Class 10 – Wednesday, September 16

Listing

Lest we go forward — We cannot come back again — But beware of baths

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

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Look both ways

• Back or ahead

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Agenda

• Make loops right

• Provide you more background on list basics

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To do list

• Review class artifacts.

• Review epistles

• Complete the big_production.py homework

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Downloads

• Program first_name.py

• Introduction to for character in a string loop.

• Program your_name.py

• Introduction to for word in text loop.

• Program esrever.py

• Performs string accumulation using a loop.

• Program on_average.py

• Performs summing accumulation using a loop.

• Program all_int.py

• Performs list accumulation using a loop.

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Looping through the characters in a string — program first_name.py

• Suppose what we want do is process the letters in a string one-by-one. Such an algorithm would like

• Python provides the for loop for doing such processing. It would like the following

for current_letter in string_of_interest :

  # process the current letter

  ...

• Variable current_letter is called the loop variable or iterator.

• The block of code indented below the for statement line is the action performed each time through the loop. The block of code is called the body of the loop.

• The body of this loop is executed once for each letter in string_of_interest.

• The first time through the loop, current_letter has the value of the first letter in string_of_interest; the next time, it has the value of the second letter in string_of_interest; and so on.

• If our program task is to just print the letter one-by-one for a user-supplied first name. It would like the following

# get name

name = input( "Enter first name: " )

# print the letters in the name one-by-one

for current_letter in name :

  # process the current letter by printing it

  print( current_letter )

• Some possible program runs could be

Enter name: Annie

A

n

n

i

e

Enter name: Kim

K

i

m

Enter name: Spenser

S

p

e

n

s

e

r

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Looping through the items of a list — program your_name.py

• Suppose what we want do is process the items in a list one-by-one. Such an algorithm would like

• The for loop for can do such processing. The entity following the keyword in is not limited to strings, it can be any type of sequence (e.g., a list of words or numbers, or a range of numbers).

for current_item in list_of_interest :

  # process the current item

  ...

• For this loop, current_item is the loop variable and list_of_interest is the sequence of interest.

• The body of the loop is executed once for each item in list_of_interest.

• The first time through the loop, current_item has the value of the first item in list_of_interest; the next time, it has the value of the second item in list_of_interest; and so on.

• If our program task is to individually print the words for a user-supplied name accompanied with their lengths, the program would like the following

# get name

text = input( "Enter name: " )

# split the text into a list of words

list_of_words = text.split()

# print the words in list_of_words accompanied by their lengths

for current_word in list_of_words :

  # process the current word

  length_of_current_word = len( current_word ) # determine current word length

  print( current_word, ':', length_of_current_word ) # print wanted info

• Some possible program runs could be

Enter name: Spiro Aurelius Evander Zambini

Spiro : 5

Aurelius : 8

Evander : 7

Zambini : 7

Enter name: Willie Howard Mays Jr.

Willie : 6

Howard : 6

Mays : 4

Jr. : 3

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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, add (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.

• To build the reverse of a string, each character in the string needs to be part of the reverse. The reverse is gotten by repeatedly adding (accumulating) the characters to the reversal. For this to work out, the reversal needs to be initialized before the characters in the string are considered. A string accumulator needs to be intialized to the empty string ''.

• 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.

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

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

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String accumulation — program esrever.py

Task

• Computes the reverse of a user-suppplied string

Realizations

• Because the length of text is not know when the program is being written, a loop is needed to process the input characters.

• Each input character contributes to the reversal

• The accumulator need to be initiliazed to the empty string ''.

Some program runs

Enter text: tab

bat

Enter text: stressed

desserts

Enter text: abcdefghijklmnopqrstuvwxyz

zyxwvutsrqponmlkjihgfedcba

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Totalling accumulation — program on_average.py

Task

• 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.

Two program runs

Enter text: over the hill and through the dale

Average word length: 4.0

Enter text: row row row your boat

Average word length: 3.4

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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.

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

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Hoos got your back — food insecurity

• Nationwide more than 10% of college students are not able to afford nutrious food. This problem occurs at UVA also. I am aware of two sources at UVA to help out.

• The Engineering School has a food pantry in Thornton A121. It operates on the honor system and is meant to help students with need.

• The unversity assists with the Community Food Pantry in the Runk Green Room. See the website for details.

• If you find yourself experiencing some financial hardships, please consider talking with anyone in the Undergraduate Programs Office. They can help you find even more resources.

• If you can pay it forward, please help by donating items or money to either pantry.

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Coney Island Cyclone roller coaster

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Willy Mays

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Marcus Aurelius

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Evander of Pallantium

 

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© 2020 Jim Cohoon

Resources from previous semesters are available.