Re: MOD operator

[Phillip David <phillip_david(at)xontech.com>]

Theo Brauers wrote:
> 
> I was wondering if the result of the MOD operator in IDL changed
> from previous versions (4.x, 5.0) to the current version (5.3.1).
> Now the output is:
> 
> IDL> PRINT, (FINDGEN(8)-4.) MOD 3
>      -1.00000     0.000000     -2.00000     -1.00000     0.000000
>       1.00000      2.00000     0.000000
> 
> When I programmed a function long ago I used the MOD operator
> expecting the output
> 
> IDL> PRINT, (FINDGEN(8)-4.) MOD 3
>       2.00000     0.000000      1.00000      2.00000     0.000000
>       1.00000      2.00000     0.000000
> 

To the best of my knowledge, this is always the way IDL (and most other
languages providing a MOD operator) has worked.  If you wish to take the
mod of a negative number , the result will be a negative number between
-(n+1) and 0.  A positive input yields a positive result.  So when we're
trying to limit a value to between 0 and 2*pi, we often end up with code
that looks something like:

x = ((y mod (2*pi)) + 2*pi) mod (2*pi)

Hope this helps.

Phillip