R: Factor level manipulation

sortLevels {ff} | R Documentation

## Factor level manipulation

### Description

`appendLevels` combines `levels` without sorting such that levels of the first argument will not require re-coding.
`recodeLevels` is a generic for recoding a factor to a desired set of levels - also has a method for large `ff` objects
`sortLevels` is a generic for level sorting and recoding of single factors or of all factors of a `ffdf` dataframe.

### Usage

    appendLevels(...)
    recodeLevels(x, lev)
    ## S3 method for class 'factor'
    recodeLevels(x, lev)
    ## S3 method for class 'ff'
    recodeLevels(x, lev)
    sortLevels(x)
    ## S3 method for class 'factor'
    sortLevels(x)
    ## S3 method for class 'ff'
    sortLevels(x)
    ## S3 method for class 'ffdf'
    sortLevels(x)

### Arguments

`...` | character vector of levels or `is.factor` objects from which the level attribute is taken
`x` | a `factor` or `ff` factor or a `ffdf` dataframe (`sortLevels` only)
`lev` | a character vector of levels

### Details

When reading a long file with categorical columns the final set of factor levels is only known once the complete file has been read. When a file is so large that we read it in chunks, the new levels need to be added incrementally. `rbind.data.frame` sorts combined levels, which requires recoding. For `ff` factors this would require recoding of all previous chunks at the next chunk - potentially on disk, which is too expensive. Therefore `read.table.ffdf` will simply `appendLevels` without sorting, and the `recodeLevels` and `sortLevels` generics provide a convenient means for sorting and recoding levels after all chunks have been read.

### Value

`appendLevels` returns a vector of combined levels, `recodeLevels` and `sortLevels` return the input object with changed levels. Do read the note!

### Note

You need to re-assign the return value not only for ram- but also for ff-objects. Remember ff's hybrid copying semantics: `LimWarn`. If you forget to re-assign the returned object, you will end up with ff objects that have their integer codes re-coded to the new levels but still carry the old levels as a `virtual` attribute.

### Author(s)

Jens Oehlschlägel

### See Also

`read.table.ffdf`, `levels.ff`

### Examples

      message("Let's create a factor with little levels")
      x <- ff(letters[4:6], levels=letters[4:6])
      message("Let's interpret the same ff file without levels in order to see the codes")
      y <- x
      levels(y) <- NULL

      levels(x)
      data.frame(factor=x[], codes=y[])

      levels(x) <- appendLevels(levels(x), letters)
      levels(x)
      data.frame(factor=x[], codes=y[])

      x <- sortLevels(x) # implicit recoding is chunked were necessary
      levels(x)
      data.frame(factor=x[], codes=y[])

      message("NEVER forget to reassign the result of recodeLevels or sortLevels,
    look at the following mess")
      recodeLevels(x, rev(levels(x)))
      message("NOW the codings have changed, but not the levels, the result is wrong data")
      levels(x)
      data.frame(factor=x[], codes=y[])

      rm(x);gc()

    ## Not run:
     n <- 5e7

     message("reading a factor from a file ist as fast ...")
     system.time(
     fx <- ff(factor(letters[1:25]), length=n)
     )
     system.time(x <- fx[])
     str(x)
     rm(x); gc()

     message("... as creating it in-RAM (R-2.11.1) which is theoretically impossible ...")
     system.time({
     x <- integer(n)
     x[] <- 1:25
     levels(x) <- letters[1:25]
     class(x) <- "factor"
     })
     str(x)
     rm(x); gc()

     message("... but is possible if we avoid some  unnecessary copying that is triggered
    by assignment functions")
     system.time({
     x <- integer(n)
     x[] <- 1:25
     setattr(x, "levels", letters[1:25])
     setattr(x, "class", "factor")
     })
     str(x)
     rm(x); gc()

     rm(n)

    ## End(Not run)

[Package ff version 2.2-13 Index]