MOD operator returns wrong results for negative argument
tingtron opened this issue · 3 comments
Bug report
Problem
MOD operator returns wrong results with negative argument.
Steps
- Run PC-BASIC
- Enter the following and press
Enter
for i=-5 to 3: ? i mod 2; : next - Observed:
-1 -2 -1 -2 -1 0 1 0 1 - Expected:
-1 0 -1 0 -1 0 1 0 1 - Compare with GW-BASIC from MS-DOS (in DOSBox)
Note: Python works as expected:
>>> [x % 2 for x in range(-5,3)]
[1, 0, 1, 0, 1, 0, 1, 0]
The problem is in the source pcbasic\basic\values\numbers.py:
if dividend < 0 or mod < 0: mod -= divisor
It should be
mod = -mod
Program
This affects an existing program, which calculates coordinate offset.
Notes
PC-BASIC version: 2.0.7
Operating system version: Windows 11 64-bit
I can confirm this is a bug and I can reproduce.
Surprisingly, Python works wrong here:
-17 % 3
1
17 % 3
2
-17 % -3
-2
For reference, these are the results for Python and for GW-BASIC:
GW-BASIC:
? 17 mod 3
2
Ok
? 17 mod -3
2
Ok
? -17 mod 3
-2
Ok
? -17 mod -3
-2
Ok
Python:
>>> 17 % 3
2
>>> 17 % -3
-1
>>> -17 % 3
1
>>> -17 % -3
-2
Some background as to what Python does and why it does that, linked below. TLDR, Python's // rounds toward negative infinity, and % then does what it needs to do to ensure the relation holds that divisor*quotient + remainder = dividend.
https://stackoverflow.com/questions/3883004/how-does-the-modulo-operator-work-on-negative-numbers-in-python#3883019
http://python-history.blogspot.com/2010/08/why-pythons-integer-division-floors.html