       Using Extract where some indices are out of bounds (efficiently)

• To: mathgroup at smc.vnet.net
• Subject: [mg114926] Using Extract where some indices are out of bounds (efficiently)
• From: Julian Francis <julian.w.francis at gmail.com>
• Date: Thu, 23 Dec 2010 03:52:42 -0500 (EST)

```Dear all,

How can I arrange to use Extract to extract values from an array; but
where some of the indices may be out of bounds, in which case they
should be simply ignored?

I wrote a simple arrayInBounds function, and then did a Select on that
to retrieve only the indices which are in bounds, which are then fed
into Extract. Whilst this works it seems to lead to a 150* reduction
in speed.

FYI:
My arrayInBounds function looks like this:

arrayInBounds[array_, index_] :=
If[index == {}, True,
First[index] > 0 && First[index] < Length[array] &&
arrayInBounds[array[], Rest[index]]]

My support functions for the test are:

disks = Table[
Transpose[Position[DiskMatrix[r, 2*(r + 1) + 1], 1]], {r, 1, 30}];

positions[x_, y_, r_] := Transpose[{
disks[[r, 1]] + y - (r + 2),
disks[[r, 2]] + x - (r + 2)}]

My timing tests are:

In:= Timing[lists=Table[Extract[image,Select[positions[x,y,
10],arrayInBounds[image,#]&]],
{y,11,64-11},
{x,11,64-11}];]
Out= {17.175,Null}
In:= Timing[lists2=Table[Extract[image,positions[x,y,10]],
{y,11,64-11},
{x,11,64-11}];]
Out= {0.094,Null}
In:= lists==lists2
Out= True

image is assumed to be an array of at least 64*64.

In this particular case, the bounds are setup so that the checking
serves no purpose and can be eliminated, but I've only done this to
show the speed comparison with the no bounds checking comparison. The
speed difference is a factor of over 150! This seems surprising to me.
Is there a better way?

I don't think I can reformulate it using partitions, or the standard
imagefilter functions, because in general the functions positions will
be giving a complicated list of coordinates which doesn't naturally
fit into a square kernel.

Any help greatly appreciated.

Thanks,
Julian.

```

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