The D Programming Language: switches

[Richard Harter]

In article <18377 at think.UUCP> barmar at fafnir.think.com.UUCP (Barry Margolin) writes:

>> > 	Theoretically the switch construct is more powerful than an else-if
>> > chain because it selects in one step.  Execution is O(1) rather than O(n)
>> > where n is the number of branches.  It is, for example, considerably more
>> > efficient to use a switch with a thousand cases than an else-if chain with
>> > a thousand elses.  [This is not to be taken as an endorsement of such
>> > code. :-)]
>>  I think he meant in terms of expressability.  Which is true in a
>>practical sense, false in theorectically.  'switch' and 'else-if'
>>are equivalent in terms of expressability.

>That is obviously untrue.  A switch can only check one expression, and
>can only compare it against constant values.  An else-if chain can
>check for multiple conditions and can compare computed values.  You
>can even have side effects in the conditional expressions, which can
>affect later conditionals (I don't recommend this practice taken to
>extreme, but things like 'if ((ch = getchar()) == EOF) ... else if (ch
>== '\n') ...' aren't too bad).

	We're talking apples, oranges, and pears, here.  The switch
construct is superior to the else-if chain in computational efficiency,
in the sense that it can select between n alternatives in one step
rather than n steps, when the selection is made on the value of a
variable which has a range of length n.  An else-if chain can be
replaced by successive switches or by a single switch contained within
a while loop (in C with no additional if's), e.g.

	more = 1;
	kludge = 0;
	while (more) switch(kludge) {
	  case 0: kludge += (1 + (expr1));
	          break;
	  case 1: kludge = 3;
                  break;
	  case 2: action1;
	          more = false;
	          break;
	  case 3: kludge += (1 + (expr2));
	  ....

[This code does not have the good programming stamp of approval.]  If
we ignore questions of computational power and simply consider expressive-
ness, else-if chains can obviously replace switches.  And, of course,
if we program reasonably, we use each where it is appropriate.  (Easier
said then done.)
-- 

In the fields of Hell where the grass grows high
Are the graves of dreams allowed to die.
	Richard Harter, SMDS  Inc.