On 12/14/2011 5:09 AM, Eelco wrote:
Arguably, the most elegant thing to do is to define integer division
and remainder as a single operation;
It actually is, as quotient and remainder are calculated together. The
microprocessors I know of expose this (as does Python). 'a divmod b'
puts the quotient in one register and the remainder in another. If you
ask for just one of the two values, both are calculated and one is
grabbed while the other is returned.
which is not only the logical
thing to do mathematically, but might work really well
programmatically too.
The semantics of python dont really allow for this though. One could
have:
d, r = a // b
>>> a,b = divmod(10,3)
>>> a,b
(3, 1)
With CPython, int.__divmod__ lightly wraps and exposes the processor
operation.
But it wouldnt work that well in composite expressions; selecting the
right tuple index would be messy and a more verbose form would be
preferred.
That is why we have
>>> a == 10 // 3
True
>>> b == 10 % 3
True
In both cases, I believe CPython calls int.__divmod__ (or the lower
level equivalent) to calculate both values, and one is returned while
the other is ignored. It it the same when one does long division by hand.
However, performance-wise its also clearly the best
solution, as one often needs both output arguments and computing them
simultaniously is most efficient.
As indicated above, there is really no choice but to calculate both at
once. If one needs both a//b and a%b, one should explicitly call divmod
once and save (name) both values, instead of calling it implicitly twice
and tossing half the answer each time.
--
Terry Jan Reedy
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