Class 21 – Monday March 4

You are so controlling

While I can go on – I do not repeat myself – Always a new way

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Look both ways

• Back or ahead

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Agenda

• Introduce the while statement and foreshadow dict

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To be considered

• How would you handle the problem of getting a yes or no reponse from a user, where case of the letters do not matter. Once gotten, the valid response is echoed (printed).

• Questions

• Do we know ahead of time, how many iterations are needed?

• What action(s) need to be done multiple times?

• What action(s) need to be done once?

• Some possible runs

Enter (yes / no): yeah

Enter (yes / no): yeap

Enter (yes / no): tá

Enter (yes / no): כן

Enter (yes / no): ndiyo

Enter (yes / no): haan

Enter (yes / no): YeS

yes

Enter (yes / no): nah

Enter (yes / no): không

Enter (yes / no): nope

Enter (yes / no): aʻole

Enter (yes / no): ei

Enter (yes / no): NO

no

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Whiling away

• Like many programming languages, Python provides more than one way to iterate actions (repeatedly execute). We are concerned here with its while looping offering.

• The while loop differs from the for loop in that the number of times it iterates is not based on the size of a list or range; instead, iteration is based on a logical test expression.

• The basic while state has the form:

while ( test expression ) :

  action

• When Python reaches a while loop, it will repeatedly execute the loop's actions as long as the test expression evaluates to true. When the test expression evaluates false, the program drops down to the statement following the loop. For example, consider yes_or_no.py

looking_for_a_yes_or_no = True

while ( looking_for_a_yes_or_no ) :

  reply = input( 'Enter (yes / no): ' )

  reply = reply.lower()

  if ( reply in [ 'yes', 'no' ] ) :

  looking_for_a_yes_or_no = False

print( reply )

• FYI: Python is one of the languages whose while statement also allows an optional else. The else action is executed once the test expression evaluates to false.

while ( test expression ) :

  action

else :

  action

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Program guess_what.py

• Play a guessing name with the user. The program is to determine what number the user has guessedl, where the possible numbers are from 0 to 100,000.

• Questions

• How would you solve the problem if you were the guesser?

• Can we do this with a for loop? If, so what are some possible ways?

• Can we do this with a while loop? If, so what are some possible ways?

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Program even_odd.py

• Introduce the Python dict data structure

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Program que_dijiste.py

• Reads and prints the babelfish csv-based translation dictionary

Problems

• Program guess_what.py

• Program even_odd.py

• Program que_dijiste.py

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To do

• Homework 18 — due Thursday

• Homework 19 — due Tuesday after Spring break