How to force Return[] to return from an arbitrary point (like in C++)?

Question

I need to test some complicated conditions to calculate a Return value. But MMa doesn't let you Return[] from anywhere the way C++ does. The structure of my Module is similar to this:

foo[x_] := Module[{},
 Do[
  If[x == (n^2), Return[0]];
  If[x == (n^4), Return[1]],
  {n, 1, 5}];
 Return[2]];

If Return[] actually Return'd, foo[4] would return 0, but instead it returns 2.

Is there a way to do a return from an arbitrary place in the code?

I know that effete programmers will dislike my program structure, but I'm asking how to force the Return[], not how to restructure the program.

Answer 1

As pointed out in the comments, Return only exits from the inner most construct. So, even though foo[4] hits the True clause in the first If statement, that only exits Do. Instead of using Return, use Throw/Catch, e.g.

bar[x_] := Module[{$myTag},
     Catch[
      Do[ 
       If[x == (n^2), Throw[0, $myTag]];
   If[x == (n^4), Throw[1, $myTag]]
       ,
       {n, 1, 5}
      ];
      Throw[2, $myTag]
  ,
  $myTag
 ]
];

Note, I use $myTag to insulate Catch against other throws. Then,

bar /@ {1, 2, 4, 7, 8, 16}
(* {0, 2, 0, 2, 2, 1} *)

Answer 2

If your Module were not inert you could use the second parameter of Return as follows:

foo[x_] :=
 Module[{n},
   Do[
    If[x == (n^2), Return[0, Module]];
    If[x == (n^4), Return[1, Module]],
    {n, 1, 5}
   ];
   Return[2]
 ]

foo[4]

0

Alternatively you could Return to CompoundExpression if you eliminate it from within the Do loop:

foo[x_] :=
  (
   Do[
    {If[x == (n^2), Return[0, CompoundExpression]],
     If[x == (n^4), Return[1, CompoundExpression]]},
    {n, 1, 5}
   ];
   Return[2]
  )

foo[4]

0

For a more complete explanation see:

• What can I use as the second argument to Return in my own functions?