[R] if() command

[Huntsinger, Reid]

Looping would look like:

group <- vector(length=length(age))

for (i in 1:length(age)) {
  group[i] <- if (age[i] <= 10) 1 else if (age[i] <= 20) 2 else 3
}

Another way to do this is to write a function, say "category", like

category <- function(x) if(x <= 10) 1 else if (x <= 20) 2 else 3

and then apply the function to all elements of "age" like

group <- sapply(age,category)

(This is a common way to vectorize a function.)

Reid Huntsinger

-----Original Message-----
From: Carlos Mauricio Cardeal Mendes [mailto:mcardeal at ufba.br] 
Sent: Wednesday, September 14, 2005 1:21 PM
To: Huntsinger, Reid
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] if() command


Hello reid ! About your third explanation, could you please write the 
complete code including that option: a loop ?

Forgiveme,  I'm trying to learn R and my mind is full of  other 
statistical program syntax. And I'd like very very much to improve my 
knowledge using R and maybe contribute to someone, someday, somehow.

Thanks, again

Mauricio

Huntsinger, Reid escreveu:

>First, "==" is logical comparison, so if you want to create a variable
based
>on both "age" and "group" you can do that. However, it looks like you want
>to define the variable "group", so you want to use "<-" or "=" for that. 
>
>Second, if you're typing this at a command prompt, you need to make sure
you
>tell R you're not finished when it looks like you could be. There are
>several ways to do this. One is to put everything inside braces; another is
>to deliberately leave lines incomplete, like
>
>if (age <= 10) {
>   group <- 1
>} else {
>   if (age <= 20) {
>      group <- 2
>   } else group <- 3
>}
>
>Third, this will work for a vector of length 1. If you want to take a
vector
>"age" and produce a corresponding vector "group", you'll need to put this
in
>a loop, or use "lapply", or some iteration.
>
>Fourth, you can also write the above as 
>
>  
>
>>group <- if (age <= 10) 1 else if (age <= 20) 2 else 3
>>    
>>
>
>that is, if() returns a value you can assign.
>
>Finally, besides "ifelse" you can use "cut" for this particular task.
>
>Reid Huntsinger
>
>
>-----Original Message-----
>From: r-help-bounces at stat.math.ethz.ch
>[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Carlos Maurício
>Cardeal Mendes
>Sent: Tuesday, September 13, 2005 9:29 AM
>To: r-help at stat.math.ethz.ch
>Subject: [R] if() command
>
>
>Hi everyone !
>
>Could you please help me with this problem ?
>
>I´ve trying to write a code that assign to a variable the content from 
>another, but all I´ve got is a message error. For example:
>
>if (age <=10) {group == 1}
>else if (age > 10 & age <= 20) {group == 2}
>else {group == 3}
>
>Syntax error
>
>Or
>
>if (age <=10) {group == 1}
>else (age > 10 & age <= 20) {group == 2}
>else {group == 3}
>
>Syntax error
>
>I know that is possible to find the solution by ifelse command or even 
>recode command, but I´d like to use this way, because I can add another 
>variable as a new condition and I believe to expand the possibilites.
>
>Thanks,
>Mauricio
>
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