# 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
[download]

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)
[download]

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?