Function with Return

Question

A simple function that adds 3 to all input and return a list containing the input and the output:

test[h_] := {
   g = h + 3;
   Return[{h,g}];
   };

When I evaluate it with test[5], I get:

{Return[{5, 8}]}

but I would like to get: {5, 8}

Answer 1

You have the wrong type of brackets. Your version is returning a List (which retains the Return object).

Also, Return is used to break out of control structures. Basic return of variables is automatic; you don't need to use Return in your example.

Here the bracket are corrected.

test[h_] := (
  g = h + 3;
  Return[{h, g}];)

test[5]

{5, 8}

The above is actually returning from the expression group before it hits the suppressing semi-colon.

Without Return the semi-colon suppresses output.

test[h_] := (
  g = h + 3;
  {h, g};)

test[5]

null

To fix this you can just omit the final semi-colon.

test[h_] := (
  g = h + 3;
  {h, g})

test[5]

{5, 8}

Why did Return appear?

Comparing two simple cases

test[h_] := Return[{h, h + 3}]

test[5]

{5, 8}

test[h_] := {Return[{h, h + 3}]}

test[5]

{Return[{5, 8}]}

The reason is mentioned here

The very last step of the evaluation loop is ...

"Discard the head Return, if present, for expressions generated through application of user-defined rules."

In the second case the head is List so the Return is not discarded.

Answer 2

You can use modules:

test[h_] := Module[{g},
   g = h + 3;
   Return[{h, g}]];

As suggested by @m_goldberg the function can be simplified:

test[h_] := Module[{g},
   g = h + 3;
   {h, g}];