Class 20 – October 9

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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Problems

• Program spell_check.py

• Program yes_or_no.py

• Program guess_what.py

• Program even_odd.py

• Program que_dijiste.py

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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. Example:

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

• The while statement also allows an optional else.

while ( test expression ) :

  action

else :

  action

• The else action is executed once the test expression evaluates to false. Example usage is below.

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

else :

  print( 'your reply of', reply, 'is accepted' )

Note, I have never needed to use else for a while statement in practice.

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To do

• Homework 15