[GRASSLIST:4582] Re: if and docell

[Eric G. Miller]

On Tue, Sep 24, 2002 at 06:04:59PM +0200, Alexander Fritz wrote:
> Is there a possibility to do if then calculations in GRASS and is there
> something similar to the DOCELL command in ARCINFO?

Nothing exactly like DOCELL, but you can use the 
if(<cond>,<when true>,<when false>) and eval() with temp variables, to
do quite a few things.  See the mapcalc algegra doc (PS) for examples.
uLtimately, every expression results in a single output.

For others:  DOCELL, basically lets you write a mini procedure which may
have multiple inputs and outputs usually in conjunction w/ Arc Macro
Language constructs.  Don't have a handy example but something like:

  DOCELL {
      a = b + c
      e = if (a, a * atan(4.0), 1.0) 
  }

would translate to something like:

  for (i = 0; i < nrows; i++)
     for (j = 0; j < ncols; j++)
     {
        a[i][j] = b[i][j] + c[i][j];
	e[i][j] = (a[i][j]) ? a[i][j] * atan(4.0) : 1.0;
     }

So, it's how many times do you want to pass over the data...

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