[R] ifelse versus ...

Ott Toomet siim at obs.ee
Mon Apr 29 09:40:08 CEST 2002


On Sun, 28 Apr 2002, Francisco J Molina wrote:

  |I read somewhere that, instead of the structure if(...) else(...)
  |it is better to use the command ifelse(...,...,...).

  |The question is:
  |Am I actually optimizing the performance?

I am sorry that I cannot answer exactly your question.  But:

-- you can find the speed of the alternate expressions using syste.time()

-- if() + else() and ifelse() are not quite the same.  The first form is for
scalar logic, the second for vector logic.  I think if you are using scalars
as your choice variables (e.g. block of expressions is conditional to a
scalar logical variable), then if()-else() and ifelse() do equally well.  If
you are looking for vector logic, i.e. the expressions for every element in
a vector depend on the corresponding element in a logical vector, then
ifelse should be faster, you need a cycle when doing the same with if() and

When there are many conditions, one may get clearer code when writing as is
common in gauss, something like

a <- (b == 1)*expression1 + (b == 2)*expression2 + ....

I hope you got an idea.

Ott Toomet

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