[R] else if statement error

[Gabor Grothendieck]

2009/10/6 Uwe Ligges <ligges at statistik.tu-dortmund.de>:
>
>
> Gabor Grothendieck wrote:
>>
>> 2009/10/6 Uwe Ligges <ligges at statistik.tu-dortmund.de>:
>>>
>>> The first rule is easy: As long as you are using scalar valued (i.e.
>>> length
>>> 1 vectors in R) "cond", you should prefer
>>>  if(cond) cons.expr  else  alt.expr
>>> rather than
>>>  ifelse(cond, yes, no)
>>> because the latter one evaluates both "yes" and "no" while the former one
>>> evaluates exactly one of both expressions.
>>
>> I don't think that that is true.  The false leg was not evaluated here:
>>
>>> ifelse(TRUE, { cat("a"); 1}, {cat("b"); 2})
>>
>> a[1] 1
>
>
> Ah, indeed that changed at some point and I forgot that the code checks for
> the length of cond nowadays. Thanks for pointing it out.
>

Modulo NAs, I think it checks whether cond is all TRUEs or all FALSEs
and in either of those cases it only evaluates one or the other.  Of
course if cond is length 1 then it necessary is all TRUE or all FALSE.
 If its a mixture of TRUE and FALSE then it evaluates both.

> ifelse(c(TRUE, TRUE), { cat("a"); 1}, {cat("b"); 2})
a[1] 1 1
> ifelse(c(FALSE, FALSE), { cat("a"); 1}, {cat("b"); 2})
b[1] 2 2
> ifelse(c(TRUE, FALSE), { cat("a"); 1}, {cat("b"); 2})
ab[1] 1 2