[R] arrays emerging from tapply

[BXC (Bendix Carstensen)]

The code below illustrates some points about results from tapply that
I find strange. I wonder if they are intended and if so why it is so.

1) When you make a table the dimnames is a *named* list, tapply
   returns an unnamed list.

2) data.frame behaves differently on an array and a table. Is this
   an intended feature?

3) For tables class(TAB) and attr(TAB,"class") both return "table",
   but class(ARR) returns "array" whereas attr(TAB,"class") returns
   NULL.

Bendix Carstensen
----------------------
Bendix Carstensen
Senior Statistician
Steno Diabetes Center
Niels Steensens Vej 2
DK-2820 Gentofte
Denmark
tel: +45 44 43 87 38
mob: +45 30 75 87 38
fax: +45 44 43 07 06
bxc at steno.dk
www.biostat.ku.dk/~bxc
----------------------

Code:
------------------------------------------------------------------------
-
Aye <- sample( c("Yes","Si","Oui"), 177, replace=TRUE )
Bee <- sample( c("Hum","Buzz"), 177, replace=TRUE )
Sea <- sample( c("White","Black","Red","Dead"), 177, replace=TRUE )
pyr <- rnorm( 177, 57, 14 )
TAB <- table( Aye, Bee, Sea )
ARR <- tapply( pyr, list( Aye, Bee, Sea ), sum )
dimnames( TAB )
dimnames( ARR )

data.frame( TAB )
data.frame( ARR )
data.frame( as.table( ARR ) )

class( TAB )
attr( TAB, "class" )
class( ARR )
attr( ARR, "class" )
------------------------------------------------------------------------
---

Result:
------------------------------------------------------------------------
---

> version
         _              
platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    2              
minor    0.1            
year     2004           
month    11             
day      15             
language R              
> Aye <- sample( c("Yes","Si","Oui"), 177, replace=TRUE )
> Bee <- sample( c("Hum","Buzz"), 177, replace=TRUE )
> Sea <- sample( c("White","Black","Red","Dead"), 177, replace=TRUE )
> pyr <- rnorm( 177, 57, 14 )
> TAB <- table( Aye, Bee, Sea )
> ARR <- tapply( pyr, list( Aye, Bee, Sea ), sum )
> dimnames( TAB )
$Aye
[1] "Oui" "Si"  "Yes"

$Bee
[1] "Buzz" "Hum" 

$Sea
[1] "Black" "Dead"  "Red"   "White"

> dimnames( ARR )
[[1]]
[1] "Oui" "Si"  "Yes"

[[2]]
[1] "Buzz" "Hum" 

[[3]]
[1] "Black" "Dead"  "Red"   "White"

> 
> data.frame( TAB )
   Aye  Bee   Sea Freq
1  Oui Buzz Black    2
2   Si Buzz Black   11
3  Yes Buzz Black   10
4  Oui  Hum Black    7
5   Si  Hum Black   11
6  Yes  Hum Black    3
7  Oui Buzz  Dead    3
8   Si Buzz  Dead   11
9  Yes Buzz  Dead    5
10 Oui  Hum  Dead   11
11  Si  Hum  Dead    3
12 Yes  Hum  Dead    5
13 Oui Buzz   Red    5
14  Si Buzz   Red    4
15 Yes Buzz   Red    8
16 Oui  Hum   Red   11
17  Si  Hum   Red    9
18 Yes  Hum   Red    8
19 Oui Buzz White   10
20  Si Buzz White    7
21 Yes Buzz White    6
22 Oui  Hum White    8
23  Si  Hum White   11
24 Yes  Hum White    8
> data.frame( ARR )
  Buzz.Black Hum.Black Buzz.Dead Hum.Dead Buzz.Red  Hum.Red Buzz.White
Hum.White
1   130.5520  377.0875  177.9144 590.1498 346.4448 633.0975   492.9589
424.1464
2   607.4388  635.3015  651.3304 162.9175 223.1840 531.5512   347.7185
622.0213
3   555.8886  177.8216  295.0027 297.4550 417.6824 461.2143   359.4606
432.6165
> data.frame( as.table( ARR ) )
   Var1 Var2  Var3     Freq
1   Oui Buzz Black 130.5520
2    Si Buzz Black 607.4388
3   Yes Buzz Black 555.8886
4   Oui  Hum Black 377.0875
5    Si  Hum Black 635.3015
6   Yes  Hum Black 177.8216
7   Oui Buzz  Dead 177.9144
8    Si Buzz  Dead 651.3304
9   Yes Buzz  Dead 295.0027
10  Oui  Hum  Dead 590.1498
11   Si  Hum  Dead 162.9175
12  Yes  Hum  Dead 297.4550
13  Oui Buzz   Red 346.4448
14   Si Buzz   Red 223.1840
15  Yes Buzz   Red 417.6824
16  Oui  Hum   Red 633.0975
17   Si  Hum   Red 531.5512
18  Yes  Hum   Red 461.2143
19  Oui Buzz White 492.9589
20   Si Buzz White 347.7185
21  Yes Buzz White 359.4606
22  Oui  Hum White 424.1464
23   Si  Hum White 622.0213
24  Yes  Hum White 432.6165
> 
> class( TAB )
[1] "table"
> attr( TAB, "class" )
[1] "table"
> class( ARR )
[1] "array"
> attr( ARR, "class" )
NULL