sequence multiplied by -1
Steven D'Aprano
steve-REMOVE-THIS at cybersource.com.au
Mon Sep 27 07:17:27 CEST 2010
On Sun, 26 Sep 2010 09:30:08 +0000, Seebs wrote:
>> There's nothing obscure or unintuitive about "spam"*3 = "spamspamspam",
>> and the fact that it doesn't do the same thing as int("spam")*3 is a
>> foolish argument.
>
> The languages in which it's surprising are mostly things like perl,
> where there's a certain amount of implicit coercion going on, so it's
> ambiguous whether "3"*3 ought to mean "333" or 9. (Except it isn't,
> because repetition is a separate operator, written "3" x 3 rather than
> "3" * 3...) People coming from that kind of background may be expecting
> * to stay an arithmetic operator, rather than having it change form when
> applied to non-arithmetic objects.
And people with no programming background at all will be surprised that *
and x aren't two different symbols for the same operation.
> I'm not sure either way. I think on the whole, I like the notion of a
> repetition operator which is distinct from multiplication, but I also
> definitely prefer the lack of implicit coercion, and without implicit
> coercion, there's a lot less underlying ambiguity to worry about.
>
> From the top, I guess my analysis is:
>
> * It seems clear that, given two sequences x and y, "x + y" ought to
> be the concatenation of these sequences.
> * Thus, "x + x" should be x concatenated to itself. * Idiomatically, it
> is not unreasonable to assert that "x * 2" and "x + x"
> could be the same value.
> * It thus makes some sense for <sequence> * <number> to indicate
> repetition of the sequence.
> * Since a string is a kind of a sequence, it also makes sense for
> <string> * <number> to indicate repetition.
My analysis as well, except that I see repetition (repeated concatenation/
addition) as more fundamental than multiplication. Think about being back
in primary school, where you learn to do this:
21
21
+21
----
63
before you learned this:
21
x3
----
63
> So I think I'm pretty much convinced that Python's behavior makes sense,
> but it makes sense only because I'm going into this expecting operators
> to be defined by types, so I don't expect every '*' to mean
> 'multiplication'.
Perhaps you should say "real-valued multiplication" rather than just
multiplication, because of course matrix multiplication A*B behaves quite
differently from integer multiplication a*b as well.
> Helps, perhaps, that I got exposed to group theory early enough to be
> used to redefining + and * to be any two operations which have
> interesting properties*.
>
> -s
> [*] What's purple and commutes? An abelian grape.
Heh :)
--
Steven
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