Negativ nearest interger?

[joni]

Have a simple question in the Integer calculator in Python 2.65 and
also 2.7..

The consol showing:

Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.

-3 are more wrong than -2. Negativ number seems not to round to
nearest interger, but the integer UNDER the anwser!! Or?

Why?

 

[Jussi Piitulainen]

Have a simple question in the Integer calculator in Python 2.65 and
also 2.7..

The consol showing:

Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) ....

-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.

 

[joni]

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.

Thanx. See if I can understand it.... /Cheers

 

[Terry Reedy]

joni writes:

In late Python 2 you *can* and in Python 3 you *must* use // rather than
/ to get an int result.

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 signas 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."