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Practical 04 - Lists

Warm-Up

File: lists_warmup.py

In your prac_04 directory, create a new Python file called lists_warmup.py and enter the following line:

numbers = [3, 1, 4, 1, 5, 9, 2]

What values do the following expressions have? Without running the code, record your answers to these questions in your Python file as a comment.
Then use the Python console to see if you were correct.

numbers[0]
numbers[-1]
numbers[3]
numbers[:-1]
numbers[3:4]
5 in numbers
7 in numbers
"3" in numbers
numbers + [6, 5, 3]

In the same Python file, write statements to achieve the following:

  1. Change the first element of numbers to "ten" (the string, not the number 10)
  2. Change the last element of numbers to 1
  3. Print all the elements from numbers except the first two (slice)
  4. Print whether 9 is an element of numbers

Walkthrough Example

Calculating a List of Cumulative Totals

Please read the whole question before starting the work!

Accountant Annie wants you to write a program to calculate the monthly cumulative totals for incomes.
The program should ask for the number of monthly incomes to enter, then get and store the incomes in a list.

When the incomes have been entered, the program should display a list of the month's income with cumulative totals.
Here's some sample output (for 5 months in this case):

How many months? 5

Enter income for month 1: 120
Enter income for month 2: 245.4
Enter income for month 3: 900
Enter income for month 4: 1205.56
Enter income for month 5: 12.35

Income Report
-------------    
Month  1 - Income: $    120.00         Total: $    120.00
Month  2 - Income: $    245.40         Total: $    365.40
Month  3 - Income: $    900.00         Total: $   1265.40
Month  4 - Income: $   1205.56         Total: $   2470.96
Month  5 - Income: $     12.35         Total: $   2483.31

Think about how to do this before reading on...

We need a list to store the incomes.
How do you add values to a list?

We need a counter variable (int) for the month number.
Remember that list indexes start at 0, but we want to print from 1.

How many loops will we need? What kind of loops?

We need a cumulative total to update as we loop through the list to display the incomes.

And lastly we need to format the output nicely, which we can use f-strings for.

Things to do:

  1. Copy the starter code from total_income.py (remember to use Raw mode) and commit it to your own prac repo:

  2. Study it to see how this code answers those questions so far.

  3. Change the line that gets the income input so that it uses an f-string instead of string concatenation (+).

  4. Problem: We have two variables that sound very similar: incomes and months. They're both plural, so they sound like they're both lists. incomes is a list of incomes, so we might assume that months is a list of months, but it's actually a scalar value that stores the number of months - an int not a list.
    Refactor the months variable to a better name.
    DO NOT just change it in 3 places or use find and replace... but use refactoring in PyCharm by clicking anywhere on the variable and pressing Shift+F6 (or use the context menu). Then rename it to something more meaningful, that sounds like a number not a list.
    When naming variables, we can say, "This variable stores..." and the completion of that statement is usually a good name.
    In this case, "This variable stores the... number of months". :)

  5. Run the program again with some sample data and make sure it still works.
    This kind of "regression testing" is important. Make sure you don't break anything!

  6. Now, let's refactor (move) the report printing into its own function. Select those 6 lines and turn them into a new function with a good name.
    When naming functions, we can say, "This function will..." and the completion of that statement is usually a good name for the function.
    In this case, "This function will... print report". :)

  7. Test again and make sure it's all good.

  8. Double-check the report printing function you just wrote. Is it well-designed according to SRP? Does it take in _ only_ what it needs to know? Refactor it if you can make it better.

Converting Data Strings to Lists

A common way of storing data is in files, which means the data is in string form. Even numbers are stored as string.
So, a data file storing subjects, lecturer and student numbers might look like:

CP1401,Ada Lovelace,192
CP1404,Alan Turing,98

We can read a file like this line-by-line (for line in file) but we also need to process each line to get the parts. A good way to do that is with the string split method.
So, line.split(',') would give us a list that contained the parts of this line... but each part would still be a string.

Things to do:

  1. Copy the starter code and data file from subject_reader.py and subject_data.txt
    Remember to commit these before modifying them any further.
    Study the starter code and run it. There are comments and print calls to show you what's happening.

  2. The code reads the file data and processes it into the parts variable, but we want the load_data function to return the data as a list of lists (nested list), like:
    [['CP1401', 'Ada Lovelace', 192],['CP1404', 'Alan Turing', 98]]
    Modify the function so that it does this.

  3. Currently, main just prints data. Add a new function to display subject details that produces something like the following:

    CP1401 is taught by Ada Lovelace and has 192 students
CP1404 is taught by Alan Turing  and has  98 students

Intermediate Exercises

Here are some small problems to give you more practice working with lists.
Feel free to check the solutions for these so that you can make sure you're on track.

Basic list operations

File: list_exercises.py

Write a program that prompts the user for 5 numbers and then stores each of these in a list called numbers.
The program should then output information about these numbers, as in the output below.
You can use the functions min, max, sum and len, and you can use the append method to add a number to a list.

   Number: 5
   Number: 20
   Number: 1
   Number: 2
   Number: 3
   The first number is 5
   The last number is 3
   The smallest number is 1
   The largest number is 20
   The average of the numbers is 6.2

Woefully inadequate security checker

Please use the same file, list_exercises.py

Copy the following list of usernames:

   usernames = ['jimbo', 'giltson98', 'derekf', 'WhatSup', 'NicolEye', 'swei45', 'BaseInterpreterInterface', 'BaseStdIn', 'Command', 'ExecState', 'InteractiveConsole', 'InterpreterInterface', 'StartServer', 'bob']

Ask the user for their username. If the username is in the above list of authorised users, print "Access granted", otherwise print "Access denied".

List comprehensions

File: list_comprehensions.py

Download/copy list_comprehensions.py
and see how the example list comprehensions work.
Write more list comprehensions (not loops) by completing the TODO instructions in the code.

Do-from-scratch Exercises

"Quick Pick" Lottery Ticket Generator

File: quick_picks.py

Write a program that asks the user how many "quick picks" they wish to generate. The program then generates that many lines of output. Each line consists of 6 random numbers between 1 and 45.
These values should be stored as CONSTANTS.

  • Each line (quick pick) should not contain any repeated number.
  • Each line of numbers should be displayed in sorted (ascending) order.
  • Study the formatting below so that numbers align neatly.

Note

Python's random module has a sample() function, which returns a selection from a list.
This is a nice way to solve this problem... but it's too easy :)
Do not use it for this program.

Your code should produce output that matches this sample output (except the numbers are random):

How many quick picks? 5
 1 12 14 15 30 36
 2  5  8 33 38 41
 2 12 19 22 29 43
13 21 28 29 42 43
 3  4 10 11 32 44

Practice & Extension Work

We know from many years of teaching programming, that to get good at this, you need to spend lots of time writing programs, so here's some more to write.

Practice

  1. (If you haven't already)
    Refactor the report function in total_income.py to use enumerate.
    Any time you need both the index and value from a list, enumerate is a useful function. In the original version, we already had the variable for the number of months, but after we refactored to make this report function, we don't need to pass two variables.
    Why not? The number of months is just the length of the list...

  2. Extend the first intermediate exercise (list_exercises.py) so that the user can enter any number of numbers until a number less than zero is entered. Adjust the prompt so that it prints like "Number 1: " then "Number 2: "...

  3. Write a program that asks the user for an indefinite set of strings - until an empty string is entered - then prints all of the strings that were entered more than once.
    E.g. if the user entered: "hello", "world is good", "hello", "john", "good world"... the program would print "Strings repeated: hello".
    If no repeated strings are entered, the program should print "No repeated strings entered".

  4. Memberwise Addition...
    In Python, the + operator concatenates lists, so for example:
    [1, 2, 3] + [4, 5, 6] = [1, 2, 3, 4, 5, 6]
    What if we want to add the elements together instead?
    Write a function, add_memberwise, that takes two lists, and returns the list that contains the sum of elements that are in the same index in the two lists. For example:
    add_memberwise([1, 2, 3], [4, 5, 6]) would return [5, 7, 9] and
    add_memberwise([1, 2, 3], [1, 2, 3, 4]) would return [2, 4, 6, 4]

Extension

  1. Download the scores reading program: scores.py and its data file: scores.csv

    The data file stores the scores for each subject for 10 people.
    This code reads the lines into lists, saves the first line as a list of subject codes and converts the rest of the lines from a list of strings into a list of numbers, which it then prints with the maximum value for that subject. Nice... Except, it's broken! It reads the lists per user not per subject, so the results are incorrect.

    a. Save the CSV file to your project folder and copy the code into a new file and run it to see how it currently works.

    b. Can you fix it so that it prints the list of (10) scores per subject with the maximum per subject?

    c. When you've done that, also print the minimum and average for each subject.

    d. Then print the results in a nicely-formatted table.

  2. Download/copy the sorting demonstration: sorting.py
    Study how this works and complete the TODO instructions.

  3. Copy your word generator program into another file. Commit.
    Add error-checking so that you repeatedly validate the user's input until it is a valid sequence of just c's and v's. Create and use a function is_valid_format(...) to return True or False for if the word format is valid or not.
    Tip: use a for loop to iterate through each character in the format sequence and return false if you see one that is not valid.

  4. Copy your ascii_table.py file from last week's prac into this week's folder.
    Create a function called get_number(lower, upper) to get a number, making sure that user input is numeric and within the given range.
    You can use exceptions to check the string is a valid number.
    Repeatedly re-prompt for a number until a valid one is entered, then return it.
    Example:

    Enter a number (10-50):  
>>>abc  
Please enter a valid number!  
Enter a number (10-50):  
>>>75  
Please enter a valid number!  
Enter a number (10-50):  
>>>30
(then the function should return 30)

    When this function works, use it in your program in place of the code you used to get the number.
    Test it with both invalid and valid inputs.

Solutions to Selected Exercises

Remember that we have many solutions available in the solutions branch of the Practicals repository. Change the branch on GitHub to solutions, or visit:
https://github.com/CP1404/Practicals/tree/solutions

Remember also that the solutions are there to help you learn after you have done your best to complete the tasks on your own and using the teaching resources we have provided.

Deliverables

This section summarises the expectations for marking in this practical.
Please follow the submission guidelines to ensure you receive marks for your work.

  • Type the URL of your GitHub practicals repository in the text box when you submit your practical.
  • lists_warmup.py
  • total_income.py
  • subject_reader.py
  • list_exercises.py
  • list_comprehensions.py
  • quick_picks.py