R help - Oct 2011 - Unexpected behavior of extract (`[`) or sapply functions

Dear folks--
The function below is a snippet of a larger function that is not doing what
it is supposed to do, and I do not understand its behavior.  The larger
function is supposed to produce an array containing the results of a
user-specified function applied to groups of data defined by the
intersection of one or more factors, and return them in an array with a
dimension for each factor and a dimension level for each factor level.  This
snippet is supposed to take a data frame, a vector of column numbers
containing factors, and a column number for the data, and return (in the
test function below, just print) a list of character vectors of the level
names (one vector per dimension) and the length of those vectors.

It works fine so long as I give it more than one factor column, but if I
give it a vector of factor columns of length 1, it behave differently and
when I try to assign the names from test.levels to the dimnames of the
array,  I end up with an error message:

Error in dimnames(data) <- dimnames : 
  length of 'dimnames' [1] not equal to array extent

The example below shows the function output for a test data frame
(?test.df?) when run first of a vector of two column number for factors and
then on just one. You can see how the structure of the output shifts.  I can
not understand what is happening. What I want it to do when given just
factor cols =c(1)  is to give me back exactly what it gives me bact for
factor colum 1 in factor.cols = c(1,2).

Any help or suggestions would be greatly appreciated.

Sincerely, 
   andrewH

# Test Data
test.df <- data.frame(AA=rep(LETTERS[1:2], c(6,6)),BB=rep(LETTERS[3:5],
c(4,4,4)), 
                              CC=rep(LETTERS[6:9],c(3,3,3,3)), DD=c(1:12))

# The function
getLevels <- function(data.df, factor.cols, data.col){
test.levels <- sapply(test.df[,factor.cols, drop=F], levels)
cat("test.levels:\n"); print(test.levels)
no.levels <- sapply(sapply(data.df[,factor.cols, drop=F], levels), length)
cat("no.levels:\n"); print(no.levels)
} 

# Run it with two factors and again with 1, Output below
cat("\nTest 2 factors:\n")
getLevels(test.df, c(1,2), 4)
cat("\nTest 1 factor:\n")
getLevels(test.df, c(1), 4)

Test 2 factors:
> getLevels(test.df, c(1,2), 4)
test.levels$AA
[1] "A" "B"

$BB
[1] "C" "D" "E"

no.levels=AA BB 
 2  3 
> cat("\nTest 1 factor:\n")
Test 1 factor:
> getLevels(test.df, c(1), 4)
test.levels=     AA 
[1,] "A"
[2,] "B"
no.levels=A B 
1 1 


--
View this message in context:
http://r.789695.n4.nabble.com/Unexpected-behavior-of-extract-or-sapply-functions-tp3881176p3881176.html
Sent from the R help mailing list archive at Nabble.com.

Hi

Is it necessary to use sapply? With lapply you will get what you want.

Regards
Petr

> 
> 
> Dear folks--
> The function below is a snippet of a larger function that is not doing 
what
> it is supposed to do, and I do not understand its behavior.  The larger
> function is supposed to produce an array containing the results of a
> user-specified function applied to groups of data defined by the
> intersection of one or more factors, and return them in an array with a
> dimension for each factor and a dimension level for each factor level. 
This
> snippet is supposed to take a data frame, a vector of column numbers
> containing factors, and a column number for the data, and return (in the
> test function below, just print) a list of character vectors of the 
level
> names (one vector per dimension) and the length of those vectors.
> 
> It works fine so long as I give it more than one factor column, but if I
> give it a vector of factor columns of length 1, it behave differently 
and
> when I try to assign the names from test.levels to the dimnames of the
> array,  I end up with an error message:
> 
> Error in dimnames(data) <- dimnames : 
>   length of 'dimnames' [1] not equal to array extent
> 
> The example below shows the function output for a test data frame
> (?test.df?) when run first of a vector of two column number for factors 
and
> then on just one. You can see how the structure of the output shifts.  I 
can
> not understand what is happening. What I want it to do when given just
> factor cols =c(1)  is to give me back exactly what it gives me bact for
> factor colum 1 in factor.cols = c(1,2).
> 
> Any help or suggestions would be greatly appreciated.
> 
> Sincerely, 
>    andrewH
> 
> # Test Data
> test.df <- data.frame(AA=rep(LETTERS[1:2], c(6,6)),BB=rep(LETTERS[3:5],
> c(4,4,4)), 
>                               CC=rep(LETTERS[6:9],c(3,3,3,3)), 
DD=c(1:12))
> 
> # The function
> getLevels <- function(data.df, factor.cols, data.col){
> test.levels <- sapply(test.df[,factor.cols, drop=F], levels)
> cat("test.levels:\n"); print(test.levels)
> no.levels <- sapply(sapply(data.df[,factor.cols, drop=F], levels), 
length)
> cat("no.levels:\n"); print(no.levels)
> } 
> 
> # Run it with two factors and again with 1, Output below
> cat("\nTest 2 factors:\n")
> getLevels(test.df, c(1,2), 4)
> cat("\nTest 1 factor:\n")
> getLevels(test.df, c(1), 4)
> 
> Test 2 factors:
> > getLevels(test.df, c(1,2), 4)
> test.levels> $AA
> [1] "A" "B"
> 
> $BB
> [1] "C" "D" "E"
> 
> no.levels=AA BB 
>  2  3 
> > cat("\nTest 1 factor:\n")
> 
> Test 1 factor:
> > getLevels(test.df, c(1), 4)
> test.levels=     AA 
> [1,] "A"
> [2,] "B"
> no.levels=A B 
> 1 1 
> 
> 
> --
> View this message in context: http://r.789695.n4.nabble.com/Unexpected-
> behavior-of-extract-or-sapply-functions-tp3881176p3881176.html
> Sent from the R help mailing list archive at Nabble.com.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.