mjhjmtu

I just learned about Mr. Wizard's very efficient code here.

list=RandomChoice[True,False,10^6];
AbsoluteTiming[l2=Developer`ToPackedArray@With[True=1,False=0,Evaluate@list];]
(*0.066533,Null*)

Compare that to Boole.

AbsoluteTiming[l3=Boole[list];]
l2===l3
(*0.262770,Null*)
(* True *)

It seems we can make a faster Boole. I tried to use the code above to do that, but it doesn't work.

boole[list_]:=Developer`ToPackedArray@With[True=1,False=0,Evaluate@list];
list=True,True,False,True,False;
boole[list]
(*True,True,False,True,False*)

Why doesn't boole above work, and how can it be fixed?

The problem is the automatic module variable renaming that happens with With. You can use TracePrint to see this:

TracePrint[boole[list], _With]

With[True$=1,False$=0,True,True,False,True,False]

True, True, False, True, False

One idea to circumvent this renaming is to use a pure function:

Clear[boole]
boole = Function[Developer`ToPackedArray @ With[True = 1, False = 0, #]];

Then:

boole[list]

1, 1, 0, 1, 0

See Using With to scope over pure functions, then try:

boole[list_] := With @@ Hold[True = 1, False = 0, list] // Developer`ToPackedArray