1
00:00:01,150 --> 00:00:02,550
- [Instructor] In this
video we're going to

2
00:00:02,550 --> 00:00:05,570
take a look at the Boolean Operators

3
00:00:05,570 --> 00:00:08,380
called and, or, and not.

4
00:00:08,380 --> 00:00:11,620
Now these operators enable
you to create more complex

5
00:00:11,620 --> 00:00:14,380
conditions then what we've
been looking at so far.

6
00:00:14,380 --> 00:00:17,210
So for example we've done
conditions like great,

7
00:00:17,210 --> 00:00:19,070
greater then or equal to 60

8
00:00:19,070 --> 00:00:20,970
and that's just a simple condition

9
00:00:20,970 --> 00:00:23,770
and it's true if the great is
greater then or equal to 60

10
00:00:23,770 --> 00:00:25,060
and false otherwise,

11
00:00:25,060 --> 00:00:28,050
but sometimes we need
to test multiple values

12
00:00:28,050 --> 00:00:31,140
and form a more compound
or complex condition

13
00:00:31,140 --> 00:00:33,940
in order to get the
results that we require.

14
00:00:33,940 --> 00:00:36,330
So lets say we go and define a variable,

15
00:00:36,330 --> 00:00:39,940
we'll call it gender and we'll
set it to the string female

16
00:00:39,940 --> 00:00:41,700
and lets create another variable,

17
00:00:41,700 --> 00:00:43,910
we'll call it age and we'll set it to 70

18
00:00:43,910 --> 00:00:47,460
and lets say we want to print
the strings senior female,

19
00:00:47,460 --> 00:00:51,220
only if the gender is female
and the age is greater then or

20
00:00:51,220 --> 00:00:54,680
equal to 65, so we can go
and write such an expression.

21
00:00:54,680 --> 00:00:58,480
So if gender double equal sign female

22
00:00:58,480 --> 00:01:00,860
and of course you have
to spell it correctly.

23
00:01:00,860 --> 00:01:04,200
By the way if we were to use
different key sensitivities

24
00:01:04,200 --> 00:01:06,740
in the two strings they
would not compare as equals,

25
00:01:06,740 --> 00:01:08,670
so please keep that in mind.

26
00:01:08,670 --> 00:01:12,650
And then, we want to combine
this with another condition,

27
00:01:12,650 --> 00:01:15,860
age greater then or equal to 65,

28
00:01:15,860 --> 00:01:18,700
so if both of those are
conditions are true,

29
00:01:18,700 --> 00:01:21,210
then we would like a
print statement to execute

30
00:01:21,210 --> 00:01:23,856
that displays senior female.

31
00:01:23,856 --> 00:01:25,750
(typing)

32
00:01:25,750 --> 00:01:28,530
And in this case, both
conditions are true,

33
00:01:28,530 --> 00:01:31,890
so indeed, senior female gets displayed.

34
00:01:31,890 --> 00:01:34,410
Now the way this expression works

35
00:01:34,410 --> 00:01:38,160
is the condition to the left
of the and gets evaluated.

36
00:01:38,160 --> 00:01:40,050
If this is false then,

37
00:01:40,050 --> 00:01:42,460
the if statement terminates immediately

38
00:01:42,460 --> 00:01:44,900
and we would move onto the next statement

39
00:01:44,900 --> 00:01:46,770
after the if statement.

40
00:01:46,770 --> 00:01:48,040
If it's true,

41
00:01:48,040 --> 00:01:51,540
I still don't yet know if
the whole condition is true

42
00:01:51,540 --> 00:01:54,560
because I need both of
these conditions to be true

43
00:01:54,560 --> 00:01:56,500
for the entire condition to be true,

44
00:01:56,500 --> 00:01:58,560
so if this is true

45
00:01:58,560 --> 00:02:01,980
and age is also greater
then or equal to 65,

46
00:02:01,980 --> 00:02:04,740
which it is, then and only then,

47
00:02:04,740 --> 00:02:09,740
will this print statement
actually execute as it did

48
00:02:09,790 --> 00:02:11,580
in this particular case.

49
00:02:11,580 --> 00:02:15,940
So, what's happening in the
evaluation of this expression

50
00:02:15,940 --> 00:02:20,400
is a case of short circuit
evaluation in python.

51
00:02:20,400 --> 00:02:23,560
If the left side of this
expression were false,

52
00:02:23,560 --> 00:02:27,050
the rest of the expression
would not evaluate.

53
00:02:27,050 --> 00:02:30,050
So that's a little bit
of a performance feature

54
00:02:30,050 --> 00:02:33,040
of the and operator and that's true of the

55
00:02:33,040 --> 00:02:36,630
and operator in most sea based languages.

56
00:02:36,630 --> 00:02:38,410
Now we also have the or operator.

57
00:02:38,410 --> 00:02:41,730
So for this one, let's go
and create another variable,

58
00:02:41,730 --> 00:02:43,780
we'll call it semester average

59
00:02:43,780 --> 00:02:45,960
and we'll give it the value of 83

60
00:02:45,960 --> 00:02:49,160
and we'll create another
one called final exam

61
00:02:49,160 --> 00:02:50,870
and we'll give that 95,

62
00:02:50,870 --> 00:02:53,260
by the way, notice the naming convention

63
00:02:53,260 --> 00:02:54,980
on these two variables.

64
00:02:54,980 --> 00:02:58,170
This is from the style
guide from python code.

65
00:02:58,170 --> 00:03:01,390
When you have multi word variable names,

66
00:03:01,390 --> 00:03:03,520
they recommend for python,

67
00:03:03,520 --> 00:03:06,600
that you separate the
words with underscores

68
00:03:06,600 --> 00:03:11,600
unless you are coding for
some library that already has

69
00:03:12,270 --> 00:03:16,150
or some code base I should say,
that already has a different

70
00:03:16,150 --> 00:03:18,640
naming convention in
which case you should use

71
00:03:18,640 --> 00:03:21,990
the naming convention of that code base,

72
00:03:21,990 --> 00:03:24,570
but generally when you
have mulit word names

73
00:03:24,570 --> 00:03:28,680
you're going to separate them
with underscores for variable

74
00:03:28,680 --> 00:03:32,570
names and function names in
the python programing language.

75
00:03:32,570 --> 00:03:35,020
So let's say we want to
use these two variables

76
00:03:35,020 --> 00:03:37,790
in a condition that says,

77
00:03:37,790 --> 00:03:41,500
if the semester average is
greater then or equal to 90

78
00:03:41,500 --> 00:03:45,570
and, or, excuse me, or
the final exam grade

79
00:03:45,570 --> 00:03:49,530
is greater then or equal to 90
in either of those two cases

80
00:03:49,530 --> 00:03:52,130
we're going to give the student an A.

81
00:03:52,130 --> 00:03:53,140
So lets try that,

82
00:03:53,140 --> 00:03:56,830
so if, oops I didn't mean
to hit the tab there,

83
00:03:56,830 --> 00:04:00,950
so if semester average is
greater then or equal to 90

84
00:04:00,950 --> 00:04:02,440
or

85
00:04:02,440 --> 00:04:04,490
final exam

86
00:04:04,490 --> 00:04:07,360
is greater then or equal to 90

87
00:04:07,360 --> 00:04:10,810
in either of those two
cases or both of those cases

88
00:04:10,810 --> 00:04:12,333
we would like to print,

89
00:04:13,700 --> 00:04:15,960
quote student,

90
00:04:15,960 --> 00:04:17,580
I cannot type,

91
00:04:17,580 --> 00:04:19,653
student grade,

92
00:04:20,590 --> 00:04:23,313
or student gets and A, sorry about that.

93
00:04:24,270 --> 00:04:27,710
All right, so let's go
ahead and execute that,

94
00:04:27,710 --> 00:04:30,640
now in this case, semester average is not

95
00:04:30,640 --> 00:04:33,600
greater then or equal
to 90, so this is false.

96
00:04:33,600 --> 00:04:37,740
But it's either or, either
of the two can be true

97
00:04:37,740 --> 00:04:39,850
in order for the entire thing to be true.

98
00:04:39,850 --> 00:04:42,250
So if semester average
is false, I'm sorry,

99
00:04:42,250 --> 00:04:44,780
if the semester average is
greater then 90, is false

100
00:04:44,780 --> 00:04:45,810
or

101
00:04:45,810 --> 00:04:47,310
gets evaluated

102
00:04:47,310 --> 00:04:49,920
and I then look at the
condition to the right of that

103
00:04:49,920 --> 00:04:52,880
to see if the final exam is
greater then or equal to 90.

104
00:04:52,880 --> 00:04:57,250
In this case it is, so student
gets an A, is displayed.

105
00:04:57,250 --> 00:04:59,280
If both of these are false,

106
00:04:59,280 --> 00:05:02,290
then the entire expression is false.

107
00:05:02,290 --> 00:05:05,060
Now again, we get short
circuit evaluation,

108
00:05:05,060 --> 00:05:08,070
so if we already know
the left side is true,

109
00:05:08,070 --> 00:05:09,131
then the right side

110
00:05:09,131 --> 00:05:12,800
would not get evaluated
in that particular case.

111
00:05:12,800 --> 00:05:14,410
So with the or operator,

112
00:05:14,410 --> 00:05:17,150
if the left is true or the right is true

113
00:05:17,150 --> 00:05:20,550
or both are true, the
entire expression is true

114
00:05:20,550 --> 00:05:24,740
and the body will execute,
otherwise if both are false,

115
00:05:24,740 --> 00:05:27,173
the entire expression is false.

116
00:05:28,180 --> 00:05:30,620
Now there's one other logical operator

117
00:05:30,620 --> 00:05:33,380
or Boolean operator as
their called in python,

118
00:05:33,380 --> 00:05:36,240
which is the not operator.

119
00:05:36,240 --> 00:05:39,300
So, let's go and create a grade variable.

120
00:05:39,300 --> 00:05:42,550
We'll give it the value
87 for the moment here

121
00:05:43,420 --> 00:05:48,420
and lets do kind of a sentinel
control loop type of test.

122
00:05:49,420 --> 00:05:50,900
I'll do it with an if statement here

123
00:05:50,900 --> 00:05:53,330
just cause we don't need
a loop for the moment.

124
00:05:53,330 --> 00:05:54,940
But let's say we have the condition,

125
00:05:54,940 --> 00:05:57,970
not grade double equals minus one.

126
00:05:57,970 --> 00:05:59,260
So you may remember

127
00:05:59,260 --> 00:06:02,250
when we were doing our
sentential controlled iteration,

128
00:06:02,250 --> 00:06:06,660
we used a flag value to
indicate when to terminate

129
00:06:06,660 --> 00:06:10,040
a given loop, so in this case,
we have the not operator.

130
00:06:10,040 --> 00:06:13,560
So what's going to happen is
the not operator will reverse

131
00:06:13,560 --> 00:06:15,600
the condition to its right,

132
00:06:15,600 --> 00:06:19,200
so if grade is equal to minus one,

133
00:06:19,200 --> 00:06:21,490
this highlighted piece would be true

134
00:06:21,490 --> 00:06:24,440
and not true would evaluate to false,

135
00:06:24,440 --> 00:06:26,210
if the highlighted piece is false,

136
00:06:26,210 --> 00:06:28,280
not false would evaluate to true.

137
00:06:28,280 --> 00:06:31,740
So it just reverses the
value of the condition,

138
00:06:31,740 --> 00:06:33,590
so in this case I would print

139
00:06:34,834 --> 00:06:37,992
(typing)
the next grade

140
00:06:37,992 --> 00:06:39,020
(typing)

141
00:06:39,020 --> 00:06:42,420
is and then we'll simply output the grade.

142
00:06:42,420 --> 00:06:44,200
We could use a format string

143
00:06:44,200 --> 00:06:46,580
or an f-string if we want to here as well.

144
00:06:46,580 --> 00:06:47,480
Now at the moment,

145
00:06:47,480 --> 00:06:48,890
grade of course is

146
00:06:48,890 --> 00:06:50,960
not equal to minus one,

147
00:06:50,960 --> 00:06:53,580
so grade double equals
minus one will be false,

148
00:06:53,580 --> 00:06:55,030
not false will be true,

149
00:06:55,030 --> 00:06:57,650
and therefore this print
statement will execute

150
00:06:57,650 --> 00:07:02,650
and you see the next grade is
87 does indeed get displayed

151
00:07:04,020 --> 00:07:05,693
in that case, so,

152
00:07:06,850 --> 00:07:10,180
this is just a way to reverse
the value of a condition,

153
00:07:10,180 --> 00:07:15,180
you can also apply the not
operator to other values,

154
00:07:15,370 --> 00:07:16,203
so remember,

155
00:07:16,203 --> 00:07:20,340
every object has a true or
false value associated with it

156
00:07:20,340 --> 00:07:24,660
any numeric value that is zero
is considered to be false,

157
00:07:24,660 --> 00:07:28,930
any numeric that is nonzero
is considered to be true,

158
00:07:28,930 --> 00:07:32,540
so the not operator would
simply reverse those conditions

159
00:07:32,540 --> 00:07:37,180
and as I told you, you'll
see more complex objects

160
00:07:37,180 --> 00:07:40,160
be used as true false values in

161
00:07:40,160 --> 00:07:42,560
number of scenarios in python

162
00:07:42,560 --> 00:07:45,383
and will encounter a lot of
those as we move forward.