Surely there's a better way?

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Views: 267 | Replies: 5

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Join Date: Apr 2008

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FreezeBlink FreezeBlink is offline

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Surely there's a better way?

Jun 3rd, 2008

Iterating over a list, possibly deleting elements. It's the bane of my existence. Suppose, for instance, we have a list of numbers, and want to iterate over it and delete each and every odd one. Sounds simple, right?

Sorta. It's trivial, but the code you end up with is pretty gruesome.

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> for n in range(len(nums)-1,-1,-1):
	# Same as saying "if nums[n] is odd"
	if nums[n] % 2 != 0:
		del nums[n]
 
 
>>> print nums
[2.0, 4.0]
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> for n in range(len(nums)-1,-1,-1):
	# Same as saying "if nums[n] is odd"
	if nums[n] % 2 != 0:
		del nums[n]

		
>>> print nums
[2.0, 4.0]

Surely there's a cleaner way to do that loop? Anyone?

Last edited by FreezeBlink : Jun 3rd, 2008 at 2:26 am.

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Join Date: May 2007

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BearofNH

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Re: Surely there's a better way?

Jun 3rd, 2008

Try a list comprehension:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> evens = [x for x in nums if x%2==0]
>>> evens
[2.0, 4.0]
>>>
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> evens = [x for x in nums if x%2==0]
>>> evens
[2.0, 4.0]
>>>

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Join Date: Jun 2008

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slate

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Re: Surely there's a better way?

Jun 3rd, 2008

Can you tell me, why it is needed to delete from the list?

Is it not good enough to construct another list with the desired members, and assign it to the original list?

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Join Date: Apr 2008

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FreezeBlink FreezeBlink is offline

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Re: Surely there's a better way?

Jun 3rd, 2008

Because if you create a new list and assign it to the old one, you're creating an un-needed variable.

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slate

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Re: Surely there's a better way?

Jun 3rd, 2008

how about:

del nums[1::2]

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jrcagle

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Re: Surely there's a better way?

Jun 4th, 2008

If you are determined to iterate through the original list ... and there are times when that's appropriate ... then you must iterate through a copy of the list:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> for n in nums[:]:
	if n % 2 == 1: # better way to say 'is odd'
		nums.remove(n)
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> for n in nums[:]:
	if n % 2 == 1: # better way to say 'is odd'
		nums.remove(n)

By iterating through the copy, you avoid the problem of trying to delete from a list under iteration.

But in general, I endorse BearofNH's solution as the cleanest. It's scarcely more expensive to use a list comprehension than to iteration through a list to begin with, and a new variable need not be created:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> nums = [x for x in nums if x % 2 == 1]
>>> nums = [1.0,2.0,3.0,4.0,5.0]
>>> nums = [x for x in nums if x % 2 == 1]