Getting and setting factor levels

levels.ff<- sets factor levels, levels.ff gets factor levels

Usage

# S3 method for class 'ff'
levels(x)
# S3 method for class 'ff'
levels(x) <- value
 is.factor(x)
 is.ordered(x)
# S3 method for class 'ff'
is.factor(x)
# S3 method for class 'ff'
is.ordered(x)
# Default S3 method
is.factor(x)
# Default S3 method
is.ordered(x)

Arguments

x: an ff object
value: the new factor levels, if NA is an allowed level it needs to be given explicitely, nothing is excluded

Details

The ff object must have an integer vmode, see .rammode. If the mode is unsigned – see .vunsigned – the first factor level is coded with 0L instead of 1L in order to maximize the number of codable levels. Usually the internal ff coding – see ram2ffcode – is invisible to the user: when subscripting from an ff factor, unsigend codings are automatically converted to R's standard factor codes starting at 1L. However, you need to be aware of the internal ff coding in two situtations.
1. If you convert an ff integer object to an ff factor object and vice versa by assigning levels and is.null(oldlevels)!=is.null(newlevels).
2. Assigning data that does not match any level usually results in NA, however, in unsigned types there is no NA and all unknown data are mapped to the first level.

Value

levels returns a character vector of levels (possibly including as.cha racter(NA)).

Author

Jens Oehlschlägel

Note

When levels as assigned to an ff object that formerly had not levels, we assign automatically ramclass == "factor". If you want to change to an ordered factor, use virtual$ramclass <- c("ordered", "factor")

Examples

  message("--- create an ff factor including NA as last level")
#> --- create an ff factor including NA as last level
  x <- ff("a", levels=c(letters, NA), length=99)
  message('    we expect a warning because "A" is an unknown level')
#>     we expect a warning because "A" is an unknown level
  x[] <- c("a", NA,"A")
#> Warning: unknown factor values mapped to NA
  x
#> ff (open) integer length=99 (99) 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 <NA>
#>  [1]  [2]  [3]  [4]  [5]  [6]  [7]  [8]      [92] [93] [94] [95] [96] [97] [98] 
#>   a    NA   NA   a    NA   NA   a    NA    :   NA   NA   a    NA   NA   a    NA 
#> [99] 
#>   NA 
  levels(x)
#>  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
#> [20] "t" "u" "v" "w" "x" "y" "z" NA 

  message("--- create an ff ordered factor")
#> --- create an ff ordered factor
  x <- ff(letters, levels=letters, ramclass=c("ordered","factor"), length=260)
  x
#> ff (open) integer length=260 (260) 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]       [253] [254] [255] [256] 
#>     a     b     c     d     e     f     g     h     :     s     t     u     v 
#> [257] [258] [259] [260] 
#>     w     x     y     z 
  levels(x)
#>  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
#> [20] "t" "u" "v" "w" "x" "y" "z"

  message("    make it a non-ordered factor")
#>     make it a non-ordered factor
  virtual(x)$ramclass <- "factor"
  x
#> ff (open) integer length=260 (260) 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]       [253] [254] [255] [256] 
#>     a     b     c     d     e     f     g     h     :     s     t     u     v 
#> [257] [258] [259] [260] 
#>     w     x     y     z 
  rm(x); gc()
#>           used (Mb) gc trigger  (Mb) max used  (Mb)
#> Ncells 1251169 66.9    2239654 119.7  2239654 119.7
#> Vcells 2303625 17.6    8388608  64.0  8387257  64.0

 if (FALSE) { # \dontrun{
  message("--- create an unsigned quad factor")
  x <- ff(c("A","T","G","C"), levels=c("A","T","G","C"), vmode="quad", length=100)
  x
  message("  0:3 coding usually invisible to the user")
  unclass(x[1:4])
  message("    after removing levels, the 0:3 coding becomes visible to the user")
  message("    we expect a warning here")
  levels(x) <- NULL
  x[1:4]
  rm(x); gc()
 } # }