[GRASS-dev] eval option in r.mapcalc in a python script

[Paulo van Breugel]

On 14-01-16 09:57, Moritz Lennert wrote:
> On 14/01/16 09:34, Paulo van Breugel wrote:
>>  From the r.mapcalc manual, the 'eval' option is described to run more
>> complex expressions. But I wonder how I apply the example below in a
>> python script?
>
>
>> eval(elev_200 = elevation - 200, \
>> elev_5 = 5 * elevation, \
>> elev_p = pow(elev_5, 2))
>> elevation_result = (0.5 * elev_200) + 0.8 * elev_p
>
> Can't you just put the above in a string variable and feed it to the 
> 'expression' parameter of grass.mapcalc (or alternatives 
> grass.run_command('r.mapcalc', ...)?

Thanks, I'll try that
>
>
>>
>> Somewhat related, if I have a large number of maps. For each map I need
>> to compute a value. At the end, the resulting maps need to be summed up.
>> One way is to run a loop, and use r.series to sum up the results. E.g.,
>>
>> for i in xrange(len(IN)):
>>      out = 'shi' + str(i)
>>      out.append(out)
>>      grass.mapcalc("$out = log($in)", out=out[i], in=in[i])
>> grass.run_command("r.series", output="endresult", input=out, 
>> method="sum")
>>
>>
>> This generates a lot of intermediate layers, which can potentially take
>> up a lot of space on HD. To avoid that, I could do
>>
>> grass.mapcalc("$out = 0", out=out)
>> for i in xrange(len(IN)):
>>      grass.mapcalc("$out = $out + log($in)", out=out, in=in[i]),
>> overwrite=True
>>
>
> ISTR that modifying the contents of a map in-place might be possible, 
> but not recommended.
I think your solution below is perfectly fine, but out of curiosity, and 
to get a better grasp of the possible pitfalls, what is the reason the 
above in-place modification is not recommended?
>
> As a compromose you could do something like this:
>
> grass.mapcalc("$oldout = 0")
> for i in xrange(len(IN)):
>      grass.mapcalc("$newout = $oldout + log($in)", out=out, in=in[i])
>      grass.run_command('g.rename', '$newout,$oldout', overwrite=True)
>
>
>> An alternative is to construct one large expression along the lines of
>> expression = "log(in[1]) + log(in[2] + ....". I assume this will be
>> faster? It would result in a unwieldy large expression though, and I
>> remember something about a limit to the length of the expression?  So
>> that is why I wonder if using the 'eval' option would be useful.
>>
>> Any tips for the best approach?
>
>
> Maybe one option, as I could image others to have this kind of issue, 
> would be to add a "modifier" (or similar name) parameter to r.series 
> which would allow to calculate the statistic defined by "method" on a 
> modified version of the maps (e.g. squared, square root, log, sin, 
> cos, etc.).

The log in the example above was for illustrative purposes, the real 
expression is a bit more complex. Nevertheless, I think it would be very 
useful to have such a modifier, really good idea!
>
>
> Moritz