1
00:00:05,400 --> 00:00:08,630
So continuing on with our
discussion on sets (inaudible),

2
00:00:08,630 --> 00:00:12,090
we finish off on our last video just
talking about the different methods, and

3
00:00:12,090 --> 00:00:16,770
as you can see on line 61 and 62, when
you run this, you get the same results.

4
00:00:16,770 --> 00:00:21,530
You get the same results using dot
difference or using the minus sign.

5
00:00:21,530 --> 00:00:24,400
So which one you use depends, again,
on which one you find more readable.

6
00:00:24,400 --> 00:00:27,960
In our opinion, the difference method does
make it clear that you're working on sets,

7
00:00:27,960 --> 00:00:31,890
which you wouldn't necessarily see
just looking at line 62 in isolation.

8
00:00:31,890 --> 00:00:35,900
So again, looking at line 61,
it's pretty obvious that .difference

9
00:00:35,900 --> 00:00:39,930
is calling a method, so That's giving
you a that it's actually a set.

10
00:00:39,930 --> 00:00:43,440
Now, there's also an update
difference method and

11
00:00:43,440 --> 00:00:45,860
that performs the subtraction in place.

12
00:00:45,860 --> 00:00:47,670
In other words,
it doesn't return a new set, but

13
00:00:47,670 --> 00:00:52,350
what it does it modifies the set
upon which it is called on.

14
00:00:52,350 --> 00:00:54,050
As a result it returns none.

15
00:00:54,050 --> 00:00:57,080
And if you think about that,
remember the list dot sort method and

16
00:00:57,080 --> 00:00:59,610
the dictionary dot update methods
cause they did the similar thing.

17
00:00:59,610 --> 00:01:01,540
So see how that works.

18
00:01:01,540 --> 00:01:04,209
So lets continue our code down here,
so I'll make a bit more space,

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00:01:04,209 --> 00:01:05,980
it's a little higher up on the screen.

20
00:01:07,000 --> 00:01:11,000
So we'll just make a bit
of space in our output.

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00:01:13,480 --> 00:01:21,850
Then we're gonna
print(sorted[even]) print(squares).

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00:01:21,850 --> 00:01:24,230
And we'll do even .difference.

23
00:01:24,230 --> 00:01:26,510
This time we're doing difference and
the score update.

24
00:01:26,510 --> 00:01:28,290
And we're gonna select squares.

25
00:01:28,290 --> 00:01:31,040
So, again we're passing squares
as a perimeter and even.

26
00:01:31,040 --> 00:01:34,720
So, that updates a curve
on the even variable.

27
00:01:34,720 --> 00:01:36,410
Now, on the even set.

28
00:01:36,410 --> 00:01:38,710
So, we're gonna print and sort it even and

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00:01:40,090 --> 00:01:43,040
you should find that the changes have
been made to it, so let's run that.

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00:01:44,620 --> 00:01:47,480
So I can say that we started
out with this long set, and

31
00:01:47,480 --> 00:01:50,330
then we printed out these squares.

32
00:01:50,330 --> 00:01:54,020
I don't know if you can see them now
being removed all the contents from

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00:01:54,020 --> 00:01:57,790
these squares, cuz we've used Is
the difference update to remove them from

34
00:01:57,790 --> 00:02:00,710
the even set and
we printed a sort of even set.

35
00:02:00,710 --> 00:02:03,360
You can see those numbers
clearly aren't there anymore.

36
00:02:03,360 --> 00:02:06,780
So, moving on the next thing we want
to talk about is the the symmetric

37
00:02:06,780 --> 00:02:11,450
difference of two sets, so What that is
a symmetric difference of two sets is all

38
00:02:11,450 --> 00:02:15,260
the members that are in one set or
the other but not both.

39
00:02:15,260 --> 00:02:18,570
So being symmetric also means
that it doesn't matter which way

40
00:02:18,570 --> 00:02:20,500
round you calculate the difference.

41
00:02:20,500 --> 00:02:23,970
And once again what I'll, what we'll do is
we'll sort the result before printing it

42
00:02:23,970 --> 00:02:26,710
just to make it easy to
interpret The result

43
00:02:26,710 --> 00:02:30,640
you don't normally need to do that in
your coding for processing reasons.

44
00:02:30,640 --> 00:02:32,230
We're just making it easy
on the screen for you.

45
00:02:32,230 --> 00:02:35,350
So, let's just take this bit of code
here because we're going to use that and

46
00:02:35,350 --> 00:02:39,070
post that down here and
I'll just comment all that code out.

47
00:02:39,070 --> 00:02:43,080
Okay, so
we're gonna stand a code there for sets.

48
00:02:43,080 --> 00:02:47,960
So, what we're going to do now is the
symmetric difference so we're gonna Print

49
00:02:50,370 --> 00:02:54,430
symmetric in there.

50
00:02:56,850 --> 00:02:58,920
Symmetric even minus squares.

51
00:03:02,050 --> 00:03:04,540
And we'll do print sorter.

52
00:03:04,540 --> 00:03:08,380
Sorter is just for our eyes but we have to
do this to get the symmetric difference

53
00:03:08,380 --> 00:03:12,940
Symmetric difference squares.

54
00:03:12,940 --> 00:03:19,930
Thought so.
And we can also do a print symmetric

55
00:03:19,930 --> 00:03:25,510
squares minus even.

56
00:03:25,510 --> 00:03:30,560
Print squares.symmetric

57
00:03:30,560 --> 00:03:35,840
underscore difference even.

58
00:03:35,840 --> 00:03:38,500
We could do the same thing there if
we want to, we can sort that as well.

59
00:03:40,680 --> 00:03:43,580
Just to make sure that it's
going to come out the same way.

60
00:03:43,580 --> 00:03:44,450
Like I say, we run that.

61
00:03:44,450 --> 00:03:47,520
The output for
both is absolutely identical.

62
00:03:47,520 --> 00:03:51,410
So it doesn't matter which order we
run it on we get the same results.

63
00:03:53,220 --> 00:03:56,940
And I just remove sort of the,
if we run that again.

64
00:03:56,940 --> 00:03:59,680
It says symmetric difference
can be thought of

65
00:03:59,680 --> 00:04:02,910
as the opposite of intersection
which we covered earlier.

66
00:04:02,910 --> 00:04:05,590
The intersection of the two sets was 4,
6, and 16.

67
00:04:05,590 --> 00:04:10,980
So the symmetric difference is the set
of everything except those three values.

68
00:04:10,980 --> 00:04:14,120
Now symmetric difference can also
be performed using the carrot.

69
00:04:14,120 --> 00:04:16,260
You know, the sort of up arrow symbol,

70
00:04:16,260 --> 00:04:19,399
which is just really another way
of doing exactly the same thing.

71
00:04:19,399 --> 00:04:22,000
There's no performance advantage
with either method, so

72
00:04:22,000 --> 00:04:24,240
again it's really just
a matter of preference.

73
00:04:24,240 --> 00:04:27,070
What we'd suggest generally is using
the methods to make it clear that you're

74
00:04:27,070 --> 00:04:30,950
dealing with sets unless your program
was only or mainly using sets,

75
00:04:30,950 --> 00:04:35,290
in which case the shorter operators, again
we're talking about In a carrot the and,

76
00:04:35,290 --> 00:04:38,700
and the mono sign will be clearing
up because of the context.

77
00:04:38,700 --> 00:04:40,220
Now it may seem to be a quirky you know,

78
00:04:40,220 --> 00:04:45,070
so I took the sorted out and
when we ran it we got the same results.

79
00:04:45,070 --> 00:04:47,940
So, that's a little bit of a quirk
with a symmetric difference.

80
00:04:47,940 --> 00:04:51,390
When we did even Dot symmetric
dot difference squares

81
00:04:51,390 --> 00:04:53,710
we have to sort the result to
say the number is in order.

82
00:04:53,710 --> 00:04:57,070
When we did these, the symmetric
difference between squares and

83
00:04:57,070 --> 00:04:59,760
even result set printed out in order.

84
00:04:59,760 --> 00:05:02,720
So why that happens
we're not really sure so

85
00:05:02,720 --> 00:05:05,910
we're not sure if it happens with
all Python implementations and

86
00:05:05,910 --> 00:05:08,780
it's certainly not documented behavior so
if you did notice it.

87
00:05:08,780 --> 00:05:10,190
But don't rely on this for your program.

88
00:05:10,190 --> 00:05:13,130
So in other words,
if you did want that to be sorted.

89
00:05:13,130 --> 00:05:16,420
And for some reason it's sorting
without actual use of the sort method.

90
00:05:16,420 --> 00:05:19,540
You may actually want to put that in there
anyway because it might be a bug they fix

91
00:05:19,540 --> 00:05:25,180
in the future versions of Python So,
the last

92
00:05:25,180 --> 00:05:30,490
set difference method that we mentioned is
symmetric and it's called difference in

93
00:05:30,490 --> 00:05:34,870
It performs really just like the line 67
with a difference update, but obviously in

94
00:05:34,870 --> 00:05:38,750
this case it's using the symmetric
difference instead of the difference

95
00:05:38,750 --> 00:05:42,300
It updates the set that it's been called
upon rather than reset as you saw when we

96
00:05:42,300 --> 00:05:45,590
talked about the difference updates
as you saw again on line 67.

97
00:05:45,590 --> 00:05:48,020
Now there's two ways to
remove items from the set.

98
00:05:48,020 --> 00:05:50,780
We can use discard and remove.

99
00:05:50,780 --> 00:05:54,070
The only difference between them is
that removes would raise an error

100
00:05:54,070 --> 00:05:56,860
if the island to be
removed doesn't exist and

101
00:05:56,860 --> 00:05:59,970
whereas discard won't actually
raise an error in that scenario.

102
00:05:59,970 --> 00:06:02,180
So let's see some code that showing that.

103
00:06:02,180 --> 00:06:03,840
I'm just going to comment
this last bit of code out.

104
00:06:05,950 --> 00:06:10,030
And I'll just remove this sort of there.

105
00:06:10,030 --> 00:06:13,690
We don't really need it anymore so
I'll get rid of those.

106
00:06:13,690 --> 00:06:19,390
See what we're starting out with and
now we can start removing some.

107
00:06:19,390 --> 00:06:20,560
So I'm going to do squares.

108
00:06:20,560 --> 00:06:23,510
.discard 4.

109
00:06:23,510 --> 00:06:28,200
Squares dot remove, remove 16.

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00:06:28,200 --> 00:06:30,180
And squares.discard.

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00:06:31,560 --> 00:06:33,490
8, it should

112
00:06:39,530 --> 00:06:42,800
show no,
it should be no error does nothing.

113
00:06:42,800 --> 00:06:47,820
And obviously we're Because obviously

114
00:06:47,820 --> 00:06:50,770
eight isn't in the set as you can
see the set defined on line 73 and

115
00:06:50,770 --> 00:06:55,700
we can do print squares to see
that the 416 were removed but

116
00:06:58,760 --> 00:07:05,190
then if we do squares Dot remove
eight you should get an error.

117
00:07:06,790 --> 00:07:10,270
First thing I need to fix is error
up online, 75, that should fix.

118
00:07:10,270 --> 00:07:14,070
So if we run that now you can
see we did get the three.

119
00:07:14,070 --> 00:07:19,260
As you can see that we
started out we had 16,

120
00:07:19,260 --> 00:07:24,120
9, 4, 6 and 25 and you can see that 16 and
4 are now removed from the last attempt

121
00:07:24,120 --> 00:07:27,730
which was from line 86, and
then we tried to use squares remove.

122
00:07:27,730 --> 00:07:30,970
We actually got an error and again that's
the difference between the discard on line

123
00:07:30,970 --> 00:07:34,490
89 because the silent didn't return,
didn't raise an error.

124
00:07:34,490 --> 00:07:39,440
But the one on line 87 you can see
the remove does raise an error.

125
00:07:39,440 --> 00:07:41,990
That's the main difference,
remove does raise an error.

126
00:07:41,990 --> 00:07:43,460
whereas discard doesn't.

127
00:07:43,460 --> 00:07:46,540
If you want to use remove,
to safely use that,

128
00:07:46,540 --> 00:07:49,480
you should check the items in the set
before attempting to remove it.

129
00:07:49,480 --> 00:07:54,070
We would do something
like this to make sure.

130
00:07:54,070 --> 00:08:00,240
We put "if 8 in squares" and squares
don't remove 8, as you can see there.

131
00:08:00,240 --> 00:08:04,100
If you run that This time
we don't get an error and

132
00:08:04,100 --> 00:08:08,040
that's because we're checking first to
see where that eights in the set and

133
00:08:08,040 --> 00:08:12,760
it wasn't in this case so therefore
the code on line 88 didn't get executed.

134
00:08:12,760 --> 00:08:15,150
So probably the next
question you're asking

135
00:08:15,150 --> 00:08:18,620
Now it would be why do you wanna use
remove if discard seems much safer.

136
00:08:18,620 --> 00:08:21,780
Well the reason is,
sometimes you may want an error

137
00:08:21,780 --> 00:08:25,620
to be raised if an operation is
being attempted that shouldn't be.

138
00:08:25,620 --> 00:08:27,330
So when attempting to discard eight,

139
00:08:27,330 --> 00:08:29,620
there's no way indication
that anything's wrong.

140
00:08:29,620 --> 00:08:32,799
And it's not possible to
take any action if eight

141
00:08:32,799 --> 00:08:35,330
should not be deleted from the zero or
could not be deleted from the set.

142
00:08:35,330 --> 00:08:38,230
So we're gonna look at exceptions later
in the course so don't worry too much

143
00:08:38,230 --> 00:08:42,760
about this next bit of code it's really
just to show that we can trip an error and

144
00:08:42,760 --> 00:08:47,280
take some other action if we tip to remove
an item that doesn't exist from a set.

145
00:08:47,280 --> 00:08:49,710
Sometimes removing them for
that reason is more useful so

146
00:08:49,710 --> 00:08:53,250
to do that we can come back here And
we can put try.

147
00:08:53,250 --> 00:08:56,930
>> [NOISE]
>> And

148
00:08:56,930 --> 00:09:01,880
the codes going to be squares dot
remove eight, and put accept.

149
00:09:01,880 --> 00:09:07,480
Key error, print,

150
00:09:07,480 --> 00:09:11,890
item eight.

151
00:09:11,890 --> 00:09:15,130
Is not member of sets

152
00:09:16,790 --> 00:09:21,190
now if you fan run that to
the awesome eighties of the set.

153
00:09:21,190 --> 00:09:24,670
Okay so this is try try and receptors.

154
00:09:24,670 --> 00:09:28,790
Exceptions that are used in path in which
we won't be going through the details now,

155
00:09:28,790 --> 00:09:31,080
but later in the course
you'll see more about those.

156
00:09:31,080 --> 00:09:34,190
The point is we're able to trap
the error and do some processing,

157
00:09:34,190 --> 00:09:36,450
which in this case consisted
only of printing a message but

158
00:09:36,450 --> 00:09:40,220
it could be anything if the removal
failed for some reason.

159
00:09:40,220 --> 00:09:44,830
Now the same is true when attempting
to access keys in a dictionary

160
00:09:44,830 --> 00:09:47,610
using the square brackets
rather than using a get, so

161
00:09:47,610 --> 00:09:50,340
in other words using [NOISE] When we use.

162
00:09:50,340 --> 00:09:53,460
Trying to get access to keys in
the dictionary using the square

163
00:09:53,460 --> 00:09:54,930
brackets instead of using get.

164
00:09:56,400 --> 00:09:58,420
Or they get is very useful,

165
00:09:58,420 --> 00:10:00,880
because we might need to take
action if the key is not present.

166
00:10:00,880 --> 00:10:05,010
So that's the reason that partham also
provides a way to raise an exception

167
00:10:05,010 --> 00:10:07,020
in error if the key is not present so.

168
00:10:07,020 --> 00:10:09,690
Basically the same concept here for a set.

169
00:10:09,690 --> 00:10:14,390
Okay, so we've now seen how to create
a set using either a set [INAUDIBLE] and

170
00:10:14,390 --> 00:10:17,980
closing the members in clearly brackets or
the set constructors.

171
00:10:17,980 --> 00:10:20,560
You have also seen how to add and
remove members.

172
00:10:20,560 --> 00:10:25,230
As well as creating new sets that are the
union and intersection of two other sets.

173
00:10:25,230 --> 00:10:28,700
We can subtract sets from each
other as well, which you say.

174
00:10:28,700 --> 00:10:32,730
And in the same way that you can create
a set from a list by passing the list

175
00:10:32,730 --> 00:10:35,710
to the set constructor,
you can actually go the other way.

176
00:10:35,710 --> 00:10:40,330
So in other words you can create a list
from a set using the list constructor.

177
00:10:40,330 --> 00:10:43,900
In fact we did something similar
by passing the sets to the sorted

178
00:10:43,900 --> 00:10:46,160
function which will turn to list.

179
00:10:46,160 --> 00:10:49,860
So the last two operations with
sets are is subset and is superset.

180
00:10:49,860 --> 00:10:54,950
Now those will be used to test if one set
is a sub or super Set of the other set.

181
00:10:54,950 --> 00:10:58,100
Now one set is a sub set of
another if all the members

182
00:10:58,100 --> 00:10:59,480
are contained in the other set.

183
00:10:59,480 --> 00:11:01,660
Now similarly,

184
00:11:01,660 --> 00:11:07,200
one set is a super set of another if it
contains all the other sets and members.

185
00:11:07,200 --> 00:11:11,040
So if we remove the odd squares from our
project, which we're about to do now.

186
00:11:11,040 --> 00:11:12,730
We can see these two methods in action.

187
00:11:12,730 --> 00:11:18,050
So what I'm going to do is just
comment this out Encrypt this code.

188
00:11:18,050 --> 00:11:23,540
But we'll just code that as, well so
we can stop down at the bottom.

189
00:11:27,500 --> 00:11:31,120
Now what we're going to do for
these square.

190
00:11:31,120 --> 00:11:33,160
We'll just make that 4, 6, and 16.

191
00:11:33,160 --> 00:11:34,660
We'll leave the other ones out.

192
00:11:37,240 --> 00:11:43,470
And we can do something like
if squares dot is subset even.

193
00:11:43,470 --> 00:11:48,600
We're gonna

194
00:11:48,600 --> 00:11:53,950
do print squares is a subset Copy and we

195
00:11:56,130 --> 00:12:02,580
can also do even dot superset
is superset squares.

196
00:12:02,580 --> 00:12:09,270
Print is a super set of squares.

197
00:12:11,970 --> 00:12:12,530
So you can run that.

198
00:12:13,930 --> 00:12:15,880
Squares is a subset of even.

199
00:12:15,880 --> 00:12:17,890
Even is a superset of squares.

200
00:12:17,890 --> 00:12:19,610
So again, a subset.

201
00:12:19,610 --> 00:12:23,850
One set is a subset of another if all its
members are contained in the other set.

202
00:12:23,850 --> 00:12:28,600
And for superset, one set is a superset
of another If it contains all the other

203
00:12:28,600 --> 00:12:31,150
set's members then you can just
say that we've used the example

204
00:12:31,150 --> 00:12:35,910
of inverting the results to be able to
show that squares is a subset of even and

205
00:12:35,910 --> 00:12:37,760
even is a superset of squares.

206
00:12:39,200 --> 00:12:41,460
So sets are really not
used as often as lists.

207
00:12:41,460 --> 00:12:42,530
Dictionaries, but

208
00:12:42,530 --> 00:12:47,120
they can still be used, can be useful
as hopefully you've seen in this code.

209
00:12:47,120 --> 00:12:49,140
So our challenge we're about to
do is gonna show you how they

210
00:12:49,140 --> 00:12:50,620
can provide a quick way to modify data.

211
00:12:50,620 --> 00:12:52,840
Now before we start the challenge though,

212
00:12:52,840 --> 00:12:56,990
there's one more type of set available to
us in Python It's called a frozen set.

213
00:12:56,990 --> 00:13:01,300
And a frozen set is a immutable set
which of course can't be changed.

214
00:13:01,300 --> 00:13:06,220
So, when being unmutable at least we can
use a frozen set as a dictionary key,

215
00:13:06,220 --> 00:13:07,220
which is good.

216
00:13:07,220 --> 00:13:10,600
And we can also add a frozen
set as a member of a set.

217
00:13:10,600 --> 00:13:13,980
Now keep in mind there's no add methods,
there's no add, remove, or

218
00:13:13,980 --> 00:13:16,280
discard methods available
with a frozen set.

219
00:13:16,280 --> 00:13:17,800
Once created, it can't be changed.

220
00:13:17,800 --> 00:13:20,470
And obviously that's the reason
they haven't added those methods.

221
00:13:20,470 --> 00:13:28,040
So to use those, It'll

222
00:13:28,040 --> 00:13:34,370
give you something like even
equals frozen set range 0 100 2.

223
00:13:34,370 --> 00:13:38,770
And print even

224
00:13:40,870 --> 00:13:46,580
even dot add three notice that
the doesn't come up and if we run

225
00:13:46,580 --> 00:13:51,520
that we're getting an error because frozen
set object has no attribute for add.

226
00:13:51,520 --> 00:13:54,140
And as I mentioned, it hasn't got
those methods because it's immutable.

227
00:13:54,140 --> 00:13:57,170
Once we create it, we can't modify it.

228
00:13:57,170 --> 00:13:59,770
Now other than an inability to add or
remove members,

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frozen sets behave just like regular
sets and can be used to create unions,

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intersections, and
can be subtracted from other sets.

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Now you can't call difference_update or
symmetric_difference_update

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on a frozen set, of course,
because A frozen set isn't mutable.

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What you can do is you can pass a
parameter to a different underscore update

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core on a regular set.

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Now before we move onto the challenge,

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we looked at many operators that
do the same job as the methods.

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And there's a nice table in
the path documentation for

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2.7 showing the methods and
the equivalent operators

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Not only use the table that was much of
the text no longer applies to Python 3.

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So that's important so ignore the text and
just see the refer to the table.

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So I'm just gonna bring that up on
the screen so you can see There and

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we're gonna put this link
in the resources section.

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So refer to that as well, but you can see
that's giving you a good overview of what

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the various operations are for sense.

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And just out of interest, the same
information is available in the Tree

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documentation but it's not laid out in
a helpful table than it is there and

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I think it is really quite helpful being
able to see that all getting there.

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So probably I could link for
you to bookmark.

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All right, it's time for a challenge.

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Let's go back to our code and
we're gonna create a new file.

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[NOISE]
>> We're gonna copy and paste challenging.

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So, here's the challenge.

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Create a program that takes some text and

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returns a list of all the characters
in the text that are not vowels.

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Sorted in alphabetical order.

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And you can either answer
the text from the keyboard or

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initialize a string
variable with a the string.

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Now obviously we wanted to use sets.

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So this is where the whole idea
of this is we're using sets.

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So you should be using
a set with this challenge.

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Okay so, go away and
see if you can figure out that challenge.

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And when you're ready to see the solution,
come back and we're make a start on it.

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Let's have a look at that.

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Let's type some text in.

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Sampletext equals "python is
a very powerful language".

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So, how would we do that?

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We would type "vowels equals Frozenset,
aeiou.

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Alternatively, you could have
done something like this

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vowels = ("a",

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"e", "i", "o", "u").

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But thank you, that takes more topics so
the one that for

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instance we dont that it was way quicker.

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So, then we do funnel set, equals set.

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Sample text, dot difference

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Print final set [NOISE]
>> And

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then we can finally do something
like final lists equals sorted.

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FInal set and print.

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[NOISE] On a list.

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So if you run that.

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And I think I need to
[INAUDIBLE] some [INAUDIBLE].

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So I need to go back to my other file.

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I'm actually running
the wrong program file.

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Unless you just run this
one here as well first.

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Run challenge.

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Just to confirm that it works.

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So you can see that the first line
shows the answer to the sorted version

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of the sort.

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And the second line as you can see.

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That's in alphabetical order.

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Noting the upper case and lower case and

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spaces have got a different ordering
than the lower case equivalence.

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So because sets don't contain duplicates,
we get the list that we wanted with each

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character only occurring once on
the list as you can see there.

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And we've used a frozen set for

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the vals because the members
shouldn't change in English.

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If you wanted to test say for the worst
language and we add w and y to the set.

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Then the frozen wouldn't be
appropriate in this scenario.

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Just to go back to that other area,
that was coming up because

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I hadn't come into that line out which was
invalid because we're using a frozen set.

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So that's actually the challenge.

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That's any few lines of code and

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again I just want to reinforce how
powerful Python is to be able to do things

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that are relatively complex
in such a few lines of code.

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It's really a fantastic language for that.

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00:18:10,180 --> 00:18:14,130
Ok, so that's the end of this video and
the end of this section, actually.

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So I've worked on quite a few
things in this section trying

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to get dictionaries and
sets out of the way.

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And we're now done with that.

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In the next section we're going to start
working on input and output in Python, so

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see you in the next section.