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Can list comprehension do this?

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Can list comprehension do this?

 
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  #1
Jan 17th, 2007
This is part of a short script I wrote to find out whether students had submitted any corrections to work in December or January.

Python Syntax (Toggle Plain Text) 
  1. import os, os.path
  2.  
  3. months = ['Dec', 'Jan']  # actually a raw_input()
  4.  
  5. files = get_all_files(root_dir)  # actually a call to os.path.walk()
  6.  
  7. dates = []
  8.  
  9. for i in files:
  10.     for j in months:
  11.          if j in i:  # filenames have date-stamp on them
  12.              dates.append(i)
  13.              break

In other words, dates ends up with all of the files whose names contain any of the items in months.

Can this be done with list comprehension?

Doesn't work:

dates = [i for i in files if j in i for j in months]

Jeff
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Re: Can list comprehension do this?

 
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  #2
Jan 17th, 2007
If you want to find file date,
stats = os.stat( filename )
print time.ctime( stats[8] )
prints 'Tue Dec 26 14:28:16 2006'
string.split, etc. yields the month, although I think you should also take the year into account.
Last edited by woooee; Jan 17th, 2007 at 1:20 am.
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Re: Can list comprehension do this?

 
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  #3
Jan 17th, 2007
can you try :

Python Syntax (Toggle Plain Text) 
  1. dates = [i for i in files for j in months if j in i  ]
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Re: Can list comprehension do this?

 
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  #4
Jan 17th, 2007
Looks like Ghostdog is right ...
Python Syntax (Toggle Plain Text) 
  1. # test sample from user input
  2. months = ['Dec', 'Jan']
  3. # test sample from os.path.walk()
  4. files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat']  # test
  5. dates = []
  6. for i in files:
  7.     for j in months:
  8.          if j in i:
  9.              dates.append(i)
  10.              break
  11. print dates
  12. # ... or list comprehension ...
  13. dates2 = [i for i in files for j in months if j in i  ]
  14. print dates2
  15. """
  16. my output -->
  17. ['JanWork3.dat', 'DecWork7.dat']
  18. ['JanWork3.dat', 'DecWork7.dat']
  19. """
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Re: Can list comprehension do this?

 
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  #5
Jan 17th, 2007
Ah ... it seems obvious now. :lol:

Thanks,
Jeff
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Re: Can list comprehension do this?

 
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  #6
Jan 21st, 2007
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Originally Posted by vegaseat View Post
Looks like Ghostdog is right ...
Python Syntax (Toggle Plain Text) 
  1. # test sample from user input
  2. months = ['Dec', 'Jan']
  3. # test sample from os.path.walk()
  4. files = ['FebWork1,dat', 'JanWork3.dat', 'DecWork7.dat']  # test
  5. dates = []
  6. for i in files:
  7.     for j in months:
  8.          if j in i:
  9.              dates.append(i)
  10.              break
  11. print dates
  12. # ... or list comprehension ...
  13. dates2 = [i for i in files for j in months if j in i  ]
  14. print dates2
  15. """
  16. my output -->
  17. ['JanWork3.dat', 'DecWork7.dat']
  18. ['JanWork3.dat', 'DecWork7.dat']
  19. """
I sort of can see how list comprehension evolves from the individual for loops. Is there anything list comprehension can do that one could not write out with individual for loops?
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Re: Can list comprehension do this?

 
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  #7
Jan 21st, 2007
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Originally Posted by sneekula View Post
Is there anything list comprehension can do that one could not write out with individual for loops?
Not in principle. However, in practice, list comprehension makes the code much more comprehensible. So: list comprehension does better code maintenance.

Mathematically,

Python Syntax (Toggle Plain Text) 
  1. mylist = [x for x in S if C]

is equivalent to
Python Syntax (Toggle Plain Text) 
  1. mylist = []
  2. for x in S:
  3.     if C:
  4.        mylist.append(x)

so it must be the case that for loops can do anything list comprehensions can do.

But the expressive power ... wow!

Python Syntax (Toggle Plain Text) 
  1. average_salary = sum([p.salary for p in personnel])/len(personnel)
is exactly what I'm thinking, while

Python Syntax (Toggle Plain Text) 
  1. total = 0
  2. for p in personnel:
  3.    total += p.salary
  4. average_salary = total / len(personnel)

was only what I was trained to think over time.

Jeff

* Error-checking not included. Salary information void where prohibited by law. p.salary may be negative or complex in some states.
Last edited by jrcagle; Jan 21st, 2007 at 3:19 pm.
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