For certain non-rectangular data formats, it can be useful to parse the data into a melted format where each row represents a single token.
melt_delim_chunked(file, callback, chunk_size = 10000, delim,
quote = "\"", escape_backslash = FALSE, escape_double = TRUE,
locale = default_locale(), na = c("", "NA"), quoted_na = TRUE,
comment = "", trim_ws = FALSE, skip = 0,
progress = show_progress(), skip_empty_rows = FALSE)melt_csv_chunked(file, callback, chunk_size = 10000,
locale = default_locale(), na = c("", "NA"), quoted_na = TRUE,
quote = "\"", comment = "", trim_ws = TRUE, skip = 0,
progress = show_progress(), skip_empty_rows = FALSE)
melt_csv2_chunked(file, callback, chunk_size = 10000,
locale = default_locale(), na = c("", "NA"), quoted_na = TRUE,
quote = "\"", comment = "", trim_ws = TRUE, skip = 0,
progress = show_progress(), skip_empty_rows = FALSE)
melt_tsv_chunked(file, callback, chunk_size = 10000,
locale = default_locale(), na = c("", "NA"), quoted_na = TRUE,
quote = "\"", comment = "", trim_ws = TRUE, skip = 0,
progress = show_progress(), skip_empty_rows = FALSE)
Either a path to a file, a connection, or literal data (either a single string or a raw vector).
Files ending in .gz, .bz2, .xz, or .zip will
be automatically uncompressed. Files starting with http://,
https://, ftp://, or ftps:// will be automatically
downloaded. Remote gz files can also be automatically downloaded and
decompressed.
Literal data is most useful for examples and tests. It must contain at least one new line to be recognised as data (instead of a path) or be a vector of greater than length 1.
Using a value of clipboard() will read from the system clipboard.
A callback function to call on each chunk
The number of rows to include in each chunk
Single character used to separate fields within a record.
Single character used to quote strings.
Does the file use backslashes to escape special
characters? This is more general than escape_double as backslashes
can be used to escape the delimiter character, the quote character, or
to add special characters like \n.
Does the file escape quotes by doubling them?
i.e. If this option is TRUE, the value """" represents
a single quote, \".
The locale controls defaults that vary from place to place.
The default locale is US-centric (like R), but you can use
locale() to create your own locale that controls things like
the default time zone, encoding, decimal mark, big mark, and day/month
names.
Character vector of strings to interpret as missing values. Set this
option to character() to indicate no missing values.
Should missing values inside quotes be treated as missing values (the default) or strings.
A string used to identify comments. Any text after the comment characters will be silently ignored.
Should leading and trailing whitespace be trimmed from each field before parsing it?
Number of lines to skip before reading data.
Display a progress bar? By default it will only display
in an interactive session and not while knitting a document. The display
is updated every 50,000 values and will only display if estimated reading
time is 5 seconds or more. The automatic progress bar can be disabled by
setting option readr.show_progress to FALSE.
Should blank rows be ignored altogether? i.e. If this
option is TRUE then blank rows will not be represented at all. If it is
FALSE then they will be represented by NA values in all the columns.
melt_delim_chunked() and the specialisations melt_csv_chunked(),
melt_csv2_chunked() and melt_tsv_chunked() read files by a chunk of rows
at a time, executing a given function on one chunk before reading the next.
Other chunked: callback,
read_delim_chunked,
read_lines_chunked
# NOT RUN {
# Cars with 3 gears
f <- function(x, pos) subset(x, data_type == "integer")
melt_csv_chunked(readr_example("mtcars.csv"), DataFrameCallback$new(f), chunk_size = 5)
# }
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