[GRASSLIST:2088] Re: r.mapcalc questions

[Glynn Clements]

Miha STAUT wrote:

> I did not understand how the min, max, median and mode functions in 
> r.mapcalc work

They take a variable number of arguments.

> (is there also a function the calculation of arithmetic 
> averages? e.g. mean).

No, but it isn't much harder to write it out explicitly, e.g.:

	(x1+x2+x3)/3

> I would only like to perform an operation like this:
> 
> r.mapcalc "p=myrast1-min(myrast2)"

What do you mean by "min(myrast2)"? The minimum cell value occuring
within myrast2? If so, you can get this data from "r.info -r", e.g.

	min=`r.info -r myrast2 | sed -n '/^min=/s///p'`
	r.mapcalc "p=myrast1-$min"

> Is there any other way to insert some descriptive statistics into r.mapcalc 
> directly?

r.mapcalc evaluates the expression(s) on a per-cell basis; it doesn't
compute aggregates over the entire map. For that, r.statistics is more
appropriate.

> How do I manage rasters with number names in r.mapcalc? E.g. ortophotos 
> named with numbers. I tried the following:
> r.mapcalc "newrast=myrast-"40600370""# 40600370 is a raster layer
> But it still treats the name like a number.

Single and double quote characters are significant to the shell. To
get the right expression in to r.mapcalc you have to do one of the
following:

1. Run r.mapcalc without arguments and type the expression at the
prompt:

	GRASS:~ > r.mapcalc
	Enter expressions, "end" when done.
	mapcalc> newrast=myrast-"40600370"
	mapcalc> end

2. Store the expression in a file, and redirect r.mapcalc's input from
that file:

	r.mapcalc < expression.txt

3. Use single quotes around the expression and double quotes within
it:

	r.mapcalc 'newrast=myrast-"40600370"'

[The shell doesn't interpret double quotes (or anything else) within
single quotes.]

4. Use double quotes around the expression and single quotes within
it:

	r.mapcalc "newrast=myrast-'40600370'"

[The shell doesn't interpret single quotes within double quotes;
however it still expands shell variables, so use this form if you need
to use shell variables within an expression.]

5. Use double quotes around the expression and "escape" any double
quotes in the expression with backslashes:

	r.mapcalc "newrast=myrast-\"40600370\""

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