Re: Built-in function for `switch'

[John W. Eaton]

I wrote:

| I believe that the MathWorks web site mentions that MATLAB 5 supports
| a switch statement.  Does anyone know what syntax it uses?

On 28-Feb-1997, Guido Dietz <address@hidden> replied:

| This is an Xtract from the manual:
| switch var
|       case 1
|               disp('1');
|       case {2,3,4}                % < This is a Cell Array (new feature)
|               disp('2 or 3 or 4');
|       case 5
|               disp('5');
|       otherwise
|               disp('something else');
| end
| NOTE: Unlike C switch does not 'fall through'!

I just put a patch on ftp.che.wisc.edu in /pub/octave/switch-diffs
that provides a simple-minded implementation of this statement.

The new statement has the following form:

  switch EXPRESSION
    case LABEL COMMAND_LIST
    case LABEL COMMAND_LIST
    ...
    otherwise  COMMAND_LIST
  endswitch

For example:

  switch (x)
    case 1
      do_one ();
    case foo
      do_foo ();
    otherwise
      do_default ();
  endswitch


NOTES:

  * The identifiers `switch', `case', `otherwise', and `endswitch' are
    now keywords.

  * The LABEL may be any expression.

  * Duplicate LABEL values are not detected.  The COMMAND_LIST
    corresponding to the first match will be executed.

  * You must have at least one `case LABEL COMMAND_LIST' clause.

  * The `otherwise COMMAND_LIST' clause is optional.

  * As with all other specific `end' keywords, `endswitch' may be
    replaced by `end', but you can get better diagnostics if you use
    the specific forms.

  * Cases are exclusive, so they don't `fall through' as do the cases
    in the switch statement of the C language.

  * The COMMAND_LIST elements are not optional.  Making the list
    optional would have meant requiring a separator between the
    label and the command list.  Otherwise, things like

      switch (foo)
        case (1) -2
      ...

    would produce surprising results, as would

      switch (foo)
        case (1)
        case (2)
          doit ();
      ...

    particularly for C programmers.

  * The implementation is simple-minded and currently offers no real
    performance improvement over an equivalent if/elseif/else/endif
    block, even if all the labels are integer constants.  Perhaps a
    future variation on this could detect all constant integer labels
    and improve performance by using a jump table.

Please send any comments about this to the mailing list
address@hidden

Now for some questions I have about the MATLAB implementation of this
statement:

  * Does it require constant labels?

  * Do the labels have to be unique?

  * Are the command lists optional?

  * Is a separator (newline, comma, or semicolon) required between the
    case labels and the command lists?

  * Are the `case' and `otherwise' clauses optional (i.e., is `switch end' 
    a valid statement)?

Thanks,

jwe