Reading and writing data.frames (ffdf)

These are the main methods for reading and writing data from ffdf objects.

# S3 method for class 'ffdf'
x[i, j, drop = ncols == 1]
# S3 method for class 'ffdf'
x[i, j] <- value
# S3 method for class 'ffdf'
x[[i, j, exact = TRUE]]
# S3 method for class 'ffdf'
x[[i, j]] <- value
# S3 method for class 'ffdf'
x$i
# S3 method for class 'ffdf'
x$i <- value

Arguments

x: an ff object
i: a row subscript or a matrix subscript or a list subscript
j: a column subscript
drop: logical. If TRUE the result is coerced to the lowest possible dimension. The default is to drop if only one column is left, but not to drop if only one row is left.
value: A suitable replacement value: it will be repeated a whole number of times if necessary and it may be coerced: see the Coercion section. If NULL, deletes the column if a single column is selected with [[<- or $<-.
exact: logical: see [, and applies to column names.

Details

The subscript methods [, [[ and $, behave symmetrical to the assignment functions [<-, [[<- and $<-. What the former return is the assignment value to the latter. A notable exception is assigning NULL in [[<- and $<- which removes the virtual column from the ffdf (and the physical component if it is no longer needed by any virtual column). Creating new columns via [[<- and $<- requires giving a name to the new column (character subscripting). [<- does not allow to create new columns, only to replace existing ones.

Subscript expressions and return values

allowed expression	–	`example`	–	`returnvalue`
row selection		`x[i, ]`		`data.frame` or single row as list if `drop=TRUE`, like from data.frame
column selection		`x[ ,i]`		`data.frame` or single column as vector unless `drop=TRUE`, like from data.frame
matrix selection		`x[cbind(i,j)]`		vector of the integer-matrix indexed cells (if the column types are compatible)
virtual selection		`x[i]`		`ffdf` with the selected columns only
physical selection		`x[[i]]`		the selected `ff`
physical selection		`x$i`		the selected `ff`

Author

Jens Oehlschlägel

Examples

   d <- data.frame(a=letters, b=rev(letters), c=1:26, stringsAsFactors = TRUE)
   x <- as.ffdf(d)

   d[1,]
#>   a b c
#> 1 a z 1
   x[1,]
#>   a b c
#> 1 a z 1

   d[1:2,]
#>   a b c
#> 1 a z 1
#> 2 b y 2
   x[1:2,]
#>   a b c
#> 1 a z 1
#> 2 b y 2

   d[,1]
#>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
   x[,1]
#>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z

   d[,1:2]
#>    a b
#> 1  a z
#> 2  b y
#> 3  c x
#> 4  d w
#> 5  e v
#> 6  f u
#> 7  g t
#> 8  h s
#> 9  i r
#> 10 j q
#> 11 k p
#> 12 l o
#> 13 m n
#> 14 n m
#> 15 o l
#> 16 p k
#> 17 q j
#> 18 r i
#> 19 s h
#> 20 t g
#> 21 u f
#> 22 v e
#> 23 w d
#> 24 x c
#> 25 y b
#> 26 z a
   x[,1:2]
#>    a b
#> 1  a z
#> 2  b y
#> 3  c x
#> 4  d w
#> 5  e v
#> 6  f u
#> 7  g t
#> 8  h s
#> 9  i r
#> 10 j q
#> 11 k p
#> 12 l o
#> 13 m n
#> 14 n m
#> 15 o l
#> 16 p k
#> 17 q j
#> 18 r i
#> 19 s h
#> 20 t g
#> 21 u f
#> 22 v e
#> 23 w d
#> 24 x c
#> 25 y b
#> 26 z a

   d[cbind(1:2,2:1)]
#> [1] "z" "b"
   x[cbind(1:2,2:1)]
#> [1] z b
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z

   d[1]
#>    a
#> 1  a
#> 2  b
#> 3  c
#> 4  d
#> 5  e
#> 6  f
#> 7  g
#> 8  h
#> 9  i
#> 10 j
#> 11 k
#> 12 l
#> 13 m
#> 14 n
#> 15 o
#> 16 p
#> 17 q
#> 18 r
#> 19 s
#> 20 t
#> 21 u
#> 22 v
#> 23 w
#> 24 x
#> 25 y
#> 26 z
   x[1]
#> ffdf (all open) dim=c(26,1), dimorder=c(1,2) row.names=NULL
#> ffdf virtual mapping
#>   PhysicalName VirtualVmode PhysicalVmode  AsIs VirtualIsMatrix
#> a            a      integer       integer FALSE           FALSE
#>   PhysicalIsMatrix PhysicalElementNo PhysicalFirstCol PhysicalLastCol
#> a            FALSE                 1                1               1
#>   PhysicalIsOpen
#> a           TRUE
#> ffdf data
#>    a
#> 1  a
#> 2  b
#> 3  c
#> 4  d
#> 5  e
#> 6  f
#> 7  g
#> 8  h
#> :  :
#> 19 s
#> 20 t
#> 21 u
#> 22 v
#> 23 w
#> 24 x
#> 25 y
#> 26 z

   d[[1]]
#>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
   x[[1]]
#> ff (open) integer length=26 (26) levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
#>  [1]  [2]  [3]  [4]  [5]  [6]  [7]  [8]      [19] [20] [21] [22] [23] [24] [25] 
#>    a    b    c    d    e    f    g    h    :    s    t    u    v    w    x    y 
#> [26] 
#>    z 

   d$a
#>  [1] a b c d e f g h i j k l m n o p q r s t u v w x y z
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
   x$a
#> ff (open) integer length=26 (26) levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
#>  [1]  [2]  [3]  [4]  [5]  [6]  [7]  [8]      [19] [20] [21] [22] [23] [24] [25] 
#>    a    b    c    d    e    f    g    h    :    s    t    u    v    w    x    y 
#> [26] 
#>    z 

   d$a[1:2]
#> [1] a b
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
   x$a[1:2]
#> [1] a b
#> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z

   rm(x); gc()
#>           used (Mb) gc trigger  (Mb) max used  (Mb)
#> Ncells 1013652 54.2    1994352 106.6  1994352 106.6
#> Vcells 1864524 14.3    8388608  64.0  3877600  29.6