I'm trying to replicate Python's modulo operation in Forth and I'm not familiar with Python at all. I've noticed that Python provides specific results when using negative inputs. For instance:
-7 % 26 results in 19, and 7 % -26 results in -19. I can get these results using an online Python emulator as in this example:
d = -7
e = 26
f = d % e
print(f"{d} % {e} = {f}")
Output: -7 % 26 = 19
This behavior seems consistent with Perl as well, which also gives 19 for -7 % 26. I want to ensure that the Forth version gives the same outcomes. Can anyone explain the algorithm or method Python uses to achieve this?
3 Answers
When dealing with negative numbers, it's crucial to understand that the modulo doesn't behave the same way in all languages. In Python, the modulo will always yield a result with the same sign as the divisor. So, if you pass a negative divisor, it will return a negative result. In your case, -7 % 26 equals 19 because the divisor (26) is positive and Python adjusts the result accordingly.
If you want to see the exact implementation, check out the CPython source code. The Python int type is defined as `PyLongObject` and you’ll want to focus on the `l_divmod` function for the details on modulo behavior.
Here's the link for quick access: [CPython Source](https://github.com/python/cpython/blob/main/Objects/longobject.c#L4484). This should give you a clear understanding of how Python handles mod operations.
You can find the algorithm for modulus in Python's documentation. Essentially, Python defines the modulo operation so that the result has the same sign as the divisor.
Here's a quick reference:
a mod b = (a - b) * floor(a / b)
So, when you apply this to -7 % 26, you get a positive result because 26 is positive. It's worth noting that if you want to dive deeper into how Python implements it, you can look at the CPython source code on GitHub.

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