Class 21 — Monday March 14

Introduction to "while"

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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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Agenda

• Introduce dict

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To do

• Examine artifacts

• Examine dict module documentation

• Examine collections epistle

• Examine while epistle

• Homework

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Examples

• Program romantic.py

• Program unromantic.py

• Program floradation.py

• Program even_odd.py

• Program good_doggie.py

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Some data sets

• romans.csv

• babelfish

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Problem romantic.py

• Get a roman numeral and determine its integer equivalent

Some program runs

Enter Roman numeral: V

5

Enter Roman numeral: d

500

Enter Roman numeral: 0

Unknown

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Problem unromantic.py

• Get a word and print its English equivalent

Some program runs

RecursionError: maximum recursion depth exceeded during compilation

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Discussion

• Repeat the word carrot ten times out loud

• The human brain is great with associations; e.g., many of us associate apple with red, sky with blue, and programming with fun.

• A dict is Python's software equivalent of a collection of associations. In particular, a dict is an un-ordered collection of associations.

• The elements of a dict are called mappings. A mapping associates one object (called the key) with another object (called the value).

• The literal for an empty dict is {}.

• A literal mapping is written using the : to separate the key with its value. For example, the below variable flora_to_color is a dict that associates flora with colors.

flora_to_color = { 'spruce': 'green', 'apple': 'red', 'shamrock': 'green',

  'banana': 'yellow', 'potato': 'brown', 'eggplant': 'purple' }

• The len() function works on a dict. For example, len( flora_to_color ) is 6, because the definition of flora_to_color specified six mappings.

• The brackets [] operator in conjunction with assignment can be used to put a mapping into a dict. For example, the statement below adds the mapping 'lemon' to 'yellow'.

flora_to_color[ 'lemon' ] = 'yellow'

• The keys of a dict are always distinct. So, the following statement

flora_to_color[ 'potato' ] = 'golden'

would cause flora_to_color to lose its association of mapping potato to brown — its association with 'potato' is now only to golden.

• As our flora_to_color example shows, the values of a dict are not necessarily unique.

• Python dict's have functions keys() and values() for getting a hold of keys and values.

the_keys = flora_to_color.keys()

the_values = flora_to_color.values()

While there are never any duplicates in the keys, depending upon the dictionary there may be duplicates in the values.

• Other information about dict objects are listed below. In the below, assume that d is a dict.

• d.get(k) returns the value that key k maps to in d. If there is no such value, returns None.

• d[k] returns the value that key k maps to in d. If there is no such value, an exception (error) occurs.

• (k in d) indicates whether k is a key value in d.

• (k not in d) indicates whether k is not a key value in d.

• d.clear() removes all mappings in d.

• del d[k] removes k’s mapping in d.

• Information on a dict is available in the module documentation section of the website.

• Additional information on collections is in general is available in the collections epistile.

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Problem even_odd.py

• Introduce the Python dict data structure

Some program runs

Enter a number: 3

3 odd

Enter even or odd: even

a number is even if its remainder divided by 2 is 0

Enter a number: 6

6 even

Enter even or odd: odd

a number is odd if its remainder divided by 2 is 1

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Problem good_doggie.py

• For a series of dog information requests, the program reports what it knows about the dog.

• Its dog information data source is located at URL

http://www.cs.virginia.edu/~cs1112/datasets/csv/puppies.csv

• Brainstorming

• How should we store the dog information?

• How do we get and store the input?

• How many times do we set up the dog information

• What times of loops do we need?

• Sequential

• Nested

• Mixture of nested and sequential

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Where am I

 

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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.