Lab 8: Part 3. Reading & Writing Files

The lecture notes on files may be helpful during this part of the lab.

To get some practice reading from and writing to files, we've supplied you with the following text files in the starter directory:

• nums.txt Contains the numbers one through 10, each on its own line.
• nums2.txt An alternate version of nums.txt.
• birds.txt Contains a list of bird names, one per line.
• fruit.txt Contains a list of fruits and their categories, one per line, with the fruit and category names separated by a single space.
• metals.txt Contains a list of a few metallic elements, along with their densities, with a colon and a space in between the element name and density value.
• meetings.txt Contains a list of meetings, with a meeting name, a start time, and a duration separated by commas. The time of day is two integers (one for the hour and one for the minutes) separated by a colon, while the duration is a floating-point number representing hours.

You will have to create your own code file for this part of the lab, called fileOps.py.

Work through the exercises below...

showTop

Write a function named showTop which accepts a file name as an argument, and which reads that file and prints out the first three lines of text in the file. Test it on nums.txt, and you should see:

one
two
three

showBottom

showBottom should work like showTop, except it should print the last three lines from the file instead of the first three. Testing on nums.txt should result in:

eight
nine
10

listLines

listLines should accept a file name as an argument, and print the entire contents of that file, but it should add a line number at the start of each line, separated from the actual line by a single space. The first line number should be 1 (not 0). Here's what the result would look like if applied to nums.txt:

1 one
2 two
3 three
4 4
5 five
6 six
7 seven
8 eight
9 nine
10 10

copyFile

copyFile should take two different file names as arguments, and it should open the first file, read in the contents, then open the second file, and write out a copy of the first file (so that at the end, the two files end up as copies of each other). Be careful that you don't accidentally overwrite one of the starter files when you're testing this! It won't have any return value or printed output... but you should check that it creates an exact copy of the file you're trying to copy (you could open both files in a text editor). You could test it with:

copyFile("birds.txt", "moreBirds.txt")

writeReport

writeReport should have two parameters: a list of tuples which each consist of a string followed by a number, and a string which is a filename. It should open the target file, erase any old contents (this happens automatically if you open in write mode) and write out the strings and numbers from the report, one pair per line. The string should come first, then a colon, then a space, and then the number. So for example, if you call:

writeReport(
    [("titanium", 4.507), ("iron", 7.874), ("copper", 8.96)],
    "metals.txt"
)

Then the file 'metals.txt' should contain the following text:

titanium: 4.507
iron: 7.874
copper: 8.96

readReport

readReport does the opposite of writeReport: it reads data from a file into a list of tuples. It should accept one argument, which is a filename, and it must read information from that filename and return a list of tuples, each consisting of a string followed by a number. The format in the file will be the same as the format shown in writeReport: a string, then a colon followed by a space, and finally the associated number. If you already got writeReport working using the test above, then calling:

readReport("metals.txt")

should return the following value:

[("titanium", 4.507), ("iron", 7.874), ("copper", 8.96)]

Hint: you can use the .split method of strings with an argument to specify what character or sequence of characters should be the split point. For this part, using ": " as the split point is quite efficient.

Optional Challenge Exercises

swapFiles

swapFiles should take two filenames as arguments, and it should swap the contents of the files. This is not too hard, but it's a bit more challenging than it might seem at first :)

listEvents

listEvents should take a single argument which is a string naming a file to read data from. It should read data from that file specifying calendar events, and then print out a listing of each event in a modified format.

The events.txt and events2.txt files each have data which lists events as three comma-separated fields: an event name, a start time in hour:minute format, and a duration as a floating-point number of hours. The start times use hours in a 24-hour format, so 3 p.m. would be listed as 15. If listEvents is called like this:

listEvents("events.txt")

the table it prints should look like this:

CS 111 (9:30-10:30)
Major advising (11:00-12:00)
Department meeting (12:30-1:30)
Research time (15:30-17:00)
Home with baby (18:00-6:30)

Note that even after you've figured out how to extract information from the file and convert it to numbers, you will have to do some math to convert the duration-in-hours into an end-time... one strategy is to first convert the number of hours and minutes into a time-in-fractional-hours and then convert that back to hours-and-minutes afterwards.

Table of Contents

• Lab 8 Home
• Part 0: Warm-Up
• Part 1: Loop Tracing & Debugging
• Part 2: Testing with Optimism
• Part 3: Reading & Writing Files
• Knowledge Check