really unhappy.

Because they are going to be like, what
are you doing?

So one better way to write it would be to
make the variables very simple.

And then cognitively we humans can figure
out which is which,

because again it's still only
about matching.

The a has to match the a, the b matches
the b, and the c's match.

It's actually the exact same program.

A equals 35.
B equals 12.5.

C equals A times B.

And print C.
It is these.

Python sees these as the same program.

It doesn't care what we name them.
Now, a human will

be much appreciative if you say, here you
can either have this one or this one.

This one will make them a lot happier.

Okay?

So that is certainly cognitively easier,
but it's not really

giving you any sense of what's going on
here, right?

So an even better way to write this exact
same program to do the exact same thing

would be to choose variables
named hours, rate, and pay,

if indeed that is what you're doing.

Now you can look at this and you go,
oh well, shoot,

35 is the number of hours, and 12.5 is the
rate, and the pay is

the number of hours times the rate, and
then we are going to print out the pay.

And that makes a lot of sense.

So this is really a awesome and wonderful
characterization.

And if that's what you're doing
and those are hours,

rate, and pay, it's a great thing
to name the variables.

But, this is where beginning students
get confused.

And so sometimes I'll write it this way
and sometimes I'll write it this way.

Because you'll look at this, until you get
a little

more sophisticated, a little more skilled,
and you'll say like

does Python know something about payroll?
Is hours a reserved word?

Is rate a reserved word and pay
a reserved word?

Are these things that Python knows about?
And the answer is, no.

Python sees these three programs as
exactly the same name.

It's just this person really made a very
bad choice of variable name.

This person made a less bad choice of
variable name,

and this person made a really awesome
choice of variable name.

So the only difference between those two
things is style.

They are the exact same program.

And Python is equivalently happy with
these, but humans

are most happy when the variables are
easy to remember

and they are somewhat descriptive of what
their expected contents will be.

That's mnemonic.

To help you remember what you were meaning
to do when you write the program.

This is still a bit cryptic, having these

really short, one-character variable
names is still

a bit cryptic.
So,

You have a couple of assignments at the
end of the chapter.

One of the assignments is to write a
program to prompt

the user for hours and rate per hour and
compute pay.

So, I won't do this here, but just a
couple of sort of odd things.

You're going to be using raw_input.

But remember that hands a string in so
you're going

to have to use float.

The function to convert it to a floating

point number so you can actually do a
calculation.

And then you're going to have to use
multiplication and print.

So then multiplication, and then print.

So some combination of raw input, float,
multiplication, and print,

constructed to make your program do
exactly this.

So, this is the end of Chapter Two.

We talked about types,

reserved words, variables, the
mnemonic, how you choose variable names.

We'll hit this a couple more times

because choosing variable names is always
problematic.

Operators, operator precedence, which just
means like does multiplication happen

before plus, parentheses.
Integer division is a little weird because

it truncates, oops, right, 9 over 10.

Oops, 9 over 10 equals 0.
That's the integer division truncating.

Conversion, this is like the int()
float().

And then user input, which is raw_input.

And then comments, which are ways for you

to add human-readable text to your
program.

Okay? See you next lecture.