Description
- (100 points) Lists, Sets, and Dicts
expenses.txt(the same file we used in Homework 2) is a small text file describing business expenses. Each line (after the header) gives the money amount, category, date, and description of an expense.
- Create a Python program file named hw3.1.py. In this script, define an empty list named records(you can say: records = []), then read the lines from expenses.txt and append each line (excluding its terminating newline character) to the records list. Close the file when you have finished reading it (unless you have used the with form of open, which closes the file for you automatically). Then, add this code to display the lines from records:
for line in records:
print(line)
Confirm that the output is not double-spaced; that is, confirm that each line (str) in the recordslist does not include a terminating newline character.
- Make sure that you have closed theexpenses.txt file, and open expenses.txt again. (If you used the with form of open(), you don’t need to do close(): it is done for you automatically.) Create an empty list named records2. Read lines from expenses.txt. For each line, strip the final newline character, and split() the line at ‘:’ characters to create a list called columns. Then, append the columnslist to the records2 list: records2 will be a list of lists.
Finally, add this code to display the list of lists records2:
for row in records2:
print(row)
The output from this loop should look like:
[‘Amount’, ‘Category’, ‘Date’, ‘Description’]
[‘5.25’, ‘supply’, ‘20170222’, ‘box of staples’]
…
[‘8.98’, ‘supply’, ‘20170325’, ‘Flair pens’]
- Recall that you can sort() a list. Make a copy of records2 named r2_copy, then sort()r2_copy and display its contents with:
for row in r2_copy:
print(row)
Is r2_copy sorted in ascending order by dollar amount? Comment.
- We have two problems with sorting r2_copy by dollar amount. First of all, the header line is part of r2_copy, and sorted along with all the other lines. It turns out that in the utf-8/ASCII character set, all of the digit characters ‘0’, ‘1’, …, ‘9’ come ahead of all of the uppercase and lowercase letters, so the header line is last in the r2_copy.
(The web page www.asciitable.com is a good reference for the ASCII character set. The first column of 32 characters are what are called control characters, like tab, newline, Ctrl-C, and so on. The second column of 32 characters is mostly punctuation marks and the digits 0 through 9. The third column is mostly uppercase letters, and the fourth column is mostly lowercase letters. On our systems, sorting is done according to this character set: ‘0’ precedes ‘9’, which precedes ‘A’, which precedes ‘a’, and so forth.)
Second, the dollar amounts are represented as strings (str) in each data line, so they are sorted as strings: ‘123.45’ comes ahead of ‘8.76’ because ‘1’ comes ahead of ‘8’.
So we need to separate the header from the data records, and we need to convert the dollar amounts from strings (str) to numbers (float).
From records2 (which still contains the original data in the original order), make a copy of the first item (the list of column headers) into a list named header. Then, make acopy of aslice of records2 containing all except the first item into a list of lists named data.
Hint:records2 is a list of lists, so records2[0] is its first item, which is a list. To make a copy, use records2[0].copy(). Use a similar idea to create a copy of a slice.
If you have done this correctly, these statements:
print(header)
for d in data:
print(d)
should display:
[‘Amount’, ‘Category’, ‘Date’, ‘Description’]
[‘5.25’, ‘supply’, ‘20170222’, ‘box of staples’]
…
[‘8.98’, ‘supply’, ‘20170325’, ‘Flair pens’]
- Next, loop through data, changing the first item of each list from a str to a float. If you have done this correctly, these statements:
print(header)
for d in data:
print(d)
should display:
[‘Amount’, ‘Category’, ‘Date’, ‘Description’]
[5.25, ‘supply’, ‘20170222’, ‘box of staples’]
…
[8.98, ‘supply’, ‘20170325’, ‘Flair pens’]
Notice that the dollar amounts are now float values rather than str values.
- Finally, sort()data. Then, these statements:
print(header)
for d in data:
print(d)
should display:
[‘Amount’, ‘Category’, ‘Date’, ‘Description’]
[5.25, ‘supply’, ‘20170222’, ‘box of staples’]
[6.53, ‘meal’, ‘20170302’, ‘Dunkin Donuts, drive…’] …
[383.75, ‘travel’, ‘20170223’, ‘flight to Boston…’]
[1247.49, ‘supply’, ‘20170306’, ‘Dell 7000 …”]
The records are now in ascending order by dollar amount.
- What are the expense categories? Create a set containing these. Start with an empty set, like this:
categories = set() # an empty set
Then, loop through data adding the category from each record to the categoriesset.
When done, use these statements to display the categories:
print(‘There are’, len(categories), ‘expense categories:’)
for c in categories:
print(c)
This might display:
There are 4 expense categories:
meal
travel
suppy
util
But recall that the items in a set are not ordered, so the categories might be displayed in a different order.
Notice that an advantage of using a set this way is that new expense categories might be added into the expenses.txt file and we will not have to change our code. If new expenses are entered with categories like ‘entertain’ and ‘charity’, we can simply re-execute our program and we will get an updated list of the 6 expense categories that now exist.
- Create a dict named n2s that uses two-digit month number strings as keys and the corresponding three-letter month abbreviation strings as values. For example, two of the key/value pairs in n2s should be ’01’ : ‘Jan’ and ’09’ : ‘Sep’. Use a loop on the items of n2s to display a neatly formatted table:
Key Value
- Jan
- Feb
- Mar
…
12 Dec
When finished, put your hw3.1.py source code file into a .zip file, and upload your .zip file to Canvas.
