Newbie struggling with "factors"

To all who have offered suggestions: 
THANKS! Wow, this list has generated a lot of good
ideas for me in a very short time, and I appreciate
it.

For now, I've got some solutions to my problem. Greg's
suggestion about creating a subclass to handle the
"multi-checkbox" type of question is probably the most
flexible, in the long run. However, I've not chosen it
in the short run because my programming experience is
deeper in the procedural vein than in OOP. I'm only
just starting to see how the OO qualities of R can be
used, and I'm not yet comfortable in coding that way.

Following the suggestions of several people on the
list, I have created a few functions that proceed this
way for my multi-choice questions:
- create a matrix with as many rows as there are
responses, and as many columns as there are
"checkboxes" in the original question

- use strsplit to break up the factors based on the
separator inside the field

- for each column in the matrix that I created, fill
it with T/F (1/0) by using the is.element function to
determine which responses had each checkbox checked

- use the resulting matrix to create whatever sums,
averages and plots I want

The code I wrote is not pretty, but is working for me
at the moment. I'm an old assembly and C programmer
mainly, so I'm still getting used to the capabilities
and idioms of R. I think my code does great violence
to both and probably makes the interpreter thrash
pitifully, but for now it seems to produce the correct
result and I can understand it! I'll look for elegance
as I go along.
--- "Warnes, Gregory R"
<gregory_r_warnes at groton.pfizer.com> wrote:

Hint #1,  to do any useful transformations on your
variables you will
probably need to convert them temporarily into
character variables (aka
strings).  Do that with 

	as.character(n$OSUSE)

Probably your will want to convert each of the
variables that are in this
format into a set of numeric variables.  Something
like this:

	n <- data.frame(OSUSE = c("1","1,3","1,2,3"))	
	n$OSUSE.Windows   <- sapply( strsplit(n$OSUSE, ",")
, function(X) (
"1" %in% X ) )
	n$OSUSE.Macintosh <- sapply( strsplit(n$OSUSE, ",")
, function(X) (
"2" %in% X ) )
     	n$OSUSE.Unix      <- sapply( strsplit(n$OSUSE,
",") , function(X) (
"3" %in% X ) )

Alternatively, if you often have variables like
this, you might consider
creating a new object type that extends factor and
that includes the
operations that you need.  

Something like:

### Start Sample Code ###

checklist <- function(X, boxnames)
  {
    attr(X, "boxnames") <- boxnames
    class(X) <- c("checklist","factor")
    return(X)
  }

contains <- function(X, name)
  {
    if(is.character(name) )
      name <- pmatch( name, attr(X,"boxnames" ) )
                     
    retval <- sapply( strsplit(X, ",") , function(X)
( name %in% X ) )
    return(retval)
  }

numchecked <- function(X)
  {
    retval <- sapply( strsplit(X, ","), length )
    return(retval)
  }

summary.checklist <- function(x, ...)
  {
    sum <- apply( as.matrix(x), 2, sum )
    mean <- apply( as.matrix(x), 2, mean )
    return( rbind(sum,mean))
  }

as.matrix.checklist <- function(x, ...)
  {
    sapply( attr(x, "boxnames"), function(YY)
contains(x, YY) )
  }

### End Sample Code ##

Here's some examples of using these functions:

n <- data.frame(OSUSE = c("1","1,3","1,2,3"))

n$OSUSE <- checklist(n$OSUSE,

c("Windows","Macintosh","Unix"))
#
# Check if OSUSE includes a specific OS
#

contains( n$OSUSE, "Windows")

[1] TRUE TRUE TRUE

contains( n$OSUSE, "Macintosh")

[1] FALSE FALSE  TRUE

contains( n$OSUSE, "Unix")

[1] FALSE  TRUE  TRUE

#
# Compute the average number of checked items
# 

numchecked(n$OSUSE)

[1] 1 2 3

mean(numchecked(n$OSUSE))

[1] 2

#
# Create a matrix showing whether each box was
checked or not
#

as.matrix(n$OSUSE)

     Windows Macintosh  Unix
[1,]    TRUE     FALSE FALSE
[2,]    TRUE     FALSE  TRUE
[3,]    TRUE      TRUE  TRUE

#
# Show some summary info
#

summary(n$OSUSE)

     Windows Macintosh      Unix
sum        3 1.0000000 2.0000000
mean       1 0.3333333 0.6666667		


Of course, you'll want to modify these classes to
suit your needs.  A little
time up front can help a lot.

If you like, I'll include these classes and any
enhancements that you make
in my 'gregmisc' library.


-Greg

-----Original Message-----
From: Tom Arnold

[mailto:thomas_l_arnold at yahoo.com]

Sent: Friday, March 29, 2002 8:59 AM
To: R
Subject: [R] Newbie struggling with "factors"


I am processing some survey results, and my data

are

being read in as "factors". I don't know how to
process these things in any way.

To start with, several of the survey questions are
mulit-choice check boxes on the original

(web-based)

survey, as in "check all that apply".

These are encoded as numbers. For example, if the
survey has a question:
Which operating systems have you used? (Check all

that

apply)
[ ]Windows
[ ]Macinotsh
[ ]Unix

...then the data exported for three different
responses might look like
;1;
;1,3;
;1,2,3;

...where ";" is the field delimiter. 
I use read.table to get the data in. I read all

the

survey data into a table "n" and the field above

is

called "OSUSE". When I query R about the field, it
tells me it is class "factor"

class(n$OSUSE)

[1] "factor"

mode(n$OSUSE)

[1] "numeric"

I'd like to be able to do some simple things like:
what is the most common item checked (1, 2, or 3?)
What is the average number of boxes checked?

But I can't find any way to manipulate this

"factor"

field. What's the secret?

Thanks.

=====
Tom Arnold
Summit Media Partners
Visit our web site at

http://www.summitmediapartners.com

Easter, Passover

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=====
Tom Arnold
Summit Media Partners
Visit our web site at http://www.summitmediapartners.com