Class 19 — Wednesday March 2

Introduction to "if" and "else"

"Fall in love with the process and the results will follow" -Eric Thomas

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Look both ways

• Back and ahead

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Agenda

• Being logical

• if statement

• else keyword

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Downloads

• Program logical_operators.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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Yesterclass

• Introduced the Python relational operators: <, <=, >, >=, ==, and !=.

• Introduced the Python logical operators: and, or, and not

• Today, we will wrap up with two more operators: in, and not in

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To do

• Homework

• Review artifacts

• Read epistle on decision making

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Program logical_operators.py

• Introduces the logical operators

Program run

##### Logical values (bool)

True

False

<class 'bool'>

<class 'bool'>

##### 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 code to act differently, depending upon the circumstances.

• To support this kind of problem solving, Python provides the if statement.

• 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 sequences of statements (ie lines of code).

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

• Note that the for loop allowed us to execute the same lines of code more than one time. The if and else statements allows us to completely skip lines of code depending on our inputs.

• That is, we choose whether or not to execute action₁ based on some logical expression (think True/False test question).

• 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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Problem begins_with_a_vowel.py

• Report whether user text begins with a vowel. The output message should be 'Text begins with vowel' or 'Text does not begin with vowel'.

• General problem solving strategy — keep things simple.

• First idea — clean up text by stripping and converting to lower case.

• Some program runs

Enter text: apple

Text begins with vowel

Enter text: banana

Text does not begin with vowel

Enter text: Orange

Text begins with vowel

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Problem daisy_daisy.py

• Report message based on indicated day of week.

• The message should be 'Weekend day' for Sunday or Saturday, the message should be 'Start of school week' for Monday, the message should be 'End of school week' for Friday, and 'School day', Tuesday, Wednesday, or Thursday.

• Some program runs

Enter day of week: Sunday

Weekend day

Enter day of week: monday

Start of school week

Enter day of week: TUESDAY

School day

Enter day of week: Wednesday

School day

Enter day of week: Thursday

School day

Enter day of week: Friday

End of school week

Enter day of week: Saturday

Weekend day

 

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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.