[Python-Dev] Floor division (original) (raw)

Gareth McCaughan gareth.mccaughan at pobox.com
Wed Jan 24 10:00:04 CET 2007

On Wednesday 24 January 2007 08:39, Alexey Borzenkov wrote:

[me, about complex():]

> It seems pretty clear what it thinks it's doing -- namely, > defining complex(a,b) = a + ib even when a,b are complex. > And half of why it does that is clear: you want complex(a)=a > when a is complex. Why b should be allowed to be complex too, > though, it's hard to imagine.

[Alexey:]

I think that's the right thing to do, because that is mathematically correct. j is just an imaginary number with a property that j*j = -1. So (a+bj) + (c+dj)j = (a-d) + (b+c)j.

Yes, thanks, I know what j is, and I know how to multiply complex numbers. (All of which you could have deduced from reading what I wrote, as it happens.) The question is whether it makes sense to define complex(a,b) = a+ib for all a,b or whether the two-argument form is always in practice going to be used with real numbers[1]. If it is, which seems pretty plausible to me, then changing complex() to complain when passed two complex numbers would (1) notify users sooner when they have errors in their programs, (2) simplify the code, and (3) avoid the arguably broken behaviour Tim was remarking on, where complex(-0.0).real is +0 instead of -0.

[1] For the avoidance of ambiguity: "real" is not
    synonymous with "double-precision floating-point".

Complex numbers are not just magic pairs with two numbers and have actual mathematical rules.

Gosh, and there I was thinking that complex numbers were magic pairs where you just make stuff up at random. Silly me.

-- Gareth McCaughan PhD in pure mathematics, University of Cambridge

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