Hello Monks

I'm new to PDL, giving it a whirl to speed up some 2D matrix crunching at the heart of an algorithm that has been rigorously proven to be the most efficient ~~waste~~ use of a week's worth of CPU time to solve the given problem. But I digress. :-)

I'm having the most trouble fully wrapping my head around threading, specifically how to fetch adjacent cells in a 2D matrix which match an arbitrary boolean expression.

Below follow some (annotated) `pdl>` commands hopefully showing what I'm trying to accomplish.

`# Set up initial 4x4 matrix with some arbitrary data
pdl> p $b = pdl (10..25)->reshape(4,4);
[
[10 11 12 13]
[14 15 16 17]
[18 19 20 21]
[22 23 24 25]
]
# Fetch the 2D index for the position of element with value 21
pdl> p $pos = whichND($b == 21);
[
[3 2]
]
# Simple transformation matrix to mark adjacent cells.
pdl> $xfrm = pdl [-1,0],[0,-1],[1,0],[0,1];
# Show the results of the transformation: So far so good (even though
# one of the values is out of bounds, we're expecting that.)
pdl> p $pos + $xfrm;
[
[2 2]
[3 1]
[4 2]
[3 3]
]
# Here's the rub. The following command returns the *values* of
# the adjacent cells, but I want the indicies instead. (See discussion
+)
pdl> p $b->indexND($pos + $xfrm, 'truncate');
[20 17 0 25]
# $b->range($pos + $xfrm, 1, 'truncate') gives similar results
`

### What I need:

The above code gets me halfway to where I need to be; from the `$pos + $xfrm` line, I need to basically grep those elements which are a) valid (i.e., within the piddle dimensions), and b) match a given boolean expression. Hence I tried this (obviously broken) code:

`pdl> p $indicies = $b->indexND($pos + $xfrm, 'truncate')->which($b % 2
+);
# Returns: empty
#
# Expected: Should contain two single element piddles:
# [17 25]
# OR, the indicies in the original matrix would be fine:
# [
# [3 1] # 17
# [3 3] # 25
# ]
# N.b. [2 2] would be omitted (value = 20, even), as would [4 2] (inde
+x out of bounds)
`

I'm thinking this sort of thing must be a very common pattern, but I can't seem to find a workable example anywhere. What's the most concise and efficient way to iterate over adjacent cells matching a given boolean expression?