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Forum Index : Microcontroller and PC projects : MOD (modulo) operator strangeness

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daveculp
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Joined: 02/07/2020
Location: United States
Posts: 22

Posted: 01:02am 06 Jul 2020

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Im writing a game of life and I wanted the board to wrap around. Normally I do this with the modulo operator. Example:

The left most column (0) should wrap around to the right most column (49). So, if I want to get the state of that cell I could do something like this (in Python)

num_neighbors += board[row] [ (col-1) % NUM_C]

If col = 0 and NUM_C represents the max number of columns (50),this would result in

-1%50

Which would be 49 and therefore wraps around to the last (50th) column (starting from 0).

I tried this on the CMM2 and it claims the statement

-1 MOD 50

is -1.

That doesn't seem right.

daveculp
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Joined: 02/07/2020
Location: United States
Posts: 22

Posted: 01:05am 06 Jul 2020

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I can write my own modulus function doing something like this:

PRINT modulus(-1,50)

FUNCTION modulus (numerator, denominator) AS INTEGER
� �modulus=numerator-INT(numerator/denominator)*denominator
END FUNCTION

This properly returns 49
Edited 2020-07-06 11:08 by daveculp

daveculp
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Joined: 02/07/2020
Location: United States
Posts: 22

Posted: 01:49am 06 Jul 2020

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Well, I just learned that using negative numbers with modulo is "undefined" and different in different languages.

https://en.wikipedia.org/wiki/Modulo_operation

"When either a or n is negative, the naive definition breaks down, and programming languages differ in how these values are defined."

I will just use my function above.

KeepIS

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Joined: 13/10/2014
Location: Australia
Posts: 1872

Posted: 04:25am 06 Jul 2020

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Thanks for following this one through, another thing to put in the notes.

Mike.

NANO Inverter: Full download - Only Hex Ver 8.1Ks

TassyJim

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Joined: 07/08/2011
Location: Australia
Posts: 6269

Posted: 04:56am 06 Jul 2020

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My way of catching small negative numbers is to add the divisor to the dividend first.

a = -1
b = 50
print a mod b

print (a+b) mod b

If you are likely to have big excursions, make it a multiple of the divisor.

Jim

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