data.table Printing Options

Description

print.data.table extends the functionalities of print.data.frame.

Key enhancements include automatic output compression of many observations and concise column-wise class summary.

format_col and format_list_item generics provide flexibility for end-users to define custom printing methods for generic classes.

Note also the option datatable.prettyprint.char; character columns entries exceeding this limit will be truncated, with ... indicating the truncation. Note that the truncation is done with strtrim; be cognizant of potential limitations when dealing with non-printable characters like newlines or tabs.

Usage

  ## S3 method for class 'data.table'
print(x,
    topn=getOption("datatable.print.topn"),             # default: 5
    nrows=getOption("datatable.print.nrows"),           # default: 100
    class=getOption("datatable.print.class"),           # default: TRUE
    row.names=getOption("datatable.print.rownames"),    # default: TRUE
    col.names=getOption("datatable.print.colnames"),    # default: "auto"
    print.keys=getOption("datatable.print.keys"),       # default: TRUE
    trunc.cols=getOption("datatable.print.trunc.cols"), # default: FALSE
    show.indices=getOption("datatable.show.indices"),   # default: FALSE
    quote=FALSE,
    na.print=NULL,
    timezone=FALSE, ...)

  format_col(x, ...)
  ## Default S3 method:
format_col(x, ...)
  ## S3 method for class 'POSIXct'
format_col(x, ..., timezone=FALSE)
  ## S3 method for class 'expression'
format_col(x, ...)

  format_list_item(x, ...)
  ## Default S3 method:
format_list_item(x, ...)

Arguments

Details

By default, with an eye to the typically large number of observations in a data.table, only the beginning and end of the object are displayed (specifically, head(x, topn) and tail(x, topn) are displayed unless nrow(x) < nrows, in which case all rows will print).

format_col is applied at a column level; for example, format_col.POSIXct is used to tag the time zones of POSIXct columns. format_list_item is applied to the elements (rows) of list columns; see Examples. The default format_col method uses getS3method to test if a format method exists for the column, and if so uses it. Otherwise, the default format_list_item method uses the S3 format method (if one exists) for each item of a list column.

Value

print.data.table returns x invisibly.

format_col returns a length(x)-size character vector.

format_list_item returns a length-1 character scalar.

See Also

print.default

Examples

  #output compression
  DT <- data.table(a = 1:1000)
  print(DT, nrows = 100, topn = 4)

  #`quote` can be used to identify whitespace
  DT <- data.table(blanks = c(" 12", " 34"),
                   noblanks = c("12", "34"))
  print(DT, quote = TRUE)

  #`class` provides handy column type summaries at a glance
  DT <- data.table(a = vector("integer", 3),
                   b = vector("complex", 3),
                   c = as.IDate(paste0("2016-02-0", 1:3)))
  print(DT, class = TRUE)

  #`row.names` can be eliminated to save space
  DT <- data.table(a = 1:3)
  print(DT, row.names = FALSE)

  #`print.keys` can alert which columns are currently keys
  DT <- data.table(a=1:3, b=4:6, c=7:9, key=c("b", "a"))
  setindexv(DT, c("a", "b"))
  setindexv(DT, "a")
  print(DT, print.keys=TRUE)

  # `trunc.cols` will make it so only columns that fit in console will be printed
  #    with a message that states the variables not shown
  old_width = options("width" = 40)
  DT <- data.table(thing_11 = vector("integer", 3),
                   thing_21 = vector("complex", 3),
                   thing_31 = as.IDate(paste0("2016-02-0", 1:3)),
                   thing_41 = "aasdfasdfasdfasdfasdfasdfasdfasdfasdfasdf",
                   thing_51 = vector("integer", 3),
                   thing_61 = vector("complex", 3))
  print(DT, trunc.cols=TRUE)
  options(old_width)

  # `char.trunc` will truncate the strings,
  # if their lengths exceed the given limit: `datatable.prettyprint.char`
  # For example:

  old = options(datatable.prettyprint.char=5L)
  DT = data.table(x=1:2, y=c("abcdefghij", "klmnopqrstuv"))
  DT
  options(old)

  # Formatting customization
  format_col.complex = function(x, ...) sprintf('(%.1f, %.1fi)', Re(x), Im(x))
  x = data.table(z = c(1 + 3i, 2 - 1i, pi + 2.718i))
  print(x)

  old = options(datatable.show.indices=TRUE)
  NN = 200
  set.seed(2024)
  DT = data.table(
    grp1 = sample(100, NN, TRUE),
    grp2 = sample(90, NN, TRUE),
    grp3 = sample(80, NN, TRUE)
  )
  setkey(DT, grp1, grp2)
  setindex(DT, grp1, grp3)
  print(DT)
  options(old)

  iris = as.data.table(iris)
  iris_agg = iris[ , .(reg = list(lm(Sepal.Length ~ Petal.Length))), by = Species]
  format_list_item.lm = function(x, ...) sprintf('<lm:%s>', format(x$call$formula))
  print(iris_agg)