1
00:00:00,610 --> 00:00:05,900
So, we saw in that previous thing one
block within a block.

2
00:00:05,900 --> 00:00:07,580
And, and we're going to do that.

3
00:00:07,580 --> 00:00:10,390
We can have ifs, we can have loops that

4
00:00:10,390 --> 00:00:12,360
get indented, but then we can indent even
more.

5
00:00:12,360 --> 00:00:14,940
We call that nested, where there's an
indented

6
00:00:14,940 --> 00:00:17,210
area that's in an area that's already
indented.

7
00:00:18,740 --> 00:00:20,520
So here's a nested decision.

8
00:00:20,520 --> 00:00:21,870
And it might be easier to start on

9
00:00:21,870 --> 00:00:24,880
this side, where I'm going to have
a first choice.

10
00:00:24,880 --> 00:00:27,290
Is x greater than 1, yes or no, and if

11
00:00:27,290 --> 00:00:29,740
it's yes, I'll do some work, and
then I'm going to

12
00:00:29,740 --> 00:00:32,720
ask another question, and if that's yes,
then I'm going to

13
00:00:32,720 --> 00:00:34,490
do this, then I'm going to come all the
way back in.

14
00:00:34,490 --> 00:00:38,858
And the way we encode this in Python is, x
equals 42,

15
00:00:38,858 --> 00:00:44,463
if x is greater than 1, it's true, so we
continue working in the indent.

16
00:00:44,463 --> 00:00:47,963
And now we say, oh, if x is less than 100,
which is still true,

17
00:00:47,963 --> 00:00:52,100
so we go in farther, and we do this,
and now we come out.

18
00:00:52,100 --> 00:00:54,530
We don't come out to here, we actually
keep going

19
00:00:54,530 --> 00:00:58,020
all the way to here, so that
ends both blocks.

20
00:00:58,020 --> 00:01:00,680
And so if you sort of think about this,

21
00:01:00,680 --> 00:01:03,310
again this is where I want you
to start seeing

22
00:01:03,310 --> 00:01:06,310
what's in a block of code and
what's not in

23
00:01:06,310 --> 00:01:09,520
a block of code, and how the
indents sort of,

24
00:01:09,520 --> 00:01:11,950
like, put a boundary on the
blocks of code.

25
00:01:11,950 --> 00:01:15,640
And so, the first thing you should see is,
sort of, like, that

26
00:01:15,640 --> 00:01:20,020
purple part, the, the x less than 100,
print, that's kind of a box.

27
00:01:20,020 --> 00:01:23,690
And you can see the box on the, on the
sort of flow diagram as well.

28
00:01:23,690 --> 00:01:26,080
The boxes are there. The boxes
on the flow

29
00:01:26,080 --> 00:01:29,410
diagram are places where there's one
entrance and one exit.

30
00:01:34,720 --> 00:01:37,210
And then there's also, sort of, the
larger box, right?

31
00:01:37,210 --> 00:01:40,000
There's this if box that includes the
smaller box.

32
00:01:40,000 --> 00:01:42,640
So, the, there's this nesting, which is
boxes

33
00:01:42,640 --> 00:01:46,510
within boxes, or indented areas within
indented areas.

34
00:01:52,080 --> 00:01:54,495
Now that was a, what we call a one-way
decision, where

35
00:01:54,495 --> 00:01:56,985
you're doing if, and this code either runs
or it doesn't run.

36
00:01:56,985 --> 00:02:01,320
It is extremely common to want to
basically say,

37
00:02:01,320 --> 00:02:03,860
look, I'm going to do one of two things.
I'm going to

38
00:02:03,860 --> 00:02:06,460
ask a question, if the question is true,
I'm going to do

39
00:02:06,460 --> 00:02:09,290
one thing. If the question's false, I'm
going to do another thing.

40
00:02:09,290 --> 00:02:11,590
So that's what we have shown here.

41
00:02:11,590 --> 00:02:15,100
We say, is x equals 4, is x equal to
question mark?

42
00:02:15,100 --> 00:02:17,040
If it's yes, we're going to go here.

43
00:02:17,040 --> 00:02:18,930
If it's no, we're going to go here.

44
00:02:18,930 --> 00:02:21,180
We,re going to execute one or the other, and
then we're going to continue.

45
00:02:22,390 --> 00:02:24,150
So we're really at a fork in the road
here, right?

46
00:02:24,150 --> 00:02:26,720
We're, we're at a fork in the road,
going to make

47
00:02:26,720 --> 00:02:31,000
a choice, and one or the other, but never
both, right?

48
00:02:31,000 --> 00:02:34,350
So, we're going to do one thing, or we're
going to do another thing.

49
00:02:34,350 --> 00:02:36,150
We're going to do one of the two, and

50
00:02:36,150 --> 00:02:38,460
depending on what the question
that we ask,

51
00:02:38,460 --> 00:02:41,430
the question that we ask is, which one
that we're going to do.

52
00:02:43,930 --> 00:02:45,100
So here's a little bit of code.

53
00:02:47,570 --> 00:02:51,950
x equals 4, is x greater than 2, the
answer is yes.

54
00:02:51,950 --> 00:02:55,040
Then we come out and hit this else and we
automatically

55
00:02:55,040 --> 00:02:59,140
skip, right, because we're only going to
do one of the two.

56
00:02:59,140 --> 00:03:03,350
And here's the picture, x equals 4.
Is x equal to yes?

57
00:03:03,350 --> 00:03:05,330
Print, done.

58
00:03:05,330 --> 00:03:09,190
Which means we'll never do both this and
that, never do both, both sides.

59
00:03:09,190 --> 00:03:12,250
We're going to do one or the other of the
sides.

60
00:03:12,250 --> 00:03:14,720
And just sort of
going with the box,

61
00:03:14,720 --> 00:03:16,610
that is our box,
oops, go back, go back.

62
00:03:19,410 --> 00:03:21,210
This is our box, right?

63
00:03:21,210 --> 00:03:24,100
It's sort of the indent followed by the
final indent.

64
00:03:24,100 --> 00:03:26,210
The else is really kind of part of it.

65
00:03:26,210 --> 00:03:29,670
And then we can draw the picture here.
It has one entry and one exit.

66
00:03:32,230 --> 00:03:36,524
Okay.
So we have one-way ifs, and we have

67
00:03:36,524 --> 00:03:41,610
two-way ifs, and now we have multi-way
ifs, okay?

68
00:03:41,610 --> 00:03:48,800
So, here is a multi-way if,

69
00:03:48,800 --> 00:03:56,682
and it introduces a new reserved word, elif,

70
00:03:56,682 --> 00:03:59,270
which is a combination of else and if.

71
00:03:59,270 --> 00:04:03,950
And this one, probably, is just as easy to
talk about the picture here.

72
00:04:03,950 --> 00:04:06,016
The first question is asked, there's still

73
00:04:06,016 --> 00:04:08,110
going to only be one, there's only
going to

74
00:04:08,110 --> 00:04:11,990
be one, one and only one of these three
choices are going to run.

75
00:04:11,990 --> 00:04:15,550
Once it's run one, then it's done, okay?

76
00:04:15,550 --> 00:04:19,680
So the, the way to think about this, if x
is less than 2, we're going to

77
00:04:19,680 --> 00:04:22,130
run this one, and then we're going to
kind of

78
00:04:22,130 --> 00:04:24,700
flush all the way out to the bottom.

79
00:04:24,700 --> 00:04:27,300
If x is not less than 2, and it's less
than 10,

80
00:04:27,300 --> 00:04:29,812
we're going to run this one, then
flush out the bottom.

81
00:04:29,812 --> 00:04:31,900
And if x is not less than 2, and x is not

82
00:04:31,900 --> 00:04:37,690
less than 10, we're going to run this one,
and flush out the bottom.

83
00:04:37,690 --> 00:04:44,299
So, one of these three, one, two, three,
one of those three is going to run.

84
00:04:44,299 --> 00:04:48,600
And it's going to run based on the
questions that are being asked.

85
00:04:48,600 --> 00:04:49,942
The questions do get asked

86
00:04:49,942 --> 00:04:52,487
in an order, and the order
does matter, okay?

87
00:04:52,487 --> 00:04:55,298
So that is a multi-way if.

88
00:04:55,298 --> 00:05:00,650
If, else if, else.
So this is kind of like an otherwise.

89
00:05:00,650 --> 00:05:04,650
The else is like an otherwise, you know,
one way or another, we're going to run

90
00:05:04,650 --> 00:05:09,110
something, and if none of these first two
have run, we will run this one.

91
00:05:09,110 --> 00:05:14,290
We call it a multi-way if, okay?

92
00:05:14,290 --> 00:05:17,780
So, here's an example
of our multi-way if.

93
00:05:17,780 --> 00:05:21,021
That, if we say x equals 0, x equals 0.

94
00:05:21,021 --> 00:05:21,840
Is it less than 2?

95
00:05:21,840 --> 00:05:22,890
Yes, it is.

96
00:05:22,890 --> 00:05:26,570
So we run Small, print Small, and then we
flush out the bottom.

97
00:05:28,170 --> 00:05:32,930
If we switch, instead, x to 5, x is 5.

98
00:05:32,930 --> 00:05:35,336
Is it less than 2?
No, it is not less than 2.

99
00:05:35,336 --> 00:05:36,173
Is it less than 10?

100
00:05:36,173 --> 00:05:40,016
Well, 5 is less than 10.
So the answer is yes, so we print Medium,

101
00:05:40,016 --> 00:05:44,910
then we flush out the bottom.
One and only one are going to execute.

102
00:05:46,130 --> 00:05:48,515
Now, in this case, we got x is 20.

103
00:05:48,515 --> 00:05:51,480
And so we come through here.
Is it less than 2?

104
00:05:51,480 --> 00:05:52,660
No, it is not.

105
00:05:52,660 --> 00:05:54,620
Is it less than 10?
No, it is not.

106
00:05:54,620 --> 00:05:56,910
So we're going to do this one, and then
flush out the bottom.

107
00:05:56,910 --> 00:06:04,580
If we go here, it's false, false, go here,
all else being equal, we run that one.

108
00:06:04,580 --> 00:06:06,020
So this one doesn't run

109
00:06:06,020 --> 00:06:09,500
and that one doesn't run, right?
Because these are like gateways.

110
00:06:09,500 --> 00:06:11,630
If it were true, it would run it.

111
00:06:11,630 --> 00:06:13,910
But it's false, so we're going to skip it.

112
00:06:13,910 --> 00:06:16,590
This one, it's false, so we're going to
skip it.

113
00:06:16,590 --> 00:06:19,520
But then we hit the else, that's like a
catch-all.

114
00:06:19,520 --> 00:06:23,993
And then if none of these were true,
then it will run the else.

115
00:06:23,993 --> 00:06:27,575
Any questions?

116
00:06:27,575 --> 00:06:29,543
Okay.
Well,

117
00:06:29,543 --> 00:06:34,600
I'm going to ask you a question, in a
second.

118
00:06:37,600 --> 00:06:40,950
Okay, so just a couple of things that
probably you're wondering about.

119
00:06:42,050 --> 00:06:43,130
You don't actually need an else.

120
00:06:44,250 --> 00:06:49,630
You can have a multi-way, x equals 5, if x
is less than 2, there's no else here.

121
00:06:49,630 --> 00:06:51,830
You'll notice that this print just comes
back.

122
00:06:51,830 --> 00:06:55,690
And so this way, it could, if both of
these are false, it could

123
00:06:55,690 --> 00:07:00,490
skip them both and just run right through
here, and there's no else clause, okay?

124
00:07:00,490 --> 00:07:02,720
So, in this case, if this one's

125
00:07:02,720 --> 00:07:06,170
going to, the way this one's going to run
is, x equals 5

126
00:07:06,170 --> 00:07:09,440
if x is less than 2, it's, it's not.

127
00:07:09,440 --> 00:07:10,285
And it skips to here.

128
00:07:10,285 --> 00:07:15,150
Else if x is less than 10, which it
is, it will run that one and come here.

129
00:07:15,150 --> 00:07:21,090
But, for a different value of x, like 95,
boop boop.

130
00:07:22,560 --> 00:07:27,780
If x was 95, or 59, this would be false.
It would skip it.

131
00:07:27,780 --> 00:07:30,912
This would, elif, would still be false,
and it would skip it,

132
00:07:30,912 --> 00:07:34,205
and the only thing it would print out
would be, All done. Okay?

133
00:07:34,205 --> 00:07:38,870
Okay, you can also have many elifs.

134
00:07:40,290 --> 00:07:43,945
So, better change to green.

135
00:07:43,945 --> 00:07:45,828
It checks this one, if it's true, it runs
the first one.

136
00:07:45,828 --> 00:07:47,113
If it's false, it checks this one.

137
00:07:47,113 --> 00:07:51,240
If that's true, it run this one, and then
it skips, right?

138
00:07:51,240 --> 00:07:52,830
And so, so the way to think about

139
00:07:52,830 --> 00:07:56,020
this is, is, it just goes through and
checks this one

140
00:07:56,020 --> 00:07:59,310
false, this one false, false, false, oh, I
finally found one.

141
00:07:59,310 --> 00:08:01,200
And now I'm done.

142
00:08:01,200 --> 00:08:05,490
It still is going to do one and only one
of these.

143
00:08:05,490 --> 00:08:09,164
This one has an else, so that sooner or
later, it is going to do one.

144
00:08:09,164 --> 00:08:15,703
And it only will do the else if all of
these are false. All have to be false.

145
00:08:15,703 --> 00:08:18,500
Then it will, actually, come and hit the
else clause.

146
00:08:19,558 --> 00:08:21,410
It's great, because there are lots of
situations where

147
00:08:21,410 --> 00:08:24,340
you're like, oh, is it before eight in the
morning?

148
00:08:24,340 --> 00:08:26,170
Or is it between eight and noon?

149
00:08:26,170 --> 00:08:28,070
Or is it between noon and five?

150
00:08:28,070 --> 00:08:31,780
Or after five? After midnight?
I don't know.

151
00:08:31,780 --> 00:08:36,710
Okay, so, here, coming up, is a question.

152
00:08:38,370 --> 00:08:40,610
And, you, there's two puzzles
and I'm going to

153
00:08:40,610 --> 00:08:42,459
stop so you can look at them for a while.

154
00:08:42,459 --> 00:08:44,811
And I want you to figure out,

155
00:08:44,811 --> 00:08:48,143
in both sides of this, which of the lines

156
00:08:48,143 --> 00:08:52,730
will not execute, regardless of the value
for x.

157
00:08:52,730 --> 00:08:54,820
So in both sides, there is a line

158
00:08:54,820 --> 00:08:57,990
that won't execute, regardless of the
value for x.

159
00:08:57,990 --> 00:08:59,180
Which will never print?

160
00:08:59,180 --> 00:09:04,210
There's two problems, problem A and
problem B.

161
00:09:04,210 --> 00:09:08,276
Okay, I'll have some coffee while you
think.

162
00:09:08,276 --> 00:09:10,276
[NOISE].

163
00:09:20,859 --> 00:09:24,970
Okay, hopefully you paused it so that you
could actually think for a bit.

164
00:09:24,970 --> 00:09:27,480
So, so I'm going to guess you probably

165
00:09:27,480 --> 00:09:29,440
got the first one right, that's pretty
straightforward.

166
00:09:29,440 --> 00:09:32,462
I mean, actually, you're in great shape
if you got both of them right.

167
00:09:32,462 --> 00:09:34,518
If you got any of them right,
you're in great shape

168
00:09:34,518 --> 00:09:36,314
because that means
you're starting to get it.

169
00:09:36,314 --> 00:09:41,157
Starting to like, oh, I'm seeing, kind of,
this flow picture, there's a picture.

170
00:09:41,157 --> 00:09:43,044
I look at these characters that seemingly

171
00:09:43,044 --> 00:09:45,630
look like gibberish, and a picture
arrives.

172
00:09:45,630 --> 00:09:49,710
Or a pattern of access execution arises.

173
00:09:49,710 --> 00:09:51,250
That's what we want to see.

174
00:09:51,250 --> 00:09:55,280
So, the in the first one, which will
never print?

175
00:09:55,280 --> 00:09:58,890
Well, we're looking for kind of a value
for x which will be defective.

176
00:09:58,890 --> 00:10:01,260
So if x is less than 2, we're going
to do this.

177
00:10:01,260 --> 00:10:04,970
Else, if x is greater than or equal to 2,
we'll do this, else we'll do that.

178
00:10:04,970 --> 00:10:06,650
Well, here's the problem with this one.

179
00:10:06,650 --> 00:10:10,150
For all values of x, it is, is either going,

180
00:10:10,150 --> 00:10:14,670
x is less than 2 is either going to be
true or greater than equal to 2.

181
00:10:14,670 --> 00:10:17,560
Greater than or equal to be, pah,
for X to be

182
00:10:17,560 --> 00:10:20,210
greater than or equal to 2 is
going to be true.

183
00:10:20,210 --> 00:10:22,510
So it's going to run this one, or it's
going to run that one.

184
00:10:22,510 --> 00:10:23,970
So for big numbers, numbers above 2,

185
00:10:23,970 --> 00:10:27,650
it's going to run this one;
below 2, it's going to run that one.

186
00:10:27,650 --> 00:10:31,060
So this one is never going to run, okay?

187
00:10:31,060 --> 00:10:33,170
Because the, one of the first two is
going to be

188
00:10:33,170 --> 00:10:37,320
true, and so the third else situation
is not going to run.

189
00:10:37,320 --> 00:10:38,120
Hope you got that right.

190
00:10:39,550 --> 00:10:42,540
Okay, so let's take a look at the next
one, okay?

191
00:10:42,540 --> 00:10:45,560
So the question is, you know, if x is less
than 2, do this,

192
00:10:45,560 --> 00:10:49,100
if x is less 20, do that, and if x is less
than 10, do this,

193
00:10:49,100 --> 00:10:54,574
and otherwise do that.
Well, the one that will never execute

194
00:10:54,574 --> 00:11:00,009
is this one. And, x equals 15,

195
00:11:00,009 --> 00:11:04,977
no, x equals 15 is a bad one,
x equals 5 is the one that will,

196
00:11:04,977 --> 00:11:09,966
sort of, cause it to behave badly.

197
00:11:09,966 --> 00:11:14,640
And so, if x is 5, this is false.

198
00:11:14,640 --> 00:11:19,316
If x is less than 20, this is true, and
then it's done.

199
00:11:19,316 --> 00:11:24,648
So the problem is, this is the one that
will never execute, because

200
00:11:24,648 --> 00:11:30,046
if a value is less than 10, it's also
less than 20, so this will be true.

201
00:11:30,046 --> 00:11:32,626
So for a value like 5, which happens to be
less than 10,

202
00:11:32,626 --> 00:11:36,410
which you would think would cause that
line to execute, does not.

203
00:11:37,540 --> 00:11:40,160
This one executes because it's checked
first.

204
00:11:40,160 --> 00:11:43,350
Now, if we just moved this code, took this
code and

205
00:11:43,350 --> 00:11:47,050
we moved it down here, then it would make
more sense, okay?

206
00:11:47,050 --> 00:11:47,780
Oops.

207
00:11:47,780 --> 00:11:49,960
If we moved it down there, it would make
more sense.

208
00:11:49,960 --> 00:11:55,950
But basically, the answer to these is,
change color, this one won't

209
00:11:55,950 --> 00:12:01,910
execute, and this one will never execute
for any value, so there's the answer.

210
00:12:04,440 --> 00:12:05,910
Okay, so we're almost done with
conditionals.