[R] how to use if statement in function correctly

[Liaw, Andy]

You need the vectorized version of if() ... else..., which is ifelse().
Try:

fff <- function(otac,sklon)  
{
  test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
  ifelse(test<65, otac/sklon * 141.76,
         otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
}

which gives:
> fff(seq(20,50,5),70)
[1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371

Andy

> From: Petr Pikal [mailto:petr.pikal at precheza.cz]
> 
> Dear all
> 
> I try to persuade if statement in my function to work as I 
> want (but I am not very 
> successful:(
> 
> My question is why my function with if statement is evaluated 
> correctly in case of 
> atomic variables and incorrectly in case of vector variables.
> 
> Here is an example:
> 
> #function with if statement
> 
> fff <- function(otac,sklon)  
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> if(test<65)
> {otac/sklon * 141.76}
> else
> {otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
> }
> 
> > fff(20,70)
> [1] 40.50286
> # correct
> 
> > fff(50,70)
> [1] 80.42371
> #correct
> 
> > fff(seq(20,50,5),70)
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  
> 91.13143 101.25714
> #wrong
> --------------------------------------------------------------
> ---------------------------------
> It does not work because if statement is obviously evaluated 
> as TRUE in all 
> instances and computes wrongly some items in vector according 
> to the first part 
> of the fff function
> 
> # see direct computation
> > seq(20,50,5)/70*141.76
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  
> 91.13143 101.25714
> 
> > seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
> [1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371
> 
> On the other hand ifelse works
> ******************************
> fff <- function(otac,sklon)  
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> ifelse(test<65, (otac/sklon * 141.76),  (otac * 
> 3.69683+286.6*(1/sklon)-
> 0.03100345*otac*sklon))
> }
> 
> > fff(seq(20,50,5),70)
> [1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
> #correct
> 
> Can anybody help?
> 
> Thank you in advance
> Petr Pikal
> petr.pikal at precheza.cz
> p.pik at volny.cz
> 
> 
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