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CS 149

Professor: Alvin Chao

What kind of soup is made with 1’s and 0’s?� Beef Boolean

Next week:�Tues 11pm Week 3 Reading

Wed 11pm HW3

Thu Actual Quiz 1

Fri 11pm Lab 6

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Boolean Expressions

  • The data type bool has two values: True and False. Boolean expressions are built using relational operators and conditional operators.

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Relational Operators

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Comparison Operators

Comparison Operators - are used to compare two values:

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and Returns True if both statements are true

�or Returns True if one of the statements is true

�not Reverse the result, returns False if the result is true�

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Boolean Expressions

  • An expression that evaluates to true or false�Examples:
    • X < 7
    • A == B
    • 3 != 4

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Model 1 Relational Ops

Interactions

Value displayed

Relational Operator

three = 3

none

none

four = 4

none

none

print(four)

4

none

three > four

False

>

is_larger = three > four

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print(is_larger)

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three == four

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three < four

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three <= four

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three = four

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three == four

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Equality comparison

three = 3

trey = 3

three_point = 3.0

three_str = “3”

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What do these evaluate to?

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three == trey

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three == three_point

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three_point == three_point

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three == three_str��.1 + .2 == .3

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If Statements

  • Syntax:

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  • Examples:

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if(boolean_expression):� Statement or block

else:

Statement or block

if(performance > 80):

bonus_pay += 1000���if(performance > 80):

bonus_pay +=1000�else:

print(“You’re fired.”)

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If examples

x = 10

hat = "fez"

if x > 100 or hat == "fez":

print("A")

if x < 50:

print("B")

else:

print("C")

else:

print("D")

if hat == "fedora":

print("E")

print("F")

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x = 10

if x < 100:

print("A")

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elif x < 50:

print("B")

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else:

print("C")

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print("D")

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Danger…

  • What’s wrong with this code?

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if(performance > 80):

print(“Nice work”)

bonus_pay += 1000

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Prevention

  • Style guide / proper indentation.

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We indent the second line to make it a part of the if block

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if(performance > 80):

print(“Nice work”)

bonus_pay += 1000

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To prevent the mistake from the previous slide

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Empty Blocks = bad style

  • These are all functionally equivalent, which is better?

# A

if (performance <= 80):

pass

else:

bonus_pay += 1000

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# B�if (performance > 80):

bonus_pay += 1000

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# C�if (performance > 80):

bonus_pay += 1000

else:� pass

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Conditionals

Boolean expressions may also use conditional operators to implement basic logic. Relational operators are always executed first so usually no need for parentheses��Operators: not, and , or��

If all 3 operators appear in the same expression, Python will evaluate the not first the and and finally or. If there are multiples of the same operator they are evaluated from left to right.��EXAMPLE VARIABLES EXAMPLE EXPRESSIONS

a = 3 a < b and funny�b = 4 a < b and b < c�c = 5 funny and a < c�funny = True not funny or weird�weird = False���

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  • Acknowledgements

Parts of this activity are based on materials developed by Chris Mayfield and Nathan Sprague.

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