R: Melt a delimited file by chunks

melt_delim_chunked {readr} | R Documentation

## Melt a delimited file by chunks

### Description

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.

### Usage

    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)

### Arguments

`file` | 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.
`callback` | A callback function to call on each chunk
`chunk_size` | The number of rows to include in each chunk
`delim` | Single character used to separate fields within a record.
`quote` | Single character used to quote strings.
`escape_backslash` | 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`.
`escape_double` | Does the file escape quotes by doubling them? i.e. If this option is `TRUE`, the value `""""` represents a single quote, `\"`.
`locale` | 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.
`na` | Character vector of strings to interpret as missing values. Set this option to `character()` to indicate no missing values.
`quoted_na` | Should missing values inside quotes be treated as missing values (the default) or strings.
`comment` | A string used to identify comments. Any text after the comment characters will be silently ignored.
`trim_ws` | Should leading and trailing whitespace be trimmed from each field before parsing it?
`skip` | Number of lines to skip before reading data.
`progress` | 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`.
`skip_empty_rows` | 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.

### Details

`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.

### See Also

Other chunked: `callback`, `read_delim_chunked`, `read_lines_chunked`

### Examples

    # 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)

[Package readr version 1.3.1 Index]