Class 20 — Frdiay February 28

Decisions made easy

I am positive — My favorite color is hue — Easy decision

Put cell phones away — Please do no multitasking — Or undertasking

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Look both ways

• Back or ahead

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Agenda

• Introduce the if statement

• Searching accumulation

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To do list

• Really examine the artifacts

• Read epistle on decision making

• Do homework

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Examples

• Problem acid_test.py

• Problem what_color_will_my_chrysanthemums_be.py

• Problem robotomy.py

• Problem daisy_daisy.py

• Problem who_is_the_longest_of_them_all.py

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Last class (baby step) problem acid_test.py — building a better chrysanthemum

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

Some program runs

Enter pH level: 6.5

True

Enter pH level: 7.0

False

Enter pH level: 7.5

False

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Expanded problem solving

• Some problem solving requires the ability to react — depending upon the circumstances, different actions need to be performed.

• 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.

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

• The most common form of the if statement is

if ( logical expression ) :

    action₁

else :

    action₂

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

• Both action₁ and action₂ are non-empty statement lists.

• 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 is True, action₁ is executed; otherwise,

• action₂ is executed.

No matter what only one of the two actions are executed.

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Problem what_color_will_my_chrysanthemums_be.py

• Determine chrysanthemum color given an input soil pH level, where acidic soil produces pink chrysanthemums and non-acidic soil produces blue chrysanthemums.

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 daisy_daisy.py

• Report message based on indicated day of week. The message should be weekend day for Sunday and for Saturday, the message should be 'start of school week' for Monday, the message should be 'end of school week' for Friday, and 'school day', otherwise.

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

• 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.

• Observation

• 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 who_is_the_longest_of_them_all.py

• Determine the longest word in user text. A constraint on your solution is that the only permitted list is the list of input words.

• WWAHD strategy

• Scan through the words one-at-a-time and determinewhether the current word is the longest word so far.

Some possible program runs

Enter text: Think before I act — My problem solving mantra — Make it your also

problem

Enter text: Remember always — Seek to be both great and good — The world needs your aid

Remember

Enter text:

 

Brainstorming

• Because we do not know the number of words to be given when we are writing our solution, we definitely need a loop that can iterate over the list of input words.

• When we process a word we need to be able to determine whether it is in contention for being the longest word.

• If it is the longest word we seen so far, then we need to remember it in order to compare it to the remaining words.

• If it is not the longest word we seen so far, we take no further interest in it.

• For such loop processing to work, we need an accumulator variable keeping track of the longest word found so far. There are two obvious candidates for initializing that variable — making it the empty string or making it the first word. Either can work and there are good arguments for both of them.

• One thing for sure, the variable keeping track of the longest word so far needs to be initialized before the loop — because during the loop we are updating it (sometimes).

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Where am I

 

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© 2020 Jim Cohoon

Resources from previous semesters are available.