[GRASS-dev] python: get results of r.stats call into list of lists of numericals instead of list of strings

[Glynn Clements]

Moritz Lennert wrote:

> I need some help with python scripting. I'm running r.stats on two 
> raster maps such as this:
> 
>          rstats_results  = gscript.read_command('r.stats',
>                               input_=[a, b],
>                               flags='n1',
>                               separator='comma',
>                               quiet=True)
>          results = rstats_results.splitlines()
> 
> 
> This gives me
> 
> ['123,456', '456,789', '987,321']
> 
> Does anyone know an efficient way to transform this into a list of lists 
> of numbers, such as:
> 
> [[123, 456], [456, 789], [987, 321]]
> 
> I know I can do this as follows:
> 
> mylist = []
> for line in results:
> 	mylist.append([int(line.split(',')[0]), int(line.split(',')[1])])
> 
> But for very large raster maps this does not seem very efficient.
> 
> Is there a way of getting the output directly as a list of lists ? Or a 
> more efficient transformation ?

For a start, there's no reason to call the .split() method twice:

	for line in results:
	    a, b = line.split(',')
	    mylist.append([int(a), int(b)])

A list comprehension might be more efficient, e.g.:

	mylist = [[int(x)
	           for x in line.split(',')]
	          for line in results]

Also, rather than reading the output as a string, I'd use
pipe_command() and iterate over the file:

        rstats  = gscript.pipe_command('r.stats',
                             input_=[a, b],
                             flags='n1',
                             separator='comma',
                             quiet=True)
	mylist = [[int(x)
	           for x in line.strip().split(',')]
	          for line in rstats.stdout]

I don't think that you'll do much better than that in Python. If it's
still too slow, try using numpy.loadtxt() (pre-processing the data
with sed if that function can't read it directly).

-- 
Glynn Clements <glynn at gclements.plus.com>