Class 19 — Wednesday, October 6

Decisions made easy

Put cell phones away — Please do no multitasking — Or under-tasking

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Look both ways

• Back or ahead

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Agenda

• Study the if statement

• Introduce searching accumulation

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To do list

• Truly examine class artifacts

• Read epistle on decision making

• Do homework

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Examples (redownload)

• Program begins_with_a_vowel.py

• Program daisy_daisy.py

• Program robotomy.py

• Program spell_check.py

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Yesterclass

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

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

• Used relational operator < to help determine whether a soil pH sample was acidic.

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

Enter soil pH level: 6.5

pink

Enter soil pH level: 7

blue

Enter soil pH level: 7.5

blue

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The why, what, and how of an if statement — revisited and expanded.

• 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.

• There are variants of the if statement to make decisions based on one of several possibilities.

• When we need to test several conditions to determine the appropriate action to take, we can use elif's to test for the various possibilities (when you see an elif think else if ).

if ( logical expression₁ ) :

    action₁

elif ( logical expression₂ ) :

    action₂

...

elif ( logical expression_k ) :

    action_k

else : # optional

    action_k+1

• The semantics of the if-elif-if statement begins with logical expression₁ being evaluated.

• If the logical expression evaluates to True, its action is executed and the if statement is exited.

• Otherwise, the elif's are evaluated one after the other until one evaluates to True. That elif's action is executed and the if statement is exited.

• If none of the elif's logical expressions evaluate to True,

• If there is an else part, then its action is executed and the if statement is exited.

• If there is no else part, no action is taken.

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

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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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Problem robotomy.py

• Task — determine what happens when a robot comes knocking on a door

• If the door is locked, the robot should be instructed to turn around.

• If the door is instead unlocked, the robot should be instructed to open the door and enter the room. Before entering, the robot should determine whether the light is off. If it is off, the robot should be instructed to first turn on the light.

• Observations

• There is one procedure to perform if the door is locked, another to perform if it is unlocked

• The procedure to perform if the door is unlocked, requires analysis whether the light is on.

• Some program runs

Door (locked / unlocked): locked

Turn around

Door (locked / unlocked): Unlocked

Open the door

Light (on / off): on

Enter the room

Door (locked / unlocked): unlocked

Open the door

Light (on / off): Off

Turn on the light

Enter the room

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Problem spell_check.py

• Determine misspelled words in user text. To support this activity, there is a GitHub-acquired web file with the 10,000 most commonly used English words on the webs. For those interested, a more-complete list of 173,541 English words is available.

• WWAHD (what would a human do) strategy — scan through the user text and determine whether the current word is not in the correct-spelling word list.

• Some program runs

Enter text: It was the beste of times It was the wurst of times It was Oktoberfest

beste

wurst

oktoberfest

Enter text: how much wod can a woodchuck chuck

wod

woodchuck

Enter text: to be or not to be that is the question

Enter text:

• Brainstorming

• To accomplish the task there are two streams of information to process.

• One stream is the input text; the other stream is the list of word spellings. Processing these streams requires different actions, so we need to be sure when we read from a stream, that we pick the right one.

• To make a decision on a word, we need to know if it’s in the list of correctly-spelled words. This fact tell us that we should get the list of word spellings before we get the user text — it would not really help to get the words first, and it would be a real mess to interleave the stream reading.

• Together, the above tell us a plan of action.

• Import our url module.

• Specify the URL for the spelling dictionary.

• Read the contents of the URL and store it as a list of word spellings.

• Get the user text and convert it into a list of words.

• For each user word, determine whether it is correctly spelled. If it is incorrectly spelled, print it out.

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Decision making

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Where in the world am I

 

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