Class 12 – Monday, February 14

Loops and Indexing

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Look both ways

• Back or ahead

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Agenda

• Indexing a list within a loop

• Loop chrestomathics

• Extend print() function functionality

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Downloads

• Submitted Classwork: Program half_scores.py

• Determine average score for first or second half of semester.

• Program on_average.py

• Determine average word length

• Program thats_printastic.py

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

• Program c_looper.py

• Foreshadow nested looping

• Program secret_decoder.py

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

• Program going_rowing.py

• Introduce datasets

• Demonstrate looping through a dataset

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Program half_scores.py

• Problem

• The program takes 8 numbers of input, which represent a student's quiz scores in a course. Then, the program calculates the student's average score for the first 4 quizzes, and also calculates their average for the last four quizzes, and prints both averages out.

• Realizations

• To compute the average of both halves of quizzes, we cannot as easily loop through the full list of numbers.

• If you come up with an answer with no loops, try to come up with a second answer that incorporates loops.

• One Possible Algorithm

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

• Split input into a list of strings.

• Create a list to store the ints for the upcoming loop. (accumulator)

• Process each string in the string list: (loop)

• Cast string to int, then append it to the int list.

• Create a running total variable, which we'll add each of the first four tests to (accumulator)

• Process each number in the first half of the list (indices 0 through 3): (loop)

• Add that number to the running total of the first quiz scores.

• Compute average by dividing by '4.0'.

• Print average.

• **Do everything again for the second half of the quiz scores.

• Create a running total variable, which we'll add each of the last four tests to (accumulator)

• Process each number in the second half of the list (indices 4 through 7): (loop)

• Add that number to the running total of the last quiz scores.

• Compute average by dividing by '4.0'.

• Print out both averages.

• A sample program run

Enter scores: 100 90 85 60 77 44 99 100

First half score: 83.75

Second half score: 80.0

• Please make sure to submit your completed program! :)

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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 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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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.