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9.10. Parsing lines

Usually when we are reading a file we want to do something to the lines other than just printing the whole line. Often we want to find the “interesting lines” and then parse the line to find some interesting part of the line. What if we wanted to print out the day of the week from those lines that start with “From “?

From stephen.marquard@uct.ac.za Sat Jan  5 09:14:16 2008

The split method is very effective when faced with this kind of problem. We can write a small program that looks for lines where the line starts with “From “, split those lines, and then print out the third word in the line:

fhand = open('mboxShort.txt') for line in fhand: line = line.rstrip() if not line.startswith('From '): continue words = line.split() print(words[2]) # Continue is used here to skip the rest of the line if it doesn't start with "From "

Later, we will learn increasingly sophisticated techniques for picking the lines to work on and how we pull those lines apart to find the exact bit of information we are looking for.

The following code should open a file and read through the lines, splitting them when a line starts with “Hello”, then printing the second word in the line. Watch out for extra pieces of code and indentation.

        fhand = open('myFile.txt')
---
for line in fhand:
---
for line in myFile: #distractor
---
    line = line.rstrip() #remove trailing whitespace
---
    if not line.startswith('From '): continue #distractor
---
    if not line.startswith('Hello'): continue
---
    words = line.split()
---
    print(words[1])
---
    print(words[2]) #distractor
        

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