[R] Learning, "if" and "else"

[Adaikalavan Ramasamy]

You can also use switch() instead of ifelse(), which makes the code a
bit easier to read. The downside to this is that switch does not take
vectorised input and thus you need a loop. For example

 # data
 dbh  <- c(30,29,15,14,30,29)
 form <- factor(c("tree", "tree", "liana", "liana", "palm", "palm")) 
 df   <- data.frame(dbh, form)
 
 out <- numeric( nrow(df) )
 for(i in 1:nrow(df)){
 
   x <- as.numeric( df[i, 1] )
   y <- as.character( df[i, 2] )

   out[i] <- switch( y,
              "tree"  = exp( -4.898 + 4.512*log(x) - 0.319*(log(x))^2 ),
              "liana" = 10^(0.07 + 2.17 * log10(x)),
              NA
                   )
 }


Or slightly more efficient solution is

 out <- apply( df, 1, function(z){
   x <- as.numeric(z[1]); y <- as.character(z[2])
   
   switch( y,
          "tree"  = exp( -4.898 + 4.512*log(x) - 0.319*(log(x))^2 ),
          "liana" = 10^(0.07 + 2.17 * log10(x)),
          NA
          )
 })

Regards, Adai



On Mon, 2005-06-13 at 15:32 -0700, Paulo Brando wrote:
> Dear Rs,
> 
> I have tried to use conditional expressions to calculate biomass for
> different life forms (trees, lianas, and palms).
> 
> Here is an example:
> 
> > lifeform
> 
>     dbh  form
> 1   30  tree
> 2   29  tree
> 3   28  tree
> 4   27  tree
> 5   26  tree
> 6   25  tree
> 7   24  tree
> 8   23  tree
> 9   22  tree
> 10  21  tree
> 11  20  tree
> 12  15 liana
> 13  14 liana
> 14  13 liana
> 15  12 liana
> 16  11 liana
> 17  10 liana
> 18   9 liana
> 19   8 liana
> 20   7 liana
> 21   6 liana
> 22   5 liana
> 23  30  palm
> 24  29  palm
> 25  28  palm
> 26  27  palm
> 27  26  palm
> 28  25  palm
> 29  24  palm
> 30  23  palm
> 31  22  palm
> 32  21  palm
> 33  20  palm
> 
> ### I want to include biomass 
> 
> lifeform$biomass <- 
> 
> {
>       if(lifeform$form=="tree")
>        exp(-4.898+4.512*log(dbh)-0.319*(log(dbh))^2) 
>        else{ 
>         if (lifeform$form=="liana")
>        10^(0.07 + 2.17 * log10 (dbh))
>        else ("NA")
> }
> Warning message:
> the condition has length > 1 and only the first element will be used in:
> if (lifeform$form == "tree") exp(-4.898 + 4.512 * log(dbh) -
> 
> 
> ### But I always get the message warning message above. 
> 
> 
> 
> I looked for similar examples in R mail list archive, but they did not
> help a lot.
> 
> I am quite new to 'R'. Any material that covers this theme?
> 
> Thank you very much!
> 
> Paulo
> 
> PS. Sorry about the last e-mail. I did not change the message title.
> ________________________________________
> Paulo M. Brando
> Instituto de Pesquisa Ambiental da Amazonia (IPAM)
> Santarem, PA, Brasil.
> Av. Rui Barbosa, 136.
> Fone: + 55 93 522 55 38
> www.ipam.org.br
> E-mail: pmbrando at ipam.org.br
> 
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