Programmeerimise alused 2020/21 sügis

Before session 6

Files

Watch the videos. Next two videos (part 1 and part 2) show how to open data files on your computer and read throught the files using Python.

Slides
Text-book: chapter 7 - files

Test

Go to Moodle and take the sixth test on the video lecture.

Homework

The deadline for homework is Sunday. Nevertheless, you can start with homework already before the session.

Example 1. Shoe size

The file foot.txt contains data about foot lengths. The following program reads the data from the file, calculates a suitable shoe size for each foot length and prints the result out. The program prints Invalid input if the foot length cannot be converted into a number for some reason, and proceeds with the next line of the file.

def shoe_size(length):
    return 1.5 * (length + 1.5)

ffile = open("foot.txt")

for foot in ffile:
    try:
        f = float(foot)
        s = shoe_size(f)
        print("Foot length:", f, "Suitable shoe size is", round(s))
    except:
        print("Invalid input")

ffile.close()

Example 2. Upper case

The following program asks for the file name, then the program reads the file and prints the contents of the file (line by line) in the upper case. The program works nicely with all the files which exist and do not exist.

fname = input("Please enter the file name: ")

try:
    ffile = open(fname)
except:
    print("File cannot be opened:", fname)
else:
    for line in ffile:
        print(line.strip().upper())
    ffile.close()

Exercise 1. Name in URL

Write a program that reads URLs of Tartu University users' webpages from a file called urls.txt and outputs the usernames. The solution should have a function called find_name that takes one parameter (URL) and returns the name (you can reuse the solution of the previous homework to find a user name from an URL and make it as a function). You should create the file urls.txt yourself.

If the file urls.txt contains the following lines:

 http://www.ut.ee/~koit/KT/index_eng.html
 http://www.ut.ee/~vilo/
 http://www.ut.ee/~kiho/

then the output of the program should be

 koit
 vilo
 kiho

Exercise 2. Numbered lines

Write a program which prompts the user for the file name and outputs all the lines from the file with its line number. Make sure that the program works nicely with all the files which exist and do not exist (use try-except).

If the content of the file names.txt is as follows:

 Jon Snow
 Arya Stark
 Tyrion Lannister
 Eddard Stark
 Melisandre

then the program output is as follows:

 Enter the file name: names.txt
 1. Jon Snow
 2. Arya Stark
 3. Tyrion Lannister
 4. Eddard Stark
 5. Melisandre