Class 21 — Monday October 12

You are so controlling

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

---

Look both ways

• Back or ahead

---

Agenda

• Introduce while statement

• Introduce dict

---

To do

• Examine artifacts

• Examine dict module documentation

• Examine collections epistle

• Homework

---

Redownload

• Module url.py

---

Examples

• Program guess_what.py

• Program even_odd.py

• Program romantic.py

• Program unromantic.py

• Program good_doggie.py

---

Maxims (among others)

• Read the problem in its entirety before problem solving

• Determine for yourself why the sample problem solutions are as they are

• Problem solve before you code

• Check whether your solution produces the same answers as the sample runs

• Check whether your solution matches problem specifications

---

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

---

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 == True ) :

  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

---

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?

---

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

---

Problem unromantic.py

• Get a word and print its English equivalent

Some program runs

RecursionError: maximum recursion depth exceeded during compilation

---

Discussion

• Repeat the word carrot ten times out loud

• The human brain is great with associations; e.g., we typically associate apple with red and January 1st with New Year's Day.

• A dict is Python's software equivallent of a collection of associations.

• In particular, a dict is unnordered collection of associations.

• The elements of 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 variable flora_to_color defined below 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 only associaton with 'potato' is 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 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 always available in the module documentation section of the website.

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

---

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

---

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

---

 

---

© 2020 Jim Cohoon

Resources from previous semesters are available.