Python: List operations

Question

I am 100% new to python, I know I need to read more but I need to do this task right now that's why I am using python for it. Here is my code:

outputList = []

for line in open('cron.log', 'r'):
   m = line[45:47]
   outputList.append(m)

So I opened the file, read through lines, and append the 2 chars I need into a list. Now I want to go from the end (or beginning of the list), comparing the element at that position with the element right before (or behind) it. How can I do that? In C++ I would be doing iterrator, using front(), pop_front() or such but I am clueless about python :(

First, your code could be outputList = [line[45:47] for line in open('cron.log', 'r')] As for your comparison, I don't yet understand what you are trying to accomplish. Perhaps you could add an example of the desired output? — Eric Wilson
– Eric Wilson, Commented Jun 23, 2011 at 15:41
oh, I have finished that. I am sorry about the fact that my code looks crappy :( I never used python before in my life ... so yeah — Tu Hoang
– Tu Hoang, Commented Jun 23, 2011 at 16:20
Just trying to help, the line I suggested is an example of a 'list comprehension', one of Python's idiomatic conveniences. — Eric Wilson
– Eric Wilson, Commented Jun 23, 2011 at 16:23
Yeah, it's like using English >.< read the file cron.log for me line by line and chop out characters 46, 47 from those lines and store them in a list. After this I think you should only learn python if you have strong understanding about one of the other High Level Language (C++, Java, or even C) so that you know what the hell is going on behind the scene. — Tu Hoang
– Tu Hoang, Commented Jun 23, 2011 at 16:26
Many non-programmers (scientists, sys-admins, etc.) prefer Python because it allows them to not care about what is happening behind the scenes. You are right, a professional programmer needs to have a deeper understanding, though. — Eric Wilson
– Eric Wilson, Commented Jun 23, 2011 at 16:34

ciferkey · Accepted Answer · 2011-06-23 15:43:45Z

2

#This is from the beginning
for i in range(1, len(outputList)):
    # == could be what ever comparison you want.  Any one in particular?
    if outputList[i] == outputList[i-1]:
        #do whatever you need to do

answered Jun 23, 2011 at 15:43

ciferkey

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4 Comments

Tu Hoang Over a year ago

So that would be going from the beginning to the end or from the end to the beginning?

ciferkey Over a year ago

beginning to end starting from the second element and comparing it to the first. Do you need it to go the other way and what kind of comparison are you looking for?

Tu Hoang Over a year ago

Thank you sir! I was able to modify it to fit my need. Python is too flexible. It was flexible to the point that I find it pretty hard to use (since I never coded in python before)

Eric Wilson Over a year ago

This will do the job, but when 'len' of something iterable is an argument for 'range', then I think that 'enumerate' is in order.

Artsiom Rudzenka · Accepted Answer · 2011-06-23 15:57:05Z

0

Also you can use something like this:

a = [1, 2, 3, 4, 5, 6, 7, 8]
# proceed backward
for x,y in ((a[i],a[i-1]) for i in xrange(len(a)-1,0,-1)):
    if x!= y:
        # do something

answered Jun 23, 2011 at 15:57

Artsiom Rudzenka

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Comments

Eric Wilson · Accepted Answer · 2011-06-23 16:00:59Z

0

outputList = [line[45:47] for line in open('cron.log', 'r')]

for idx, item in enumerate(outputList[:-1]):
    if item == outputList[idx+1]:
        # do something

answered Jun 23, 2011 at 16:00

Eric Wilson

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