Re: st: RE: Taking averages, etc.

[Richard Williams <Richard.A.Williams.5@nd.edu>]

At 10:55 AM 12/17/2003 +0000, Nick Cox wrote:
> Non-programmers should note the crucial difference
> in principle between
>
> 1. if <condition> <command>
>
> and
>
> 2. <command> if <condition>
>
> Form 1 carries out _one_ test of the <condition>
> supplied. If it is true, <command> is carried
> out, but not otherwise.
>
> Form 2 carries a test of the <condition>
> supplied for _every_ observation specified
> and then carries out <command>
> for the observations for which it is true.
That is very interesting Nick and something I  might well have screwed up 
sooner or later.  Form 1 looks very much like the SPSS if command; but it 
works very differently.  Form 2 is what SPSS users will want as their 
equivalent to the SPSS if.

I'm probably just missing it, but it seems that Stata does not have a 
convenient if-then-else structure, i.e. the equivalent of SPSS's DO IF 
command.  In SPSS, I could do something like

DO IF X1 = 1 and X2 = 3
   Compute Y = 3.
ELSE IF X3 = 2 and X4 = 17
   Compute Y = 4.
ELSE
   Compute Y = 5.
END IF.

In SPSS, this would result in Y equaling 3, 4, 5, or missing for all 
cases.  If, say, X1 = 1, X2 = 3, X3 = 2, X4 = 17, Y would be coded 3, i.e. 
only the first compute would be executed.

In stata, it seems to me like I would have to write increasingly 
complicated if statements to achieve the same thing; something like

gen y = 3 if X1==1 & X2==3
replace y = 4 if X1 !=1 & X2 !=3 & X3==2 & X4==17
replace y = 5 if X1 !=1 & X2 !=3 & X3 !=2 & X4 !=17

or something like that.  Is that correct, or is there an easier way I am 
overlooking?

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