Re: Problem with list.remove() method
On Wed, Nov 21, 2012 at 12:56 AM, Alvaro Combo alvaro.co...@gmail.com wrote: Hi All, I'm relatively new to Python... but I have found something I cannot explain... and I'm sure you can help me. I have the following function that serves for removing the duplicates from a list... It's a simple and (almost) trivial task. I'm using WingIDE as editor/debugger and have Python 2.7.3. When running this I have an error when trying to remove cpy_lst[4]... and ONLY THAT!!! Even in interactive mode!!! Several points here. You've written a beautiful O(N^2) duplicates remover... Python has a really fast way of doing it, if you don't mind losing order: cpy_lst = list(set(lst)) But let's assume you're doing this for the exercise. Your technique is fine, if inefficient on large lists, but the remove() method looks for the first occurrence of an element by its value - what you want is: del cpy_lst[i] which will remove one element by index. With that change, you'll have a slightly odd duplicate remover that keeps the *last* of any given element. That's rather unusual. Instead, you may want to consider maintaining a set of items I've already seen, and keeping all elements that aren't in that set. I won't give you all the code, but here's the basic set operations: sighted = set() sighted.add(some_element) if some_element in sighted: # condition is True if you've already seen this element Hope that helps! ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Problem with list.remove() method
Dear Chris, Thank you very much for you reply... For a newcomer to Python the Hell is in the details... :-). You are absolutely right... and I just used the index instead of the value. Regarding you other (most relevant) comments, I absolutely agree... BUT in those cases... I was aware :-). But since it is in fact an exercise... and having N^2 operations... is not important. Still your comments are quite pertinent. Once again, Thank you and best regards ACombo -- http://mail.python.org/mailman/listinfo/python-list
Re: Problem with list.remove() method
On 11/20/2012 9:14 AM, Chris Angelico wrote: On Wed, Nov 21, 2012 at 12:56 AM, Alvaro Combo alvaro.co...@gmail.com wrote: Hi All, I'm relatively new to Python... but I have found something I cannot explain... and I'm sure you can help me. I have the following function that serves for removing the duplicates from a list... It's a simple and (almost) trivial task. I'm using WingIDE as editor/debugger and have Python 2.7.3. When running this I have an error when trying to remove cpy_lst[4]... and ONLY THAT!!! Even in interactive mode!!! Several points here. You've written a beautiful O(N^2) duplicates remover... Python has a really fast way of doing it, if you don't mind losing order: cpy_lst = list(set(lst)) But let's assume you're doing this for the exercise. Your technique is fine, if inefficient on large lists, but the remove() method looks for the first occurrence of an element by its value - what you want is: del cpy_lst[i] which will remove one element by index. With that change, you'll have a slightly odd duplicate remover that keeps the *last* of any given element. That's rather unusual. Instead, you may want to consider maintaining a set of items I've already seen, and keeping all elements that aren't in that set. I won't give you all the code, but here's the basic set operations: sighted = set() sighted.add(some_element) if some_element in sighted: # condition is True if you've already seen this element The itertools doc, in the last section, has a recipe for this problem that uses the above approach. -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Problem with list.remove() method
On Wed, Nov 21, 2012 at 1:37 AM, Alvaro Combo alvaro.co...@gmail.com wrote: Dear Chris, Thank you very much for you reply... For a newcomer to Python the Hell is in the details... :-). You're most welcome! As Adam Savage of Mythbusters is fond of saying (with an exaggerated accent), It's all a learning experience. Regarding you other (most relevant) comments, I absolutely agree... BUT in those cases... I was aware :-). But since it is in fact an exercise... and having N^2 operations... is not important. Still your comments are quite pertinent. Yep. O(N^2) isn't inherently a problem, it just means that the technique will scale poorly to large numbers of list elements. With small lists, it'll be fast enough - for all intents and purposes, the bulk of Python code executes in zero time, despite being in an oh so slow interpreted language and using ridiculously inefficient (but beautifully readable) code. That's the beauty of modern hardware :) However, I do think that people should be aware when they're writing non-scaleable code. That's not just algorithmic complexity; if you're making a system that won't handle more than one request a second (because, for instance, it uses seconds-since-1970 as a request identifier), that's something worth being aware of, even if it's unlikely ever to be a problem. Just know, so that if a problem ever _does_ occur, you know where it is! ChrisA -- http://mail.python.org/mailman/listinfo/python-list
RE: Problem with list.remove() method
Alvaro Combo wrote: Hi All, I'm relatively new to Python... but I have found something I cannot explain... and I'm sure you can help me. I have the following function that serves for removing the duplicates from a list... It's a simple and (almost) trivial task. I'm using WingIDE as editor/debugger and have Python 2.7.3. When running this I have an error when trying to remove cpy_lst[4]... and ONLY THAT!!! Even in interactive mode!!! Any suggestion is MOST welcome. Best Regards ACombo ...And the code: def remove_dup_5_10(): Remove the duplicates of a given list. The original list MUST be kept. # Set the original list lst = ['a', 1, 10.0, 2, 'd', 'b', 'b', 'b', 1, 2, 'b' ] # NEED to create a copy... See dicussion on Problem 5.6 and issue #2 cpy_lst = list(lst) # Perform an infinite loop... explained later i=0 # initialize the index while i != len(cpy_lst): if cpy_lst.count(cpy_lst[i]) != 1: cpy_lst.remove(i) else: i += 1 print The original List: , lst print List with NO duplicates: , cpy_lst return True -- Remove looks for the *value* not the *index* of the value. help([].remove) Help on built-in function remove: remove(...) L.remove(value) -- remove first occurrence of value. Raises ValueError if the value is not present. Change ` cpy_lst.remove(i)` to `cpy_lst.remove(cpy_lst[i])`. ~Ramit This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. -- http://mail.python.org/mailman/listinfo/python-list
