[R] ifelse versus ...

[Prof Brian Ripley]

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(...,...,...).
>
> Currently I am using structures like
>
> ifelse(...,{sequence; of; commands}, 0)
>
> instead of if(...){sequence; of; commands}
>
> and I am using
>
> ifelse(...,{sequence; of; commands}, {another; Sequence; Of; Commands })
>
> instead of
>
> if(...){sequence; of; commands}
> else{another; Sequence; Of; Commands })
>
> In this way the program becomes a little bit more difficult to read but
> I think that I am optimizing the performance.
>
> The question is:
>
> Am I actually optimizing the performance?

No.  ifelse is only more efficient if used to replace constructs like

for(i in seq(along=x)) if(x[i]) foo(x[i]) else bar(x[i])

by ifelse(x, foo(x), bar(x)).

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595


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