1
00:00:00,800 --> 00:00:05,540
So, here we are, talking about integers
and floating points.

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00:00:05,540 --> 00:00:09,520
These are a concept in programming
languages and in Python called type.

3
00:00:10,820 --> 00:00:13,270
Variables and constants have a type.

4
00:00:14,600 --> 00:00:17,540
We can see that if you say 1, versus 1.0,

5
00:00:17,540 --> 00:00:21,090
they have different, they, it works
different, it functions differently.

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00:00:21,090 --> 00:00:27,300
And so Python keeps track of both
variables and literals/constants, and

7
00:00:27,300 --> 00:00:31,240
having them have a type.
And we've seen this, right?

8
00:00:31,240 --> 00:00:33,920
Now, the interesting thing is, is Python
is very aware of

9
00:00:33,920 --> 00:00:38,920
the type and can use the same syntax to
accomplish different things.

10
00:00:38,920 --> 00:00:43,200
So if we look at this line here, where we
say dd equals 1.4.

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00:00:43,200 --> 00:00:44,790
Well it looks at the 1 and looks at the 4
and it says,

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00:00:44,790 --> 00:00:47,770
oh those are two integers. I will add those
together and give you a 5.

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00:00:47,770 --> 00:00:52,210
So it gives you an integer, an integer, and an
integer comes back,

14
00:00:52,210 --> 00:00:52,310
Okay?

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00:00:52,310 --> 00:00:56,820
And then ee equals 'hello ' plus 'there'.
Well these are two strings,

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00:00:56,820 --> 00:01:01,798
'hello ' and 'there'. And it says hmm, this must
be a concatenation.

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00:01:01,798 --> 00:01:06,040
Alright? So I'm going to concatenate those
together because

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00:01:06,040 --> 00:01:09,070
those are strings and I know how to
concatenate strings.

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00:01:09,070 --> 00:01:11,660
And that's kind of like string addition,
right?

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00:01:13,060 --> 00:01:17,460
And so we see a "hello there" as a result.
Now the interesting thing is, where

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00:01:17,460 --> 00:01:20,760
did this space come from?
Let me change colors here.

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00:01:20,760 --> 00:01:21,690
Oops.

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00:01:21,690 --> 00:01:26,050
Where did that space come from?
Well, the plus does not add the space.

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00:01:26,050 --> 00:01:28,750
Here's a space right there, and that's the
space.

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00:01:28,750 --> 00:01:33,930
So I can concatenate it, hello space plus
there, and that's how I got hello there.

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00:01:33,930 --> 00:01:35,690
But, the key thing is, is this plus

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00:01:35,690 --> 00:01:41,930
operator, clear, this plus operator looks
to either side

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00:01:41,930 --> 00:01:42,700
and says oh,

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00:01:42,700 --> 00:01:45,790
they're strings.
I think you mean concatenation.

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00:01:45,790 --> 00:01:48,290
Here it looks either side and says oh,

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00:01:48,290 --> 00:01:50,970
those are integers, I think you mean
addition.

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00:01:50,970 --> 00:01:56,610
So Python is very aware of type and type
informs Python what you really mean.

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00:01:56,610 --> 00:01:57,952
So, it looks like those are kind

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00:01:57,952 --> 00:02:00,270
of the same, but they're quite different
operations.

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00:02:02,990 --> 00:02:08,090
So the type can get you into trouble.
Remember Python is looking at the type.

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00:02:08,090 --> 00:02:09,840
So here we have a little problem, our

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00:02:09,840 --> 00:02:13,880
first traceback, first of many
tracebacks.

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00:02:13,880 --> 00:02:18,500
So here we have ee which is hello there
which is

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00:02:18,500 --> 00:02:21,330
exactly what we did. This is a string and
this is a string.

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00:02:21,330 --> 00:02:26,150
So ee should be a string. And then we try
to add 1 to it.

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00:02:26,150 --> 00:02:28,170
And again, Python is saying oh, I see

42
00:02:28,170 --> 00:02:31,130
a plus sign here, so I'm going to look
over here, yeah,

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00:02:31,130 --> 00:02:33,240
that's a string, and look over here, and
that's an integer.

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00:02:33,240 --> 00:02:36,780
And it's like, aaah! And this is a traceback.

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00:02:36,780 --> 00:02:40,070
Now, here's a good time to talk about
tracebacks.

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00:02:40,070 --> 00:02:42,370
Tracebacks, I color them red.

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00:02:42,370 --> 00:02:46,480
Because you might think that Python
dislikes you or

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00:02:46,480 --> 00:02:49,780
thinks that you're, you know, unworthy of
its brilliance.

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00:02:50,860 --> 00:02:53,280
And certainly the way these things are
worded it sounds like,

50
00:02:53,280 --> 00:02:57,190
you know, the, you're being scolded.
It's like, hey, type error.

51
00:02:57,190 --> 00:03:00,800
You can, cannot concatenate str and int
objects, right?

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00:03:00,800 --> 00:03:04,800
That's, I'm, I'm scolding you, you bad,
bad programmer.

53
00:03:04,800 --> 00:03:07,040
And it does feel a bit like you're
scolded.

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00:03:07,040 --> 00:03:10,740
But, if you go back to lecture one, this
is also

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00:03:10,740 --> 00:03:15,010
the moment where, really, we shouldn't
think of this as like scolding.

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00:03:15,010 --> 00:03:17,550
We should think of this as Python sort of
asking for help.

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00:03:17,550 --> 00:03:18,700
It's like,

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00:03:18,700 --> 00:03:24,510
wow, you gave me this line, and I, Python,
have no idea.

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00:03:24,510 --> 00:03:27,970
In all your greatness, could you give me
some possible

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00:03:27,970 --> 00:03:29,810
clue as to what you really
mean for me to do?

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00:03:29,810 --> 00:03:31,260
Because I'm so lost.

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00:03:31,260 --> 00:03:34,730
And given that I'm Python and I'm lost and
you are the only

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00:03:34,730 --> 00:03:40,420
purpose for my existence, I must stop
until you give me better guidance.

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00:03:40,420 --> 00:03:43,554
So, don't look at tracebacks as scolding.

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00:03:43,554 --> 00:03:49,305
They sound like scolding.
I'll stop coloring them red after a while.

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00:03:49,305 --> 00:03:53,360
So, if Python is so obsessed with the type
of things, you

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00:03:53,360 --> 00:03:56,030
should be able to ask Python what the type
of something is.

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00:03:56,030 --> 00:03:58,720
So there's a built-in function called
type.

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00:03:58,720 --> 00:04:00,410
This is part of the Python language.

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00:04:00,410 --> 00:04:03,750
Type (), and you can put a variable in
here.

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00:04:03,750 --> 00:04:05,450
What's the type of the variable ee?

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00:04:05,450 --> 00:04:09,180
And it says, oh yeah, I know what that is,
that would be a string.

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00:04:09,180 --> 00:04:11,060
And then you can also put a constant in
here.

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00:04:11,060 --> 00:04:14,640
And say what's the type of quote, hello,
quote, and that's a string too.

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00:04:14,640 --> 00:04:16,210
And what's the type of the number 1?

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00:04:16,210 --> 00:04:18,010
Well that would be an integer.

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00:04:18,010 --> 00:04:19,660
So it's picky about the type, but it will

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00:04:19,660 --> 00:04:22,630
also share with you what it believes the
type is.

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00:04:23,990 --> 00:04:27,120
And there's several types of numbers.

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00:04:27,120 --> 00:04:30,540
As I've already mentioned, there are
integers, which are the whole numbers.

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00:04:30,540 --> 00:04:32,960
They can be positive and negative and
zero.

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00:04:32,960 --> 00:04:34,310
And then there are the decimal numbers,

83
00:04:34,310 --> 00:04:41,060
the floating point numbers,
like 98.6 or negative 2.5 or 14.0.

84
00:04:41,060 --> 00:04:45,308
Python knows these as well because it does
division different if it's presented with

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00:04:45,308 --> 00:04:48,610
two integers, or an integer and a float,
or a float and a float.

86
00:04:53,100 --> 00:04:56,620
And so here we have x is 1, and we'll say
what is it?

87
00:04:56,620 --> 00:04:57,890
It's an integer.

88
00:04:57,890 --> 00:05:00,600
And we say it's 98.6, and we'll say, well,
what's that?

89
00:05:00,600 --> 00:05:01,810
It's a float.

90
00:05:01,810 --> 00:05:04,130
And you can ask for both variables and
constants.

91
00:05:04,130 --> 00:05:06,400
So what's the type of 1? It's an integer.

92
00:05:06,400 --> 00:05:08,700
And what's type of up 1.0?
And it's a float.

93
00:05:10,550 --> 00:05:11,850
You can also convert types.

94
00:05:11,850 --> 00:05:15,370
It has a bunch of type conversion
functions built into it.

95
00:05:15,370 --> 00:05:18,170
So, there's implicit conversion going on

96
00:05:18,170 --> 00:05:22,110
when you're sort of saying, you know,
divide an integer by a floating point.

97
00:05:22,110 --> 00:05:24,690
It says okay I see, I look to
the sides and

98
00:05:24,690 --> 00:05:27,890
I will make the, I will make the
conversion for you.

99
00:05:27,890 --> 00:05:29,400
But you can also be explicit.

100
00:05:29,400 --> 00:05:31,760
So in this case we're going to say,
take this

101
00:05:31,760 --> 00:05:34,848
99 and convert to a floating point
version of itself.

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00:05:34,848 --> 00:05:38,060
Which is 99.0.
And then do the division.

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00:05:38,060 --> 00:05:40,920
So Python looks out here and goes oh,
after that, that's

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00:05:40,920 --> 00:05:43,920
a float, and that's an integer if I look
over here.

105
00:05:43,920 --> 00:05:46,540
And then that means that the
result is a float.

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00:05:46,540 --> 00:05:48,360
And the division is done as a float.

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00:05:48,360 --> 00:05:54,330
So we are force converting the 99 integer
into a 99.0 float.

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00:05:56,180 --> 00:05:58,460
And we can even do this like and just
stick it in the variable.

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00:05:58,460 --> 00:06:01,790
So we can just put 42 in i and that is an
integer.

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00:06:01,790 --> 00:06:05,530
Then we can say, hey, convert float that i

111
00:06:05,530 --> 00:06:08,750
into a float and stick it into the
variable f.

112
00:06:08,750 --> 00:06:12,600
And so we can print it.
And now it's 42.0 instead of 42.

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00:06:12,600 --> 00:06:15,426
Right? They're not the same.

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00:06:15,426 --> 00:06:17,180
They're both kind of 42, but one is a

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00:06:17,180 --> 00:06:19,900
floating point 42 and the other is an
integer 42.

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00:06:19,900 --> 00:06:23,110
And we can ask, and that is a float.

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00:06:23,110 --> 00:06:25,320
And you can also do the same thing in the
middle of

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00:06:25,320 --> 00:06:29,510
a calculation, where you have 1 plus 2
times a float of 3.

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00:06:29,510 --> 00:06:33,812
This float is done quickly.
So the first thing that happens

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00:06:33,812 --> 00:06:38,130
this is 1 plus 2 times 3.0 over 4
minus 5.

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00:06:38,130 --> 00:06:38,630
So

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00:06:40,800 --> 00:06:42,540
the first thing that happens is these
floats

123
00:06:42,540 --> 00:06:44,740
are done because they are parentheses so
they matter.

124
00:06:44,740 --> 00:06:48,640
So this is a built-in function called
float that takes, as its

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00:06:48,640 --> 00:06:54,390
argument, a non-floating point number and
gives you back a floating point number.

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00:06:54,390 --> 00:06:56,390
We'll talk more about functions
in Chapter Four.

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00:07:00,230 --> 00:07:04,640
You can also convert between strings and
numbers, and if you

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00:07:04,640 --> 00:07:09,220
recall, I, we did the example where we
took a string.

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00:07:09,220 --> 00:07:11,880
In this case, I'm being a little
confusing, because

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00:07:11,880 --> 00:07:14,680
I'm making a string with the
characters 1, 2, 3.

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00:07:14,680 --> 00:07:18,110
Now, this is not the same as 123.

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00:07:18,110 --> 00:07:22,530
This is a three-character string
with 1, 2, 3 in it.

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00:07:22,530 --> 00:07:25,460
And I can ask what kind of thing is in
there, and it says,

134
00:07:25,460 --> 00:07:27,810
oh, there's a string in there.
I know about that.

135
00:07:27,810 --> 00:07:29,490
And then I can try to add 1 to it, and

136
00:07:29,490 --> 00:07:34,690
it seems intuitive that quote 123 plus 1
would be somehow 124.

137
00:07:34,690 --> 00:07:36,966
But it's not.

138
00:07:36,966 --> 00:07:39,474
Python takes a look at the plus and says,
oh there's

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00:07:39,474 --> 00:07:42,440
a string on that side, and an integer on
that side.

140
00:07:42,440 --> 00:07:44,720
I am going to freak out and tell you

141
00:07:44,720 --> 00:07:47,940
that you cannot concatenate
a string and an integer.

142
00:07:47,940 --> 00:07:50,710
Okay?
But there is an int function

143
00:07:50,710 --> 00:07:54,360
that converts various things, including
strings, to an integer.

144
00:07:54,360 --> 00:07:59,520
So we can give as its parameter, its input,
the string value, then it

145
00:07:59,520 --> 00:08:04,450
converts it to an integer, and then we'll
put the result in the variable ival.

146
00:08:04,450 --> 00:08:08,840
We can ask what the type of that is, it's
an i, it's a integer.

147
00:08:08,840 --> 00:08:12,360
And now we can use it in an expression,
print ival plus 1, and

148
00:08:12,360 --> 00:08:16,230
so now Python looks to both sides, sees an
integer, sees an integer, and

149
00:08:16,230 --> 00:08:18,530
gets 124.
Voila.

150
00:08:20,090 --> 00:08:23,880
Now, if I make a new variable and I stick
hello Bob in it, and I

151
00:08:23,880 --> 00:08:30,340
say hey, let's convert hello Bob to an
integer, as you might expect, it blows up.

152
00:08:30,340 --> 00:08:32,850
And it says, invalid literal
for int.

153
00:08:34,980 --> 00:08:41,310
These, these tracebacks again, once you
kind of get used to the kind of harsh

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00:08:41,310 --> 00:08:44,242
wording of them, because they're not
saying, sorry, comma,

155
00:08:44,242 --> 00:08:47,010
they're trying to tell you what's
going on.

156
00:08:47,010 --> 00:08:51,930
So, cannot concatenate string and integer,
and invalid literal for int.

157
00:08:51,930 --> 00:08:54,140
It's trying to be as helpful as it
possibly can

158
00:08:54,140 --> 00:08:56,710
be to give you a clue as to what to fix.

159
00:08:56,710 --> 00:08:59,270
So, again, not scolded.

160
00:09:01,030 --> 00:09:04,610
Okay, so that's variables and types and
type conversion.

161
00:09:04,610 --> 00:09:08,563
Now we'll talk a little bit about
user input.

162
00:09:08,563 --> 00:09:14,360
And there's a function that's built into
Python called raw_input.

163
00:09:14,360 --> 00:09:20,080
And what happens when raw_input runs is
it, it has as one of

164
00:09:20,080 --> 00:09:24,310
its parameters, a prompt, which is
something that shows up on the screen.

165
00:09:24,310 --> 00:09:25,020
Who are you?

166
00:09:25,020 --> 00:09:26,820
And then,

167
00:09:26,820 --> 00:09:33,520
it waits, tik, tik, tik, tik, tik.
Sits and waits, says, what next?

168
00:09:33,520 --> 00:09:35,920
And then, you type a string,
dot, dot, dot, dot, dot,

169
00:09:35,920 --> 00:09:37,740
and then you hit the Enter key.

170
00:09:39,540 --> 00:09:40,640
The Enter key.

171
00:09:40,640 --> 00:09:46,310
And then whatever you typed here goes
into a variable.

172
00:09:47,510 --> 00:09:51,650
And it is a string.
And, then you,

173
00:09:51,650 --> 00:09:52,656
then you can use it.

174
00:09:52,656 --> 00:09:54,670
So I'm going to print the string Welcome,

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00:09:54,670 --> 00:09:57,710
comma. So that means I'm printing two
things now.

176
00:09:57,710 --> 00:10:00,740
The comma adds a space between Welcome and
then nam, and so

177
00:10:00,740 --> 00:10:06,160
Welcome is a literal, and then Chuck is
coming from this nam variable.

178
00:10:06,160 --> 00:10:08,380
So this is a two-line program.

179
00:10:08,380 --> 00:10:11,430
I think this is one of your
assignments, actually,

180
00:10:11,430 --> 00:10:14,930
to well, it's one of the exercises
in the book.

181
00:10:14,930 --> 00:10:16,840
To a prompt for a user's name, and

182
00:10:16,840 --> 00:10:18,550
then welcome them, okay?

183
00:10:20,382 --> 00:10:25,380
So raw_input is a function that issues a
prompt, waits, and then takes whatever

184
00:10:25,380 --> 00:10:28,670
string that's entered, and then returns it
and then puts it into that variable.

185
00:10:32,390 --> 00:10:37,190
So, now we're going to create kind of the
first useful program.

186
00:10:37,190 --> 00:10:40,160
It's not a powerful program.

187
00:10:40,160 --> 00:10:47,630
It is a, an interesting problem of the
fact that for some reason there

188
00:10:47,630 --> 00:10:49,190
is a difference in the numbering scheme

189
00:10:49,190 --> 00:10:52,140
of United States elevators and
European elevators.

190
00:10:53,180 --> 00:10:57,440
European elevators, the floor that you
walk out on is the

191
00:10:57,440 --> 00:10:58,750
zero floor.

192
00:10:58,750 --> 00:11:00,700
The floor above that is the
one floor and the

193
00:11:00,700 --> 00:11:05,300
floor below that, the basement,
is the minus one floor.

194
00:11:05,300 --> 00:11:10,198
And so you walk in and you can go either
up the elevator or down the elevator.

195
00:11:10,198 --> 00:11:14,240
Of course, in the United States, the floor
that you walk in is the

196
00:11:14,240 --> 00:11:19,120
one and then there's the two floor above
that and then there's like, the basement.

197
00:11:19,120 --> 00:11:22,460
So this is the
imagination that the Americans

198
00:11:22,460 --> 00:11:25,230
have as to how to number floors, right?

199
00:11:25,230 --> 00:11:28,290
The Europeans go zero, one, minus one.

200
00:11:28,290 --> 00:11:34,100
So, children who go to hotels learn
instantly the notion of zero and the

201
00:11:34,100 --> 00:11:35,790
notion of positive and negative
numbers and

202
00:11:35,790 --> 00:11:38,280
the symmetry between positive and
negative numbers.

203
00:11:38,280 --> 00:11:43,520
I mean, I just wish the United States
hotels would switch to this

204
00:11:43,520 --> 00:11:48,000
to teach young people zero immediately and

205
00:11:48,000 --> 00:11:49,190
negative numbers.

206
00:11:49,190 --> 00:11:52,730
So we somehow think that numbers all in
the United States start at 1

207
00:11:52,730 --> 00:11:56,030
and then there are no
negative numbers, there's the

208
00:11:56,030 --> 00:11:56,530
basement.

209
00:11:59,100 --> 00:12:00,990
I wonder why that is, but whatever.

210
00:12:02,630 --> 00:12:06,150
For people who travel a lot, they may be
confused by this.

211
00:12:07,190 --> 00:12:08,780
They need a way to convert back and

212
00:12:08,780 --> 00:12:13,009
forth between the US and European
numbering system.

213
00:12:14,500 --> 00:12:17,170
So this is a simple program that
demonstrates

214
00:12:17,170 --> 00:12:21,160
a real classic pattern of input processing
and output.

215
00:12:21,160 --> 00:12:24,380
It's just three lines, but it has the

216
00:12:24,380 --> 00:12:27,940
essential things that all programs that
are useful.

217
00:12:27,940 --> 00:12:31,600
They generally read some data,
do some work with

218
00:12:31,600 --> 00:12:35,070
the data, and then produce some
kind of results.

219
00:12:35,070 --> 00:12:39,740
And so, so the first line is a raw_input

220
00:12:39,740 --> 00:12:44,240
that effectively, that puts out a prompt
and then it waits.

221
00:12:44,240 --> 00:12:48,190
It says, please enter your Europe floor.
It sits there.

222
00:12:48,190 --> 00:12:49,920
We type a zero,

223
00:12:49,920 --> 00:12:53,100
then zero goes into inp, but it is a
string.

224
00:12:54,250 --> 00:12:55,303
It's not a number.

225
00:12:55,303 --> 00:12:56,700
It's a string.

226
00:12:56,700 --> 00:12:59,870
So we can't add to it. But we can take

227
00:13:01,100 --> 00:13:03,800
and convert it to an integer with
the int function.

228
00:13:03,800 --> 00:13:06,710
Int of inp, thats a string being
converted to an integer

229
00:13:06,710 --> 00:13:09,370
so now its a real numeric zero.

230
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And we can add 1 to that and we
sum that together.

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And we put it into the

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

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

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00:13:24,350 --> 00:13:28,280
So we've written a very simple elevator
floor conversion

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from a European floor to a
United States floor.

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00:13:32,310 --> 00:13:34,810
Don't ask about negative numbers, it's not
really good at that.

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But from zero and positive numbers it
works great.

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00:13:41,710 --> 00:13:47,130
So another thing to think about in any
programming language is comments.

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

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

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00:14:00,480 --> 00:14:05,680
And so in Python anything after a pound
sign is ignored.

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You can have a pound sign at the beginning

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of a line and then the
whole line is ignored.

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00:14:08,545 --> 00:14:11,670
There are two or three reasons why you
could do this.

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One is sort of like paragraph headings,
where you can

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00:14:14,700 --> 00:14:20,190
say what's going to happen in English or,
or your language.

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00:14:20,190 --> 00:14:22,010
And you can write documentation says this

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00:14:22,010 --> 00:14:26,510
code was written by Charles Severence,
December 2010.

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00:14:26,510 --> 00:14:28,800
And you can also just hide
a line of code to

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00:14:28,800 --> 00:14:31,920
test and, and turn it on and off without
actually deleting

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00:14:31,920 --> 00:14:35,900
the line of code.
It's a real common thing in debugging.

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00:14:35,900 --> 00:14:42,070
So for example, here is a, here is a, the
program that we've been playing with.

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00:14:42,070 --> 00:14:44,160
This is our word counting program that

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00:14:44,160 --> 00:14:45,960
we've been talking about
from the beginning.

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00:14:45,960 --> 00:14:50,850
And here is an example of four comments,
one, two, three, four.

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00:14:50,850 --> 00:14:55,330
Four comments that basically tell us what
these paragraphs are going to do.

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00:14:55,330 --> 00:14:58,250
Now, they don't have any effect on the
program whatsoever.

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00:14:58,250 --> 00:15:00,150
But this one says get the name of the file
and open it.

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00:15:01,290 --> 00:15:02,860
Kind of helpful, right?

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00:15:02,860 --> 00:15:03,910
Count the word frequency.

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00:15:03,910 --> 00:15:06,870
That's what this little bit does.
Find the most common word.

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00:15:06,870 --> 00:15:08,490
That's what this little bit does.

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00:15:08,490 --> 00:15:10,500
And all done, print this out.

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00:15:10,500 --> 00:15:15,450
So it's really can be very helpful just to
add a little tiny bit of stuff.

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00:15:15,450 --> 00:15:17,410
And you don't want to overuse comments.

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00:15:17,410 --> 00:15:22,470
You don't want to say like x equals 12,
take 12 and put it into x.

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00:15:22,470 --> 00:15:23,530
Sometimes people teach

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00:15:23,530 --> 00:15:27,360
you and try to say, oh you need a one
comment every three lines.

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00:15:27,360 --> 00:15:28,760
I don't believe in any of those rules.

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00:15:28,760 --> 00:15:32,230
I basically say if its useful to describe
it, then describe it.

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00:15:33,920 --> 00:15:40,100
So that's comments.
So some operators apply to strings.

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00:15:40,100 --> 00:15:42,410
We've already talked about plus.

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00:15:42,410 --> 00:15:44,600
It's kind of silly, although
useful in places.

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00:15:44,600 --> 00:15:48,660
You can actually multiply strings.
The asterisk looks and

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00:15:48,660 --> 00:15:53,170
says I've got a string and an integer, and
it prints out the string five times.

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00:15:54,300 --> 00:15:55,120
Not a lot of use for that.

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00:15:56,620 --> 00:16:00,420
Now, let's talk a little bit about
choosing variable names.

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This is something that is
really confusing.

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00:16:03,280 --> 00:16:07,170
So I said like, x equals 1, x equals x
plus 1, what does x mean?

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00:16:07,170 --> 00:16:11,190
And the answer is, it doesn't
mean anything.

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00:16:11,190 --> 00:16:13,970
I chose it.
I wanted to make a variable,

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00:16:13,970 --> 00:16:15,770
and so I picked x.

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00:16:15,770 --> 00:16:18,100
We pick x a lot, probably because
we learned

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00:16:18,100 --> 00:16:22,210
in algebra in sixth grade that
x was a variable.

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00:16:22,210 --> 00:16:25,270
So, and it's short, and so,
why not call it x?

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00:16:28,400 --> 00:16:31,610
But as your programs get larger this gets
kind of frustrating

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00:16:31,610 --> 00:16:34,510
to have all your variables like x
and y and z.

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00:16:34,510 --> 00:16:38,120
And so the notion of mnemonic, it means
memory aid.

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00:16:38,120 --> 00:16:43,150
We choose our variable names wisely, so
they remind us of what the variable's

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00:16:43,150 --> 00:16:48,360
going to do internally.
And so, as I go through this lecture

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00:16:50,540 --> 00:16:54,310
in the beginning if I choose a variable
that's too clever

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00:16:54,310 --> 00:16:58,050
you're going to think that it's
part of the language.

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00:16:58,050 --> 00:17:01,560
And so I sort of switch back and forth
between well-chosen variable names

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00:17:01,560 --> 00:17:06,055
and stupid variable names to kind of
reemphasize the notion that I can choose.

295
00:17:06,055 --> 00:17:09,140
Mnemonic is a good practice, okay?

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00:17:09,140 --> 00:17:12,860
So here we go.
Let's take a look at a bit of code.

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00:17:16,210 --> 00:17:20,100
So the question is, what is
this code doing?

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00:17:20,100 --> 00:17:22,290
What will it even print out?

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00:17:22,290 --> 00:17:23,890
Is it syntactically correct?

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00:17:26,380 --> 00:17:32,330
Now you could probably cut and paste this
into your brow, into Python and figure

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00:17:32,330 --> 00:17:38,266
out that it is syntactically correct.
There are three variables.

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00:17:40,266 --> 00:17:45,670
This one here and this one here match.

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00:17:46,800 --> 00:17:51,860
This one here and that one there match.
And these two match.

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00:17:52,860 --> 00:17:54,360
So it's taking these two numbers and

305
00:17:54,360 --> 00:17:56,890
multiplying together, and then printing
out the product

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00:17:56,890 --> 00:18:02,790
of the two numbers, if you're real careful
and like look at every, every character.

307
00:18:02,790 --> 00:18:06,180
Now, this would be called
non-mnemonic variables.

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00:18:06,180 --> 00:18:08,200
They're really messy.

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00:18:08,200 --> 00:18:12,290
Now Python, it's happy, because all it
wants is to say, oh.

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00:18:12,290 --> 00:18:12,800
Here is then name that

311
00:18:12,800 --> 00:18:15,550
I, the programmer, decided I
wanted to call this

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00:18:15,550 --> 00:18:19,470
piece of memory and I'll refer to
it down here, okay?

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00:18:19,470 --> 00:18:22,320
And so Python is happy.

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00:18:22,320 --> 00:18:26,280
Now if you hand this to another human
being they are going to be really unhappy.

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00:18:26,280 --> 00:18:28,170
Because they are going to be like, what
are you doing?

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00:18:29,270 --> 00:18:34,660
So one better way to write it would be to
make the variables very simple.

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00:18:34,660 --> 00:18:38,134
And then cognitively we humans can figure
out which is which,

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00:18:38,134 --> 00:18:41,720
because again it's still only
about matching.

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00:18:41,720 --> 00:18:47,470
The a has to match the a, the b matches
the b, and the c's match.

320
00:18:47,470 --> 00:18:49,290
It's actually the exact same program.

321
00:18:49,290 --> 00:18:52,570
A equals 35.
B equals 12.5.

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00:18:52,570 --> 00:18:53,820
C equals A times B.

323
00:18:53,820 --> 00:18:56,740
And print C.
It is these.

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00:18:56,740 --> 00:18:59,980
Python sees these as the same program.

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00:18:59,980 --> 00:19:03,210
It doesn't care what we name them.
Now, a human will

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00:19:03,210 --> 00:19:07,770
be much appreciative if you say, here you
can either have this one or this one.

327
00:19:07,770 --> 00:19:10,070
This one will make them a lot happier.

328
00:19:12,450 --> 00:19:13,480
Okay?

329
00:19:13,480 --> 00:19:17,610
So that is certainly cognitively easier,
but it's not really

330
00:19:17,610 --> 00:19:22,240
giving you any sense of what's going on
here, right?

331
00:19:22,240 --> 00:19:28,260
So an even better way to write this exact
same program to do the exact same thing

332
00:19:28,260 --> 00:19:31,430
would be to choose variables
named hours, rate, and pay,

333
00:19:31,430 --> 00:19:34,810
if indeed that is what you're doing.

334
00:19:34,810 --> 00:19:37,610
Now you can look at this and you go,
oh well, shoot,

335
00:19:37,610 --> 00:19:41,730
35 is the number of hours, and 12.5 is the
rate, and the pay is

336
00:19:41,730 --> 00:19:45,090
the number of hours times the rate, and
then we are going to print out the pay.

337
00:19:45,090 --> 00:19:47,060
And that makes a lot of sense.

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00:19:47,060 --> 00:19:53,290
So this is really a awesome and wonderful
characterization.

339
00:19:53,290 --> 00:19:55,592
And if that's what you're doing
and those are hours,

340
00:19:55,592 --> 00:19:59,090
rate, and pay, it's a great thing
to name the variables.

341
00:19:59,090 --> 00:20:03,370
But, this is where beginning students
get confused.

342
00:20:03,370 --> 00:20:06,415
And so sometimes I'll write it this way
and sometimes I'll write it this way.

343
00:20:06,415 --> 00:20:08,749
Because you'll look at this, until you get
a little

344
00:20:08,749 --> 00:20:11,880
more sophisticated, a little more skilled,
and you'll say like

345
00:20:14,180 --> 00:20:18,430
does Python know something about payroll?
Is hours a reserved word?

346
00:20:18,430 --> 00:20:21,370
Is rate a reserved word and pay
a reserved word?

347
00:20:21,370 --> 00:20:25,350
Are these things that Python knows about?
And the answer is, no.

348
00:20:25,350 --> 00:20:29,270
Python sees these three programs as
exactly the same name.

349
00:20:29,270 --> 00:20:33,440
It's just this person really made a very
bad choice of variable name.

350
00:20:33,440 --> 00:20:36,740
This person made a less bad choice of
variable name,

351
00:20:36,740 --> 00:20:39,830
and this person made a really awesome
choice of variable name.

352
00:20:39,830 --> 00:20:42,010
So the only difference between those two
things is style.

353
00:20:44,100 --> 00:20:46,260
They are the exact same program.

354
00:20:46,260 --> 00:20:50,460
And Python is equivalently happy with
these, but humans

355
00:20:50,460 --> 00:20:54,280
are most happy when the variables are
easy to remember

356
00:20:54,280 --> 00:20:57,840
and they are somewhat descriptive of what
their expected contents will be.

357
00:20:58,840 --> 00:21:00,530
That's mnemonic.

358
00:21:00,530 --> 00:21:04,680
To help you remember what you were meaning
to do when you write the program.

359
00:21:04,680 --> 00:21:07,280
This is still a bit cryptic, having these

360
00:21:07,280 --> 00:21:09,340
really short, one-character variable
names is still

361
00:21:09,340 --> 00:21:11,320
a bit cryptic.
So,

362
00:21:13,460 --> 00:21:16,220
You have a couple of assignments at the
end of the chapter.

363
00:21:16,220 --> 00:21:19,740
One of the assignments is to write a
program to prompt

364
00:21:19,740 --> 00:21:24,220
the user for hours and rate per hour and
compute pay.

365
00:21:25,430 --> 00:21:31,566
So, I won't do this here, but just a
couple of sort of odd things.

366
00:21:31,566 --> 00:21:33,755
You're going to be using raw_input.

367
00:21:33,755 --> 00:21:38,528
But remember that hands a string in so
you're going

368
00:21:38,528 --> 00:21:40,980
to have to use float.

369
00:21:43,030 --> 00:21:44,690
The function to convert it to a floating

370
00:21:44,690 --> 00:21:46,755
point number so you can actually do a
calculation.

371
00:21:46,755 --> 00:21:50,170
And then you're going to have to use
multiplication and print.

372
00:21:50,170 --> 00:21:52,210
So then multiplication, and then print.

373
00:21:54,200 --> 00:21:58,540
So some combination of raw input, float,
multiplication, and print,

374
00:21:59,790 --> 00:22:03,170
constructed to make your program do
exactly this.

375
00:22:04,690 --> 00:22:07,350
So, this is the end of Chapter Two.

376
00:22:07,350 --> 00:22:08,210
We talked about types,

377
00:22:08,210 --> 00:22:13,400
reserved words, variables, the
mnemonic, how you choose variable names.

378
00:22:13,400 --> 00:22:14,835
We'll hit this a couple more times

379
00:22:14,835 --> 00:22:17,400
because choosing variable names is always
problematic.

380
00:22:17,400 --> 00:22:21,400
Operators, operator precedence, which just
means like does multiplication happen

381
00:22:21,400 --> 00:22:25,980
before plus, parentheses.
Integer division is a little weird because

382
00:22:25,980 --> 00:22:33,255
it truncates, oops, right, 9 over 10.

383
00:22:33,255 --> 00:22:40,050
Oops, 9 over 10 equals 0.
That's the integer division truncating.

384
00:22:40,050 --> 00:22:46,930
Conversion, this is like the int()
float().

385
00:22:46,930 --> 00:22:49,030
And then user input, which is raw_input.

386
00:22:49,030 --> 00:22:51,340
And then comments, which are ways for you

387
00:22:51,340 --> 00:22:54,690
to add human-readable text to your
program.

388
00:22:54,690 --> 00:22:56,560
Okay? See you next lecture.