How to simulate C style integer division?

[Steven D'Aprano]

On Thu, 21 Jan 2016 08:11 pm, Ben Finney wrote:

> Shiyao Ma <i at introo.me> writes:
> 
>> I wanna simulate C style integer division in Python3.
> 
> I'm not sure I know exactly what behaviour you want (“C style” may mean
> different things to each of us).

Surely is means "whatever the C standard defines integer division to mean".

Are their any C programmers here who can tell us what the C standard says?

According to this:

http://stackoverflow.com/questions/3602827/what-is-the-behavior-of-integer-division-in-c

the behaviour of integer division was implementation dependent in C89, but
has now been specified in detail since C99. The answer is that a/b for
integers a and b *truncate towards zero*. That is, it returns the integer
part with no fractional part:

Both arguments are positive, or both negative:

    11/2 = 5.5 so throw away the 0.5 and return 5

    -11/-2 = 5.5 so throw away the 0.5 and return 5

One argument is positive and the other negative:

    -11/2 = -5.5 so throw away the 0.5 and return -5

    11/-2 = -5.5 so throw away the 0.5 and return -5


Python's integer division // performs flooring, not truncating, so it
disagrees in the case that exactly one of the arguments are negative:

py> 11//2
5
py> -11//-2
5

py> 11//-2
-6
py> -11//2
-6


So my guess is that the fastest, and certainly the most obvious, way to get
the same integer division behaviour as C99 would be:

def intdiv(a, b):
    # C99 style integer division with truncation towards zero.
    n = a//b
    if (a < 0) != (b < 0):
        n += 1
    return n



-- 
Steven