CODEWITHSUNDEEP
rclass
Fnzh Xx
Fnzh Xx

Reputation: 681

Why is the class and mode of the object returned by matrix() and array() the same?

Below are the first few rows of my large data file:

Symbol|Security Name|Market Category|Test Issue|Financial Status|Round Lot Size
AAC|Australia Acquisition Corp. - Ordinary Shares|S|N|D|100
AACC|Asset Acceptance Capital Corp. - Common Stock|Q|N|N|100
AACOU|Australia Acquisition Corp. - Unit|S|N|N|100
AACOW|Australia Acquisition Corp. - Warrant|S|N|N|100
AAIT|iShares MSCI All Country Asia Information Technology Index Fund|G|N|N|100
AAME|Atlantic American Corporation - Common Stock|G|N|N|100

I read the data in:

data <- read.table("nasdaqlisted.txt", sep="|", quote='', header=TRUE, as.is=TRUE)

and construct an array and a matrix:

d1 <- array(data, dim=c(nrow(data), ncol(data))) 
d2 <- matrix(data, nrow=nrow(data), ncol=ncol(data))

However, even though d1 is an array and d2 is a matrix, the class and mode are the same:

> class(d1)
[1] "matrix"
> mode(d1)
[1] "list"
> class(d2)
[1] "matrix"
> mode(d2)
[1] "list"

Why is this?

Upvotes: 0

Views: 599

Answers (1)

thelatemail
thelatemail

Reputation: 93813

I'll bite and have a go at explaining my understanding of the issues.

You don't need your large test file to demonstrate the issue. A simple data.frame would do:

test <- data.frame(var1=1:2,var2=letters[1:2])

> test
  var1 var2
1    1    a
2    2    b

Keep in mind that a data.frame is just a list internally.

> is.data.frame(test)
[1] TRUE
> is.list(test)
[1] TRUE

With a list-like structure as you would expect.

> str(test)
'data.frame':   2 obs. of  2 variables:
 $ var1: int  1 2
 $ var2: Factor w/ 2 levels "a","b": 1 2

> str(as.list(test))
List of 2
 $ var1: int [1:2] 1 2
 $ var2: Factor w/ 2 levels "a","b": 1 2

When you specify a matrix call against a data.frame or a list, you end up with a matrix filled with the elements of the data.frame or list.

result1 <- matrix(test)

> result1
     [,1]     
[1,] Integer,2
[2,] factor,2

Looking at the structure of result1, you can see it is still a list, but now just with dimensions (see the last line in the output below).

> str(result1)
List of 2
 $ : int [1:2] 1 2
 $ : Factor w/ 2 levels "a","b": 1 2
 - attr(*, "dim")= int [1:2] 2 1

Which means it is now both a matrix and a list

> is.matrix(result1)
[1] TRUE
> is.list(result1)
[1] TRUE

If you strip the dimensions from this object, it will no longer be a matrix and will revert to just being a list.

dim(result1) <- NULL

> result1
[[1]]
[1] 1 2

[[2]]
[1] a b
Levels: a b

> is.matrix(result1)
[1] FALSE
> is.list(result1)
[1] TRUE

> str(result1)
List of 2
 $ : int [1:2] 1 2
 $ : Factor w/ 2 levels "a","b": 1 2

Upvotes: 7

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