Class 10 – Wednesday, September 15

Listing

Lest we go forward — Without knowing about lists — We would be wanting

---

Look both ways

• Back or ahead

---

Agenda

• Remainder of string manipulation

• Make loops right

• Provide you more background on list basics

---

Alert

• Anyone who asks me today about built-in function sum() earns negative ducks.

---

To do list

• Review epistles

• Complete homework

• Tell me about lists

• Tell me more about lists

---

Downloads

• Program countability.py

• Introduces string method count()

• Program replacement.py

• Introduces creating new strings through replacement

• Program slice_of_pie.py

• Introduces string slicing

• Program wowzers.py

• Loop through and analyze lists

• Program on_average.py

• Determine average word length

• Program all_int.py

• Converts a list of integer inputs in to a list of integers.

• Program summing.py

• Accumulate a sum of values

---

Program countability.py

• Introduces string function count()

• A possible program run

Enter text: any bananas

Enter substring: an

Enter index: 7

text.count( substring ): 3 # count all

text.count( substring, 7 ): 1 # count starting from index 7

---

Program replacement.py

• Purpose

• Introduces string function replace()

• Some possible program runs

Enter text: yellow bellied sapsucker

Enter substring (s): ll

Enter substring (r): L L

text.replace( s, r ): yeL Low beL Lied sapsucker # text's s's replaced with r's

Enter text: yellow bellied sapsucker

Enter substring (s): e

Enter substring (r):

text.replace( s, r ): yllow bllid sapsuckr # text's s's replaced with r's

---

Program slice_of_pie.py

• Purpose

• Introduce string slicing

• Three program runs

Enter favorite pie: chocolate

Enter two indices i and j: 3 8

s[ i : j ]: colat

s[ i : ]: colate

s[ : j ]: chocolat

s[ : ]: chocolate

Enter favorite pie: pizza

Enter two indices i and j: 2 3

s[ i : j ]: z

s[ i : ]: zza

s[ : j ]: piz

s[ : ]: pizza

Enter favorite pie: garlic tart

Enter two indices i and j: 1 8

s[ i : j ]: arlic t

s[ i : ]: arlic tart

s[ : j ]: garlic t

s[ : ]: garlic tart

---

Program wowzers.py

• Task

• Loop through a string, list of words, and a list of numbers

• Analyze a list using built-in functions

• Possible program run

Enter your favorite color: teal

Enter some names: willy evander aurelius ann

color = teal

names = ['willy', 'evander', 'aurelius', 'ann' ]

numbers = [12, 7, -11, -12, 59]

T

E

A

L

Willy

Evander

Aurelius

Ann

-12

-7

11

12

-59

t e a l

['willy', 'evander', 'aurelius', 'ann' ]

[5, 7, 8, 3]

length of longest name: 8

first alphabetically = ann

---

Accumulation

• Many times problem solving requires we analyze some sequence (e.g, string or list) to determine a property or build a new sequence. The determination can require each element in the sequence, contribute (accumulate) to the answer. For example, from text determine average word length or from numbers determine their absolute values or from a string determine its reverse.

• If the answer requires the items in list contribute to a total, then an accumulation is needed of the individual terms — the accumulator is updated by repeatedly adding to it. For this to work out, the accumulator needs to be initialized before the list is processed. For totaling, the accumulator is initialized to zero.

• Data processing can require a list be built out of some input data. Here, the accumulator needs to be initialized to the empty list [].

• Sometimes data processing can require the product of some numeric data. Here, the accumulator needs to be initialized to 1.

• Data processing can require a string be built character by character. For this to work out, a string accumulator needs to be intialized to the empty string ''.

---

Program on_average.py

• Problem

• Examines a list of user-supplied words and reports determine average word length.

• Realizations

• Because the number of words is not know when the program is being written, a loop is needed to process the words.

• To compute the average word length, need to know the number of words and the sum of the word lengths.

• The loop needs to two things for each word: compute its length and add its length to a running total.

• The running total needs to be initialized before the loop.

• Algorithm

• Prompt and get text (i.e., a bunch of words).

• Split input into a word list.

• Echo word list to user.

• Compute the length of the list.

• Initialize running total of word lengths to 0.

• Process each word in the word list:

• Compute length of the current word of interest.

• Add that length to running total of the word lengths.

• Compute average from the word lengths total and the number of words.

• Print average.

• Two program runs

Enter text: over the hill and through the dale

words = ['over', 'the', 'hill', 'and', 'through', 'the', 'dale']

Average word length: 4.0

Enter text: row row row your boat

words = ['row', 'row', 'row', 'your', 'boat']

Average word length: 3.4

---

List accumulation — program all_int.py

• Task

• Produce a list of integers from user-supplied data.

• Realizations

• Function int() can only be applied to one string at a time.

• List of integers will need to be accumulated by processing the numeric strings one at a time.

• For each numeric string it will need to be converted to an integer and added to the list of integers being built

• The list of integers needs to be initalized to an empty list.

• Algorithm

• Prompt and get text (i.e., a list of integers).

• Split input into a list of numeric strings.

• Echo list of numeric strings.

• Initialize running list of integers to empty list

• Process each string in the numeric strings list:

• Get integer representation of the string.

• Add integer to the list accumulation

• Print accumulation.

• Some program runs

Enter integers: 3 1 4 1 5 9

nbr_strings: ['3', '1', '4', '1', '5', '9']

numbers: [3]

numbers: [3, 1]

numbers: [3, 1, 4]

numbers: [3, 1, 4, 1]

numbers: [3, 1, 4, 1, 5]

numbers: [3, 1, 4, 1, 5, 9]

Enter integers: 58 2 37 16 99 1 9 23

nbr_strings: ['58', '2', '37', '16', '99', '1', '9', '23']

numbers: [58]

numbers: [58, 2]

numbers: [58, 2, 37]

numbers: [58, 2, 37, 16]

numbers: [58, 2, 37, 16, 99]

numbers: [58, 2, 37, 16, 99, 1]

numbers: [58, 2, 37, 16, 99, 1, 9]

numbers: [58, 2, 37, 16, 99, 1, 9, 23]

• Code

# get input

reply = input( 'Enter integers: ' )

print()

# split reply into list of numeric strings

nbr_strings = reply.split()

# see what we got

print( 'nbr_strings:', nbr_strings )

print()

# convert numeric text into list of numbers

# begin with setting up the accumulator

numbers = [] # initialize holder to prepare for adding elements

# process the strings of nbr_strings one-by-one

for ns in nbr_strings :

  # process the current numeric string

  nbr = int( ns ) # get numeric equivalent of ns

  numbers.append( nbr ) # add equivalent to list of numbers

  print( 'numbers:', numbers ) # print the updated list of numbers

 

---

🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.