Re: else condition in list comprehension
Andrey Tatarinov wrote: I presume the point of this is to avoid polluting the local namespace with "newval". I further presume you also have plans to do something about "i"? ;-) no, the point is in grouping definition of newval() with place where it is used. I'd have said the point was both :) But yeah, unfortunately the 'leaking list comp' problem won't be fixed in the 2.x series due to the compatibility problem. Fortunately, generator expressions didn't inherit the issue. Cheers, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://boredomandlaziness.skystorm.net -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Steve Holden wrote: Nick Coghlan wrote: Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i to be "i-2" if i%2 is not equal to 0? Hmm: z = [newval(i) for i in range(10)] using: def newval(x): if x % 2: return x - 2 else: return x + 2 Just some more mental twiddling relating to the thread on statement local namespaces. I presume the point of this is to avoid polluting the local namespace with "newval". I further presume you also have plans to do something about "i"? ;-) no, the point is in grouping definition of newval() with place where it is used. -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Steve Holden wrote: Nick Coghlan wrote: z = [newval(i) for i in range(10)] using: def newval(x): if x % 2: return x - 2 else: return x + 2 Just some more mental twiddling relating to the thread on statement local namespaces. I presume the point of this is to avoid polluting the local namespace with "newval". I further presume you also have plans to do something about "i"? ;-) Well, while I'm not at all a fan of the "using" syntax, getting rid of 'i' is simple: z = list(newval(i) for i in range(10)) =) Steve -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
On 9 Jan 2005 12:20:40 -0800, Luis M. Gonzalez <[EMAIL PROTECTED]> wrote: > Hi there, > > I'd like to know if there is a way to add and else condition into a > list comprehension. I'm sure that I read somewhere an easy way to do > it, but I forgot it and now I can't find it... > > for example: > z=[i+2 for i in range(10) if i%2==0] > what if I want i to be "i-2" if i%2 is not equal to 0? z = [i+2-(i%2)*4 for i in range(10)] C'mon, who needs an 'if' statement when we have maths! Stephen. -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Nick Coghlan wrote: Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i to be "i-2" if i%2 is not equal to 0? Hmm: z = [newval(i) for i in range(10)] using: def newval(x): if x % 2: return x - 2 else: return x + 2 Just some more mental twiddling relating to the thread on statement local namespaces. I presume the point of this is to avoid polluting the local namespace with "newval". I further presume you also have plans to do something about "i"? ;-) regards Steve -- Steve Holden http://www.holdenweb.com/ Python Web Programming http://pydish.holdenweb.com/ Holden Web LLC +1 703 861 4237 +1 800 494 3119 -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i to be "i-2" if i%2 is not equal to 0? Hmm: z = [newval(i) for i in range(10)] using: def newval(x): if x % 2: return x - 2 else: return x + 2 Just some more mental twiddling relating to the thread on statement local namespaces. Cheers, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://boredomandlaziness.skystorm.net -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Nick Coghlan wrote: Dan Bishop wrote: Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i [sic] to be "i-2" if i%2 is not equal to 0? z = [i + (2, -2)[i % 2] for i in range(10)] For the specific case of +/- a number, (-1) ** x works, too: z = [i + 2 * ((-1) ** i) for i in range(10)] Not that I'm claiming it's particularly readable or anything. . . just that it works :) Yet another variant: z = [i + ( (i % 2) and -2 or 2 ) for i in range(10)] -- Serhiy Storchaka -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
On Mon, 10 Jan 2005 09:13:17 -0700, Steven Bethard <[EMAIL PROTECTED]> wrote: > Luis M. Gonzalez wrote: > > It's me wrote: > >>> z = [i + (2, -2)[i % 2] for i in range(10)] > >> > >> But then why would you want to use such feature? Wouldn't that make > >> the code much harder to understand ... > >> Or are we trying to write a book on "Puzzles in Python"? > > > > Once you get used to list comprehensions (and it doesn't take long), > > they are a more concise and compact way to express these operations. > > After looking the two suggestions over a couple of times, I'm still > undecided as to which one is more readable for me. The problem is not > the list comprehensions (which I love and use extensively). The problem > is the odd syntax that has to be used for an if/then/else expression in > Python. They're both pretty unreadable, IMHO. Why not just factor out the if/then/else function like this: .def plusMinusTwo(i): . if i%2 == 0: . return i-2 . else: . return i+2 . .z = [plusMinusTwo(i) for i in range(10)] Then you can add whatever you like into the function. Anthony -- - HyPEraCtiVE? HeY, WhO aRE YoU cALliNg HypERaCtIve?! [EMAIL PROTECTED] - -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Dan Bishop wrote: Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i [sic] to be "i-2" if i%2 is not equal to 0? z = [i + (2, -2)[i % 2] for i in range(10)] For the specific case of +/- a number, (-1) ** x works, too: z = [i + 2 * ((-1) ** i) for i in range(10)] Not that I'm claiming it's particularly readable or anything. . . just that it works :) Cheers, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://boredomandlaziness.skystorm.net -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Luis M. Gonzalez wrote: It's me wrote: z = [i + (2, -2)[i % 2] for i in range(10)] But then why would you want to use such feature? Wouldn't that make the code much harder to understand then simply: z=[] for i in range(10): if i%2: z.append(i-2) else: z.append(i+2) Or are we trying to write a book on "Puzzles in Python"? Once you get used to list comprehensions (and it doesn't take long), they are a more concise and compact way to express these operations. After looking the two suggestions over a couple of times, I'm still undecided as to which one is more readable for me. The problem is not the list comprehensions (which I love and use extensively). The problem is the odd syntax that has to be used for an if/then/else expression in Python. I think I would have less trouble reading something like: z = [i + (if i % 2 then -2 else 2) for i in range(10)] but, of course, adding a if/then/else expression to Python is unlikely to ever happen -- see the rejected PEP 308[1]. Steve [1] http://www.python.org/peps/pep-0308.html -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
It's me wrote: > > z = [i + (2, -2)[i % 2] for i in range(10)] > > But then why would you want to use such feature? Wouldn't that make the > code much harder to understand then simply: > > z=[] > for i in range(10): > if i%2: > z.append(i-2) > else: > z.append(i+2) > > Or are we trying to write a book on "Puzzles in Python"? Once you get used to list comprehensions (and it doesn't take long), they are a more concise and compact way to express these operations. I think that writing 6 lines instead of 1 could be more readable of you are a beginner, but after playing a little bit with listcomps for the first time, you'll see they are very practical yet readable. -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
> z = [i + (2, -2)[i % 2] for i in range(10)] But then why would you want to use such feature? Wouldn't that make the code much harder to understand then simply: z=[] for i in range(10): if i%2: z.append(i-2) else: z.append(i+2) Or are we trying to write a book on "Puzzles in Python"? -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Matteo Dell'Amico wrote: > Luis M. Gonzalez wrote: >> Hi there, >> >> I'd like to know if there is a way to add and else condition into a >> list comprehension. I'm sure that I read somewhere an easy way to do >> it, but I forgot it and now I can't find it... >> >> for example: >> z=[i+2 for i in range(10) if i%2==0] >> what if I want i to be "i-2" if i%2 is not equal to 0? > > You could use > > [(i-2, i+2)[bool(i%2 == 0)] for i in range(10)] > > or, in a less general but shorter way > > [(i+2, i-2)[i%2] for i in range(10)] > > or even > > [i%2 and i-2 or i+2 for i in range(10)] One should note that the (cond and X or Y) construct only works if X can never produce a false value (such as 0, "", []). In this example, it is okay, but replace 2 with 1 and you will run into trouble for i = 1. Reinhold -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Luis M. Gonzalez wrote: > Hi there, > > I'd like to know if there is a way to add and else condition into a > list comprehension. I'm sure that I read somewhere an easy way to do > it, but I forgot it and now I can't find it... > > for example: > z=[i+2 for i in range(10) if i%2==0] > what if I want i [sic] to be "i-2" if i%2 is not equal to 0? z = [i + (2, -2)[i % 2] for i in range(10)] In general, the expression "T if C is true, or F if C is false" can be written as (F, T)[bool(C)]. (If you know that C will always be either 0 or 1, as is the case here, the "bool" is redundant.) Unless, of course, either F or T has side effects. For a side-effect free expression, you can use (C and [T] or [F])[0] or one of the many other ternary operator substitutes. (Search for PEP 308.) -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Thank you guys! -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Luis M. Gonzalez wrote: Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i to be "i-2" if i%2 is not equal to 0? You could use [(i-2, i+2)[bool(i%2 == 0)] for i in range(10)] or, in a less general but shorter way [(i+2, i-2)[i%2] for i in range(10)] or even [i%2 and i-2 or i+2 for i in range(10)] The "if" clause in comprehensions is used as a filter condition. -- Ciao, Matteo -- http://mail.python.org/mailman/listinfo/python-list
Re: else condition in list comprehension
Luis M. Gonzalez wrote: > Hi there, > > I'd like to know if there is a way to add and else condition into a > list comprehension. I'm sure that I read somewhere an easy way to do > it, but I forgot it and now I can't find it... > > for example: > z=[i+2 for i in range(10) if i%2==0] > what if I want i to be "i-2" if i%2 is not equal to 0? You'll have to add the condition at the front: z = [(i+2, i-2)[i%2] for i in range(10)] should do what you need. Reinhold -- http://mail.python.org/mailman/listinfo/python-list
else condition in list comprehension
Hi there, I'd like to know if there is a way to add and else condition into a list comprehension. I'm sure that I read somewhere an easy way to do it, but I forgot it and now I can't find it... for example: z=[i+2 for i in range(10) if i%2==0] what if I want i to be "i-2" if i%2 is not equal to 0? -- http://mail.python.org/mailman/listinfo/python-list
