David Elliott <[email protected]> wrote about nested braces,
> if strlen(trim("`exp'"))<6 { // Too short text field
> local error 1
> local type "unkn"
> }
> else { //#1 (counter for number of close braces needed)
>
> ...a lot of additional code...
>
> // 2digit year leading separated
> if regexm("`exp'","([0-9][0-9])[-,./\ ]+([0-9][0-9]?)[-,./\
> ]+([0-9][0-9]?)") {
> local type y2/m/d
> local order 20ymd
> local error 7
> }
> else { //#8
> // 0 to O substitution
> if regexm("`exp'","[O]") {
> local error 2
> }
> else { //#9
> // Nothing matched
> local error 3
> }
> }
> }
> }
> }
> }
> }
> }
> } // close all 9 elses
and made a case for a -case- statement. Nick Cox <[email protected]>
made a point about indentation:
> I would lay out like this:
>
> if #1 {
> something
> }
> else {
> if #2 {
> something else
> }
> else {...
I would like to remind both of you that Stata allows -else if-:
if #1 {
something
}
else if #2 {
something else
}
else if #3 {
something else again
}
...
else {
default case
}
-else if- is not, appearance wise, very different from case:
docase {
case #1 {
something
}
case #2 {
something else
}
otherwise {
and something else again
}
}
In fact, the -else if- way of coding requires less identation!
The advantage of a -case- statement has to do with constants and having a
compiler that build efficient branch tables. That is, in "case #1" above, you
might be imagining "x==y" or det(x)!=0 or some other expression. That,
however, does away with all the efficiency that -case- can bring to the table.
What you should be imagining is something like
docase (x) {
case 1 {
something
}
...
}
-case 1- code would be done when x==1. Nice, simple, and constant. The
advantage of these constants is that a compiler can build a table of of where
to go. Think of a vector with (address of routine for 1, address of routine
for 2, ...). Then vector[i] is where to branch, and the compiler just grabs
the element and off it goes. Importantly, the speed of the branch is no
longer a function of the number of alternatives, or even where in the list the
alternative appears.
Anyway, a case can be made for -case- in Mata. For Stata, being an
interpreter, the case can only be made on the basis of style and, as I've
said, -else if- is a good alternative, stylewise.
I can tell you that -case- is on the list for Mata, but not high on the list.
In my experience in statistical programming, constant branch tables do not
arise often.
Am I missing something?
-- Bill
[email protected]
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