Class 13 – Wednesday, Wednesday February 16

Loops and Indexing

An optimist thinks — Elevator close button — Really does something

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Look both ways

• Back or ahead

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Zooming

• Link

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Agenda

• Indexing a list within a loop

• Loop chrestomathics

• Extend print() function functionality

• Assign homework

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Word of the day

• saippuakivikauppias

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Shirt of the day

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Downloads

• Program on_average.py

• Determine average word length

• Program secret_decoder.py

• Chrestomathics — display a secret message using a code phrase and indices

• Program thats_printastic.py

• Introduce print() optional closing parameter end="..."

• Program c_looper.py

• Foreshadow nested looping

• Program going_rowing.py

• Introduce datasets

• Demonstrate looping through a dataset

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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

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Program secret_decoder.py

• Scenario

• Separately get an input code phrase and list of indices. Determine the hidden message by using the indices to peek into the code phrase. For example, suppose the code phrase equals

'abcdefghijklmnopqrstuvwxyz-'

and the indices are

[ 4, 8, 26, 4, 8, 26, 14 ]

• Then the hidden message is

'ei-ei-o'

Because s[ 4 ] equals 'e';   s[ 8 ] equals 'i';   s[ 14 ]; equals 'o';   and s[ 26 ] equals '-'.

• Algorithm

• Get the code phrase that contains hidden message.

• Clean up reply to get code phrase with no leading or trailing blank space.

• Print the code phrase.

• Get the list of indices (as string) for peeking into code phrase.

• Convert reply into a list of numeric strings.

• Build one-by-one the list of indices out of the numeric strings .

• Print the list of indices.

• Build secret message (string) by peeking into code phrase using the indices.

• Print secret message.

• Some program runs

Enter code phrase: computer

Code phrase: computer

Enter indices: 3 6 5

Indices: [3, 6, 5]

Hidden message: pet

Enter code phrase: gosh look what the cat is up to now

Code phrase: gosh look what the cat is up to now

Enter indices: 24 5 17 17 27 23 32 0

Indices: [24, 5, 17, 17, 27, 23, 32, 0]

Hidden message: sleeping

Enter code phrase: ‐abcdefghijklmnopqrstuvwxyz‐

Code phrase: ‐abcdefghijklmnopqrstuvwxyz-

Enter indices: 12 15 22 5 27 9 19 0 1 12 12 27 14 5 5 4

Indices: [12, 15, 22, 5, 27, 9, 19, 0, 1, 12, 12, 27, 14, 5, 5, 4]

Hidden message: love‐is‐all‐need

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Program thats_printastic.py

• Introduce print() optional closing parameter end="..."

• The optional end="..." parameter for a print() function allows changing from its default action of starting the next print on a new line.

• Program run

Wa-Hoo-Wa

Rah-Rah-Rah

Wa-Hoo-Wa!Rah-Rah-Rah

Wa-Hoo-Wa! Rah-Rah-Rah

s-a-i-p-p-u-a-k-i-v-i-k-a-u-p-p-i-a-s-.

s a i p p u a k i v i k a u p p i a s .

saippuakivikauppias.

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Program c_looper.py

• Foreshadow nested looping

• For a user-supplied n, prints a line of the form

row : 0 1 2 ... n-1

• Some program runs

Enter number of columns: 5

row : 0 1 2 3 4

Enter number of columns: 12

row : 0 1 2 3 4 5 6 7 8 9 10 11

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Program dataset_intro.py

• Demonstrate looping through a dataset in different ways

• Program run

table: [['A', 'B', 'C'], ['D', 'E', 'F'], ['G', 'H', 'I'], ['J', 'K', 'L', 'M']]

the table has 4 rows

row ['A', 'B', 'C'] has 3 columns

row ['D', 'E', 'F'] has 3 columns

row ['G', 'H', 'I'] has 3 columns

row ['J', 'K', 'L', 'M'] has 4 columns

row 0 : ['A', 'B', 'C']

row 1 : ['D', 'E', 'F']

row 2 : ['G', 'H', 'I']

row 3 : ['J', 'K', 'L', 'M']

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Lutefisk

 

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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.