negative numbers and integer division

[Matthew Wilson]

Hi-

I just discovered this:

-1 // 12 -1 1 // 12 0

I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any ideas?

[Martin v. Löwis]

"Matthew Wilson" <mw*****@sarcastic-horse.com> writes:

Hi-

I just discovered this:

-1 // 12 -1 1 // 12 0

I thought that -1 // 12 would be 0 also.

Why did you think that? Dividing -1 by 12 gives -1, with a remainder
of 11: -1*12 + 11 == -1.
I'm writing a simple monthly date class and i need (-1,2001) to be
translated to (11,2000). Any ideas?

def norm_month(month, year):
delta_year, month = divmod(month, 12)
return month, year+delta_year

print norm_month(-1, 2001)

Regards,
Martin

[Mark Hahn]

That definitely seems wrong to me, even though it is a correct "floor"
function.

I was given the impression that (int // int) was going to be the replacement
for (int / int) when (int / int) is changed to return a float, but -1/12 now
gives 0, not -1, so (int // int) is not a replacement for (int / int).

"Matthew Wilson" <mw*****@sarcastic-horse.com> wrote in message
news:ma**********************************@python.o rg...

Hi-

I just discovered this:

-1 // 12 -1 1 // 12 0

I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any ideas?

[Mark Hahn]

oops -- ignore that last post -- i was totally wrong

"Mark Hahn" <ma**@hahnca.com> wrote in message
news:HWjfb.5187$hp5.1152@fed1read04...

That definitely seems wrong to me, even though it is a correct "floor"
function.

I was given the impression that (int // int) was going to be the replacement for (int / int) when (int / int) is changed to return a float, but -1/12 now gives 0, not -1, so (int // int) is not a replacement for (int / int).

"Matthew Wilson" <mw*****@sarcastic-horse.com> wrote in message
news:ma**********************************@python.o rg...

Hi-

I just discovered this:

>> -1 // 12

-1

>> 1 // 12

0

>>

I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any ideas?

[Rainer Deyke]

Matthew Wilson wrote:

I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any
ideas?

Luckily for you, Python integer division does the right thing. You
*want* -1 // 12 to be -1 for your application.

def normalize_date(month, year):
return (month % 12, year + month // 12)

normalize_date(-1, 2001) # Returns (11, 2000)
'normalize_date' as given assumes 'month' is in the range from 0 to 11; use
this if your months are in the range from 1 to 12:

def normalize_date(month, year):
return ((month - 1) % 12 + 1, year + (month - 1) // 12)

--
Rainer Deyke - ra*****@eldwood.com - http://eldwood.com

[Skip Montanaro]

Mark> I was given the impression that (int // int) was going to be the
Mark> replacement for (int / int) when (int / int) is changed to return
Mark> a float, but -1/12 now gives 0, not -1, so (int // int) is not a
Mark> replacement for (int / int).

from __future__ import division
-1/12 -0.083333333333333329 -1//12

-1

Skip

[Peter Abel]

"Matthew Wilson" <mw*****@sarcastic-horse.com> wrote in message news:<ma**********************************@python. org>...

Hi-

I just discovered this:

-1 // 12 -1 1 // 12 0
I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any ideas?

Maybe the following could help:

def normalize_date(month,year): .... return (((month+11) % 12)+1,year+((month-1)/12))
.... normalize_date(1,2003) (1, 2003) normalize_date(12,2003) (12, 2003) normalize_date(0,2003) (12, 2002) normalize_date(-1,2003) (11, 2002) normalize_date(13,2003) (1, 2004)

Regards
Peter

[mensanator]

"Matthew Wilson" <mw*****@sarcastic-horse.com> wrote in message news:<ma**********************************@python. org>...

Hi-

I just discovered this:

-1 // 12 -1 1 // 12 0
I thought that -1 // 12 would be 0 also. I'm writing a simple monthly
date class and i need (-1,2001) to be translated to (11,2000). Any ideas?

-1 % 12

11