[GRASSLIST:37] Re: r.mapcalc evaluation order

[Glynn Clements]

Jan Hartmann wrote:

> I am running r.mapcalc on the LandScan world population database 
> (www.ornl.gov/gist/projects/LandScan). This is a 4 byte, 43.200*20.800 
> raster, and I use mapcalc to recategorize it to a one byte, 256 valued 
> raster, according to some different categorizing algorithms (essentially 
> variations on histogram equalisation). The category boundaries have been 
> computed separately; the only thing r.mapcalc has to do is to apply them 
> to the input raster. The mapcalc script only consists of a few dozens of 
> "if" rules, but these take a very long time to execute (days). My 
> impression is that r.mapcalc evaluates every rule for every cell. Does 
> anyone know if this is the case, and if so, is there a way to stop 
> evaluation after a match has been found (something like an "else" 
> statement)?

The overall evaluation strategy is that r.mapcalc processes one row at
a time. Each function (e.g. "if") takes a row of cells (i.e. a
one-dimensional array) as input, and stores the results in a result
array. For nested expressions, the array which holds the result from
an inner term is passed as an input to the enclosing term. Once the
final result has been computed for a row, the row is written out and
computation starts on the next row.

I suspect that your problem stems from the fact that the "if" function
is strict; i.e. all of the arguments are computed, passed to the
function, then one of the arguments is returned. Any time spent
computing the unused argument is effectively wasted.

This contrasts with general-purpose programming languages, where the
"if" construct is lazy; i.e. the condition is evaluated first, and
only one of the alternatives is evaluated.

r.mapcalc doesn't provide a lazy conditional mechanism. For
general-purpose programming languages, a lazy conditional is
absolutely necessary in order to be able to define recursive
functions. As r.mapcalc doesn't allow you to define your own functions
(recursive or otherwise), this isn't an issue.

For r.mapcalc, a lazy conditional would only be an optimisation; it
wouldn't extend the set of possible computations, but would just make
some of them faster.

I appreciate that this particular optimisation might be of substantial
benefit to you personally, but it would require significant changes to
r.mapcalc. Currently, *all* functions are strict; the core evaluation
code would need to be extended to allow for lazy functions generally
before you could implement any specific lazy function.

Consequently, such an extension isn't likely to be implemented any
time soon. In the absence of any other options, you could just add
your categorisation function to r.mapcalc itself. Adding new functions
is relatively straightforward; I can provide assistance if you wish to
take that route.

-- 
Glynn Clements <glynn.clements at virgin.net>