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# subexpressions

 P: n/a Hello all! Please help, is there way to use sub-expressions in lambda? For example, if I want to calculate sin(x^2)+cos(x^2) I must code: lambda x: sin(x*x)+cos(x*x) How to make x*x to be evaluated once? Jun 1 '07 #1
20 Replies

 P: n/a Sergey Dorofeev wrote: Please help, is there way to use sub-expressions in lambda? For example, if I want to calculate sin(x^2)+cos(x^2) I must code: lambda x: sin(x*x)+cos(x*x) How to make x*x to be evaluated once? >>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5) + cos(.5*.5) True The real answer is of course: Use a function. Peter Jun 1 '07 #2

 P: n/a "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... Sergey Dorofeev wrote: >Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once? >>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5) + cos(.5*.5) True The real answer is of course: Use a function. But what about something like lambda x: sin(y)+cos(y) where y=x*x ? May be this could be a PEP? If there is no straight way to do this. Jun 1 '07 #3

 P: n/a Sergey Dorofeev wrote: "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... >Sergey Dorofeev wrote: >>Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once? >>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5) + cos(.5*.5)TrueThe real answer is of course: Use a function. But what about something like lambda x: sin(y)+cos(y) where y=x*x ? May be this could be a PEP? If there is no straight way to do this. def f(x): y = x*x return sin(y) + cos(y) What is not straightforward about that? Peter Jun 1 '07 #4

 P: n/a "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... >>>Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once?>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5)>+cos(.5*.5)TrueThe real answer is of course: Use a function. But what about something likelambda x: sin(y)+cos(y) where y=x*x?May be this could be a PEP? If there is no straight way to do this. def f(x): y = x*x return sin(y) + cos(y) What is not straightforward about that? This code is needed once in a map, so I don't want 3+ extra lines. Solution seemed so simple... I always considered python as languague, where simple things do not require extensive coding. Moreover, this construction is common thing in functional programming. Jun 1 '07 #5

 P: n/a Sergey Dorofeev wrote: > "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... >>>>Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once?>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5)>>+cos(.5*.5)TrueThe real answer is of course: Use a function.But what about something likelambda x: sin(y)+cos(y) where y=x*x?May be this could be a PEP? If there is no straight way to do this. def f(x): y = x*x return sin(y) + cos(y)What is not straightforward about that? This code is needed once in a map, Perhaps you like [sin(y)+cos(y) for y in (x*x for x in items)] then. so I don't want 3+ extra lines. What syntax would you suggest for a lambda enhanced to cover your use case? I suppose you will end up with roughly the same number of characters, all crammed in one line -- or broken into lines at a random position as it happens with overambitious list comprehensions. Solution seemed so simple... It /is/ simple. I always considered python as languague, where simple things do not require extensive coding. In Python, when conciseness and readability compete, readability tends to win (with the inline if...else as a notable exception). Moreover, this construction is common thing in functional programming. I can write Haskell in any language :-) Peter Jun 1 '07 #6

 P: n/a "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... What syntax would you suggest for a lambda enhanced to cover your use case? I suppose you will end up with roughly the same number of characters, all crammed in one line -- or broken into lines at a random position as it happens with overambitious list comprehensions. Agree, this argument is strong. Jun 1 '07 #7

 P: n/a On 2007-06-01, Sergey Dorofeev

 P: n/a Sergey Dorofeev wrote: "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... >Sergey Dorofeev wrote: >>Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once?(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5) + cos(.5*.5)TrueThe real answer is of course: Use a function. But what about something like lambda x: sin(y)+cos(y) where y=x*x ? May be this could be a PEP? If there is no straight way to do this. Or maybe it could be made a part of some other language. When straightforward mechanisms (in rhis case, function definitins) exist to avoid repeated computations it's very unlikely that such mangled constructions will be made a part of Python. If it *were* considered, you should at least change the "where" to "for", and extend it to unpacking assignment to allow lambda x, y: (sin(xx+yy) + cos(xx+yy) for xx, yy = x*x, y*y regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden ------------------ Asciimercial --------------------- Get on the web: Blog, lens and tag your way to fame!! holdenweb.blogspot.com squidoo.com/pythonology tagged items: del.icio.us/steve.holden/python All these services currently offer free registration! -------------- Thank You for Reading ---------------- Jun 1 '07 #9

 P: n/a Sergey Dorofeev wrote: "Peter Otten" <__*******@web.dewrote in message news:f3*************@news.t-online.com... >>>>Please help, is there way to use sub-expressions in lambda?For example, if I want to calculate sin(x^2)+cos(x^2) I must code:lambda x: sin(x*x)+cos(x*x)How to make x*x to be evaluated once?>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]])(.5) == sin(.5*.5)>>+cos(.5*.5)TrueThe real answer is of course: Use a function.But what about something likelambda x: sin(y)+cos(y) where y=x*x?May be this could be a PEP? If there is no straight way to do this. def f(x): y = x*x return sin(y) + cos(y)What is not straightforward about that? This code is needed once in a map, so I don't want 3+ extra lines. Solution seemed so simple... I always considered python as languague, where simple things do not require extensive coding. Moreover, this construction is common thing in functional programming. Stop thinking of three lines as "extensive coding" and your problem disappears immediately. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden ------------------ Asciimercial --------------------- Get on the web: Blog, lens and tag your way to fame!! holdenweb.blogspot.com squidoo.com/pythonology tagged items: del.icio.us/steve.holden/python All these services currently offer free registration! -------------- Thank You for Reading ---------------- Jun 1 '07 #10

 P: n/a Steve Holden a écrit : (snip) Stop thinking of three lines as "extensive coding" and your problem disappears immediately. Lol ! +1 QOTW Jun 1 '07 #11

 P: n/a On 1 Jun, 12:55, Steve Howell FWIW there's the possibility that even without a subexpression syntax, some Python implementations would detect the duplication of x*x and optimize that for you. It would have to know that x*x had no side effects, which I think is a safe assumption even in a dynamic language like Python. On the basis of you believing that x is one of the built-in numeric types, yes, but how does the compiler know that? Paul Jun 1 '07 #12

 P: n/a "Sergey Dorofeev" (params): return expression except that there is no external binding of the otherwise illegal ..func_name ''. The resulting function objects are otherwise identical. After years of discussion, Guido has decided to leave lambda alone for 3.0. It will not be neither expanded, nor removed, nor renamed. Terry Jan Reedy Jun 1 '07 #13

 P: n/a On Jun 1, 9:51 am, "Sergey Dorofeev"

 P: n/a Sergey Dorofeev wrote: Please help, is there way to use sub-expressions in lambda? For example, if I want to calculate sin(x^2)+cos(x^2) I must code: lambda x: sin(x*x)+cos(x*x) [and later] This code is needed once in a map, Peter Otten wrote: Perhaps you like [sin(y)+cos(y) for y in (x*x for x in items)] then. Just wanted to emphasize this suggestion so that it doesn't get lost in the flood of lambda recommendations. If your code really looks like:: map(lambda x: sin(x * x) + cos(x * x), items) you should be using a list comprehension instead. Using map() here is not only more obscure and more verbose, but slower than:: [sin(x * x) + cos(x * x) for x in items] From there, it's a simple nested generator comprehension to pull out the subexpression: [sin(y) + cos(y) for y in (x * x for x in items)] If you aren't yet familiar with list and generator comprehensions, you should take a few minutes to look at some of your uses of map() and filter and see if you can simplify them using comprehensions instead. STeVe Jun 1 '07 #15

 P: n/a Steve Howell wrote: > The compiler doesn't know the types up front, but if you wanted to do this kind of optimization (and you believed that 95% of x*x cases would benefit from it, and you're willing to sacrifice performance for the 5% of folks that overload multiply), then the compiler could generate bytecode that set the stage for later conditional caching of the first execution of x*x. True. You'd then need the execution of the bytecodes at runtime (ceval.c or something called by it) to work in such a way that they only cache the result when side effects are not an issue. At runtime you can reliably detect whether something is still a virgin builtin, correct? I've no idea, but I imagine that psyco knows whether or not it has a proper built-in number object when it generates specialised code for similar cases. To my disclaimer, you would only undertake such an optimization if multiplication were really, really expensive (which I don't think is even true for floats today), and even then you'd proceed cautiously. Indeed. Some believe that for "full Python" you can only introduce such measures at run-time, although extensive enough analysis of the code could perhaps suggest suitable specialisations in advance, as presumably demonstrated by Shed Skin. Paul Jun 1 '07 #16

 P: n/a On Fri, 01 Jun 2007 07:09:50 -0400, Steve Holden wrote: >>>>The real answer is of course: Use a function.But what about something likelambda x: sin(y)+cos(y) where y=x*x?May be this could be a PEP? If there is no straight way to do this.def f(x): y = x*x return sin(y) + cos(y)What is not straightforward about that? This code is needed once in a map, so I don't want 3+ extra lines.Solution seemed so simple...I always considered python as languague, where simple things do not requireextensive coding.Moreover, this construction is common thing in functional programming. Stop thinking of three lines as "extensive coding" and your problem disappears immediately. The F-bot once suggested adding a clause to the Zen of Python about "writing two lines of code is not a sin" or "cramming two lines of code into one is not a virtue" (my paraphrases). Check the two alternatives: def f(x): y = x*x return sin(y) + cos(y) 44 key presses, including tabs and newlines and a blank line after the function, but excluding counting the shift key separately. lambda x: (lambda y: sin(y) + cos(y))(x*x) 42 key presses. Apart from the extremely minor issue of "namespace pollution", I think that speaks for itself. -- Steven. Jun 2 '07 #17

 P: n/a .... After years of discussion, Guido has decided to leave lambda alone for 3.0. It will not be neither expanded, nor removed, nor renamed. But it still will be as ugh, ugh, ugh-lee as a mule walking backwards ..... ;-) -- Stanley C. Kitching Human Being Phoenix, Arizona ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Jun 2 '07 #18

 P: n/a "Cousin Stanley" | It will not be neither expanded, nor removed, nor renamed. | | But it still will be as ugh, ugh, ugh-lee | as a mule walking backwards ..... ;-) Then pretend it was eliminated, as Guido once thought to do, and do not use it. And look away when others do ;-) tjr Jun 2 '07 #19

 P: n/a > > Check the two alternatives: def f(x): y = x*x return sin(y) + cos(y) 44 key presses, including tabs and newlines and a blank line after the function, but excluding counting the shift key separately. lambda x: (lambda y: sin(y) + cos(y))(x*x) 42 key presses. Apart from the extremely minor issue of "namespace pollution", I think that speaks for itself. and now I've only 60 lines on my screen, so what about def f(x): y = x*x; return sin(y)+cos(y); cheers, Stef Mientki Jun 2 '07 #20

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