On 9/22/2011 7:44 AM, Jussi Piitulainen wrote:
joni writes:
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
...
-7/3
-3
In late Python 2 you *can* and in Python 3 you *must* use // rather than
/ to get an int result.
-3 are more wrong than -2. Negativ number seems not to round to
nearest interger, but the integer UNDER the anwser!! Or?
Why?
It simply does not round to the nearest integer. It floors. This has
nicer mathematical properties. In particular, it allows the remainder
(notated as "per cent") operation (n % m) to return a number that
differs from n by a multiple of m ("is congruent to n modulo m").
These two operations go together.
The Python doc set has a FAQ collection. I recommend you read the
questions for those you might be interested in. In the Programming FAQ:
"Why does -22 // 10 return -3?
It’s primarily driven by the desire that i % j have the same sign as j.
If you want that, and also want:
i == (i // j) * j + (i % j)
then integer division has to return the floor. C also requires that
identity to hold, and then compilers that truncate i // j need to make i
% j have the same sign as i.
There are few real use cases for i % j when j is negative. When j is
positive, there are many, and in virtually all of them it’s more useful
for i % j to be >= 0. If the clock says 10 now, what did it say 200
hours ago? -190 % 12 == 2 is useful; -190 % 12 == -10 is a bug waiting
to bite."
--
Terry Jan Reedy
--
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