Class 18 — Monday October 4

Logical values and decision

May the light of peace — For you and your families — Always shine brightly

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Look both ways

• Back and ahead

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Agenda

• Being logical

• if statement

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Transitioning

• Some problem solving requires the ability to react — depending upon the circumstances different conditions need actions to be performed. Python provides the if statement for decision making.

• The if statement uses logical test expressions to determine which action to perform. There are variants of the if statement to make decisions based on one, two, or several possibilities.

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Downloads

• Program logical_operators.py

• Program can_you_relate.py

• Program acid_test.py

• Program what_color_will_my_chrysanthemums_be.py

• Program begins_with_a_vowel.py

• Program robotomy.py

• Program daisy_daisy.py

• Program spell_check.py

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To do

• Homework

• Review artifacts

• Read epistle on decision making

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Program can_you_relate.py

• Introduces the comparison operators

Three program runs

Enter number: 12

Enter number: 11

12 < 11 : False

12 > 11 : True

12 <= 11 : False

12 >= 11 : True

12 == 11 : False

12 != 11 : True

Enter number: 3

Enter number: 14

3 < 14 : True

3 > 14 : False

3 <= 14 : True

3 >= 14 : False

3 == 14 : False

3 != 14 : True

Enter number: 7

Enter number: 7

7 < 7 : False

7 > 7 : False

7 <= 7 : True

7 >= 7 : True

7 == 7 : True

7 != 7 : False

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Program logical_operators.py

• Introduces the logical operators

Program run

##### Logical values (bool)

True

False

##### Logical and

True and True = True

True and False = False

False and True = False

False and False = False

##### Logical or

True or True = True

True or False = True

False or True = True

False or False = False

##### Logical negation

not True = False

not False = True

##### Logical includes

1 in [3, 1, 4, 1, 5] = True

2 in [3, 1, 4, 1, 5] = False

##### Logical exclusion

1 not in [3, 1, 4, 1, 5] = False

2 not in [3, 1, 4, 1, 5] = True

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The why, what, and how of the basic if statement

• General problem solving requires depending upon the circumstances, to perform different actions.

• To support this kind of problem solving, Python provides the if statement. Python does not provide any other decision statements.

• The if statement uses logical test expressions to determine which action to perform.

• The most common form of the if statement is

if ( logical expression ) :

    action₁

else :

    action₂

• Where logical expression is a test expression that evaluates to either True or False.

• Both action₁ and action₂ are non-empty sequence of statements.

• The actions are indented one-level further than the start of the if statement.

• The else is indented at the same level as the if. Both if and else are keywords:

• The semantics of the if statement begins with the logical expression being evaluated.

• If the expression evaluates to True, action₁ is executed. Otherwise, action₂ is executed.

• No matter what only one of the two actions of an if-else statement is executed.

• The else part of the if statement is optional.

if ( logical expression ) :

    action

• If the logical expression evaluates to True, the action is executed. Otherwise, no action occurs.

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Program acid_test.py

• Determine whether its input pH level is acidic; i.e., less than 7.0

Three program runs

Enter pH level: 6.5

True

Enter pH level: 7.0

False

Enter pH level: 7.5

False

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Program what_color_will_my_chrysanthemums_be.py

• Uses an if statement to help determine the color of a chrysanthemum based on soil pH.

• Some program runs

Enter soil pH level: 6.5

pink

Enter soil pH level: 7

blue

Enter soil pH level: 7.5

blue

 

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