Class 20 — Friday, October 8

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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Joke of the day

• Why do some computer scientists die while washing their hair?

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Agenda

• Consider range() variations

• Introduce while statement

• Continue exploration of if statement

• Review test

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To do

• Examine artifacts

• Examine while epistle

• Prepare for next class by looking at dict module documentation

• Homework

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Examples

• Program arrangements.py

• Program yes_or_no.py

• Program guess_what.py

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Problem arrangements.py

• Fun fact: a range is a type of sequence. Other sequences we have used have been strings and lists.

• Up to now whenever we have had a range() loop, we always had two arguments handed to range(). range() can also handle having one or three arguments.

• If a single argument n is given to range(), then Python treats it as range( 0, n ).

• If three arguments are given to range() as in range( m, n, k ), then

• If k is positive, then

• The start value is m; the stop value is less than n; and the other values go up in steps of k

• If k is negative, then

• The start value is m; the stop value is greater than n; and the other values go down in steps of k

• Another fun fact: we can a range sequence into a list sequence using the built-in function list() on a range.

• Some program runs

Enter integers m, n, and k: 3 8 2

i: 3

i: 5

i: 7

list( range( m, n, k ) ): [3, 5, 7]

Enter integers m, n, and k: 10 6 -3

i: 10

i: 7

list( range( m, n, k ) ): [10, 7]

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For your consideration

• 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).

• Brainstorming

• Do we know ahead of time, how many often we need to loop?

• Are there action(s) that need to be done multiple times?

• Are there action(s) that need to be done once?

• Some possible runs

Enter (yes / no): YES

yes

Enter (yes / no): No

no

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.

• 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

looking_for_a_yes_or_no = True

while ( looking_for_a_yes_or_no == True ) :

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

  reply = reply.lower()

  if ( reply in [ 'yes', 'no' ] ) :

  looking_for_a_yes_or_no = False

print( reply )

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Problem yes_or_no.py

• Solicit's a yes or no response from it's user.

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Problem 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.

• Brainstorming

• 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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