# Position with Compile is acting slower than without

For a big matrix 5000 x 5000 or list2 i am trying to get positions of all entries Except[1.0]. I am using Position, however I find it quite odd that with Compile the result is comparatively slower.

Any ideas where i might be wrong in my implementation? and I would be grateful if you can let me know of an even faster implementation.

pos1 =  RandomInteger[{0, 1}, {5000, 2}];

pos2 = RandomInteger[{0, 1}, {5000, 2}];

list2 = DistanceMatrix[N@pos1, N@pos2];

(l2 = IntegerPart[
Compile[{{lis, _Real, 2}},
Position[lis, Except[1.], {2}, Heads -> False],
CompilationTarget -> "C"
][list2]
];) // AbsoluteTiming

(* {9.70626, Null} *)

(l1 = Position[list2, Except[1.], {2},

(* {5.80316, Null} *)

l1 === l2
(* True *)


Load CompiledFunctionTools and check whether Position compiled (your example probably doesn't compile because of the Except):

Needs["CompiledFunctionTools"]

CompilePrint @ Compile[
{{lis,_Real,2}},
CompilationTarget->"C"
]

(*
1 argument
2 Tensor registers
Underflow checking off
Overflow checking off
Integer overflow checking on
RuntimeAttributes -> {}

T(R2)0 = A1
Result = T(R2)1

1   T(R2)1 = MainEvaluate[ Function[{lis}, Position[lis, Except[1.], {2}, Heads \
-> False]][ T(R2)0]]
2   Return
*)


Notice the MainEvaluate. When Compile has to call MainEvaluate it will be slower than the uncompiled version.

You asked for a faster method. The following is pretty fast:

sa = SparseArray[list2, Automatic, 1.];
sa // AbsoluteTiming


{0.593123, SparseArray[< 12499892 >, {5000, 5000}, 1.]}

Then, you can use the accessor functions like:

sa["MatrixColumns"]; //AbsoluteTiming


{0.055956, Null}

to get information related to the positions. Depending on what you want to do with the positions, this may be useful.

positions can be obtained using:

sa["NonzeroPositions"]//AbsoluteTiming;
(* {0.0689321, Null} *)

sa["NonzeroPositions"] === Position[list2, Except[1.], {2}, Heads -> False];
(* True *)

• Worth to mention that simple cases will compile CompiledFunctionToolsCompilePrint @ Compile[{{lis, _Real, 2}}, Position[lis, 1.], CompilationTarget -> "C"] because one could be surprised it is mentioned in CompileCompilerFunctions[]
– Kuba
Jul 21, 2017 at 14:53
• @Carl so is there a way to make my current implementation faster. I was thinking of using a compiled version of MapIndexed but apparently i am having trouble with Compile[{{list, _Real, 2}}, MapIndexed[If[#1 != 1.0, #2] &, list, {2}]][list2] Jul 21, 2017 at 14:58