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

[Don MacQueen]

There are lots of ways to do this, and lots of sources of 
information. Go to the R website and download the "Introduction to R" 
document from the "Manuals" link.

Here is one simple way. Not the most succinct in terms of coding 
style, perhaps, but relatively easy to understand, and relatively 
good for beginning to understand the language, in my opinion.

Starting with your lifeform object as shown, with the two variables, 
dbh and form.

lifeform$biomass <- rep(NA,nrow(lifeform))
is.tree <- lifeform$form=='tree'
lifeform$biomass[is.tree] <- 
exp(-4.898+4.512*log(lifeform$dbh)-0.319*(log(lifeform$dbh))^2)[is.tree]
is.liana <- lifeform$form=='liana'
lifeform$biomass[is.liana] <- 
exp(-4.898+4.512*log(dbh)-0.319*(log(dbh))^2)[is.liana]

To better understand what is going on, type just this:

    lifeform$form=="tree"

You should see a vector of TRUE and FALSE values, 33 of them. But the 
rules for the "if()" function say that whatever you supply inside the 
parentheses should reduce to a *single* true or false value. You gave 
it 33 of them. See the help page for "if", which you can get to using 
either

   help('if')
or
   ?'if'

The ifelse() function takes a logical vector for its first argument.

At 3:32 PM -0700 6/13/05, 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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>R-help at stat.math.ethz.ch mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA