On Tue, 06 Dec 2005 15:44:33 +0000, Steve Holden <[EMAIL PROTECTED]> wrote:
>Daniel Schüle wrote: >> hi, >> >> [...] >> >> >>>># pseudo code >>>>i=2 >>>>lst=[i**=2 while i<1000] >>>> >>>>of course this could be easily rewritten into >>>>i=2 >>>>lst=[] >>>>while i<1000: >>>> i**=2 >>>> lst.append(i) >>>> >>> >>> >>>Neither of these loops would terminate until memory is exhausted. Do you >>>have a use case for a 'while' in a list comprehension which would >>>terminate? >> >> >> unless I am missing something obvious, I can not see why the loop should >> not terminate > >In that case, kindly explain how the condition i<1000 can become false >when it starts at 2 and never changes! [In other words: you *are* >missing something obvious]. > >> sure pseudo code is not executable but the other one works >> while tests the boolean expression first then decides whether to execute >> the body or not, in particular no next-iterator is involved(??) >> as it would be in >> lst=range(5) >> for i in lst: >> del lst[0] >> >Indeed. But the test condition is initially true, and can never become >false, so the loop is endless. It will probably eventually terminate by >throwing a MemoryError exception when lst and its element values use up >all available space. > >Don't you have an interpreter you could run the code in to verify that >it does indeed loop interminably? You seem to be assuming that the >expression i**2 changes the value of i. It doesn't. > No, but i**=2 does. Are you two talking about the same code? >>> i=2 >>> lst=[] >>> while i<1000: ... i**=2 ... lst.append(i) ... >>> lst [4, 16, 256, 65536] Regards, Bengt Richter
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