[GRASS-user] Strange r.mapcalc results

[Glynn Clements]

Hermann Peifer wrote:

> I am trying to find out if 2 raster layers are exactly identical. I am 
> not an experienced mapcalc user, so I am wondering, where the 3200204 
> NULL values come from, in Test1. The result in Test2 is what I expected.
> 
> My obviously wrong understanding was that I wouldn't need the special 
> operator ||| in Test1, as NULL values are already treated specially by 
> isnull()

> Any hint from the experts?

> Test1
> 
> $ r.mapcalc 'result = isnull(mapA) && isnull(mapT) || mapA == mapT ? 1 : 2'

> Test2
> 
> $ r.mapcalc 'result = isnull(mapA) && isnull(mapT) ||| mapA == mapT ? 1 : 2'

If mapA and mapT are both null, then Test 1 reduces to:

	result = isnull(mapA) && isnull(mapT) || mapA == mapT
->	result = isnull(null) && isnull(null) || null == null
->	result = 1 && 1 || null
->	result = 1 || null
->	result = null

while Test2 reduces to:

	result = isnull(mapA) && isnull(mapT) ||| mapA == mapT
->	result = isnull(null) && isnull(null) ||| null == null
->	result = 1 && 1 ||| null
->	result = 1 ||| null
->	result = 1

The || and && operators always propagate nulls (i.e. they return null
if either operand is null), while ||| and &&& return a non-null result
where possible (i.e. ||| returns 1 if either operand is non-zero, &&&
returns 0 if either operand is zero).

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