R help - Apr 2006 - simple if statement

I am ashamed to be asking this question, but I couldn't find the
solution anywhere.  Searching for "if" and "R" is not very
productive...

I cannot get a simple if statement to work.

I have data on college students.  I want to make a string variable
that has the names of the years.  That is, when the year variable i is
equal to 1, I want to have a variable called years equal to
"Freshmen".

I tried this
years <- "Freshmen" if i==1
years <- "Sophomores" if i==2

and so on, but I couldn't get it to work.  How can I get this variable to
work?

Thanks,

BQ

How about the following, if you really want characters or just leave as 
factor

i <- round(runif(10, 1, 4))
  years <- as.character(factor(i, labels = c("Freshman",
"Sophomore",
"Junior", "Senior")))

HTH,

ken

On Fri, 7 Apr 2006, Brian Quinif wrote:
> I am ashamed to be asking this question, but I couldn't find the
> solution anywhere.  Searching for "if" and "R" is not
very
> productive...
>
> I cannot get a simple if statement to work.
>
> I have data on college students.  I want to make a string variable
> that has the names of the years.  That is, when the year variable i is
> equal to 1, I want to have a variable called years equal to
> "Freshmen".
>
> I tried this
> years <- "Freshmen" if i==1
> years <- "Sophomores" if i==2
>
> and so on, but I couldn't get it to work.  How can I get this variable
to work?
Most simply:

years <- c("Freshmen", "Sophomores")[i]

What you seem to be trying to do can be written

if(i == 1) years <- "Freshmen"
if(i == 2) years <- "Sophomores"

but then 'years' is undefined if !i %in% c(1,2).  Better ways to program
that are

years <- switch(i, "Freshmen", "Sophomores")

(which gives NULL otherwise) or

years <- if(i == 1) "Freshmen" elseif(i == 2)
"Sophomores" else "unknown"

of (vectorized)

years <- ifelse(i == 1, "Freshmen",
                         ifelse(i == 2, "Sophomores",
"unknown"))

But the first solution is both vectorized and simple.


-- 
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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

On Fri, Apr 07, 2006 at 02:58:00AM -0400, Brian Quinif
wrote:
> 
> I have data on college students.  I want to make a string variable
> that has the names of the years.  That is, when the year variable i is
> equal to 1, I want to have a variable called years equal to
> "Freshmen".
> 
> I tried this
> years <- "Freshmen" if i==1
> years <- "Sophomores" if i==2
What you are looking for is not an if clause but logical indexing:

years[years=="Freshmen"] <- 1
years[years=="Sophomores"] <- 2

Of course it's still a character vector so you will have to do

years = as.numeric(years)

Have a look at the manual (Introduciotn to R) for more details.

Another question is what you have in mind. To me it looks like what you
are trying to do is make a factor on your own. Maybe this is what you
want:

factor(years)

or maybe

factor(years, levels=c("Freshmen", "Sophomores"))

if you want more control over the coding.

cu
	Philipp

-- 
Dr. Philipp Pagel                            Tel.  +49-8161-71 2131
Dept. of Genome Oriented Bioinformatics      Fax.  +49-8161-71 2186
Technical University of Munich
Science Center Weihenstephan
85350 Freising, Germany

 and

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