[R] objects of class "matrix" and mode "list"?

[Stephen Tucker]

Hello everyone,

I cannot seem to find information about objects of class "matrix" and mode
"list", and how to handle them (apart from flattening the list). I get this
type of object from using sapply(). Sorry for the long example, but the code
below illustrates how I get this type of object. Is anyone aware of
documentation regarding this object?

Thanks very much,

Stephen

===== begin example ====

# I am just making up a fake data set
df <- data.frame(Day=rep(1:3,each=24),Hour=rep(1:24,times=3),
                 Name1=rnorm(24*3),Name2=rnorm(24*3))

# define a function to get a set of descriptive statistics
tmp <- function(x) {
  # this function will accept a data frame
  # and return a 1-row data frame of
  # max value, colname of max, min value, and colname of min
  return(data.frame(maxval=max(apply(x,2,max)),
                    maxloc=names(x)[which.max(apply(x,2,max))],
                    minval=min(apply(x,2,min)),
                    minloc=names(x)[which.min(apply(x,2,min))]))
}

# Now applying function to data:
# (1) split the data table by Day with split()
# (2) apply the tmp function defined above to each data frame from (1)
#     using lapply()
# (3) transpose the final matrix and convert it to a data frame
#     with mixed characters and numbers
#     using as.data.frame(), lapply(), and type.convert()

> final <- as.data.frame(lapply(as.data.frame(t(sapply(split(df[,-c(1:2)],
+                                                           
f=df$Day),tmp))),
+                               type.convert,as.is=TRUE))
Error in type.convert(x, na.strings, as.is, dec) : 
	the first argument must be of mode character

I thought sapply() would give me a data frame or matrix, which I would
transpose into a character matrix, to which I can apply type.convert()
and get the same matrix as what I would get from these two lines (Fold
function taken from Gabor's post on R-help a few years ago):

Fold <- function(f, x, L) for(e in L) x <- f(x, e)
final2 <- Fold(rbind,vector(),lapply(split(df[,-c(1:2)],f=day),tmp))

> print(c(class(final2),mode(final2)))
[1] "data.frame" "list"  

====================================================
However, by my original method, sapply() gives me a matrix with mode, "list"

intermediate1 <- sapply(split(df[,-c(1:2)],f=df$Day),tmp)
> print(c(class(intermediate1),mode(intermediate1)))
[1] "matrix" "list"  

Transposing, still a matrix with mode list, not character:

intermediate2 <- t(sapply(split(df[,-c(1:2)],f=day),tmp))
> print(c(class(intermediate2),mode(intermediate2)))
[1] "matrix" "list"  

Unclassing gives me the same thing...

> print(c(class(unclass(intermediate2)),mode(unclass(intermediate2))))
[1] "matrix" "list"  




 
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