fread {data.table} | R Documentation

## Fast and friendly file finagler

### Description

Similar to `read.table` but faster and more convenient. All controls such as `sep`, `colClasses` and `nrows` are automatically detected. `bit64::integer64` types are also detected and read directly without needing to read as character before converting.

Dates are read as character currently. They can be converted afterwards using the excellent `fasttime` package or standard base functions.

'fread' is for regular delimited files; i.e., where every row has the same number of columns. In future, secondary separator (`sep2`) may be specified within each column. Such columns will be read as type `list` where each cell is itself a vector.

### Usage

    fread(input, sep="auto", sep2="auto", nrows=-1L, header="auto", na.strings="NA", file,
    stringsAsFactors=FALSE, verbose=getOption("datatable.verbose"), autostart=1L,
    skip=0L, select=NULL, drop=NULL, colClasses=NULL,
    integer64=getOption("datatable.integer64"),         # default: "integer64"
    dec=if (sep!=".") "." else ",", col.names,
    check.names=FALSE, encoding="unknown", quote="\"",
    strip.white=TRUE, fill=FALSE, blank.lines.skip=FALSE, key=NULL,
    showProgress=getOption("datatable.showProgress"),   # default: TRUE
    data.table=getOption("datatable.fread.datatable")   # default: TRUE
    )

### Arguments

`input` | Either the file name to read (containing no \n character), a shell command that preprocesses the file (e.g. `fread("grep blah filename"))` or the input itself as a string (containing at least one \n), see examples. In both cases, a length 1 character string. A filename input is passed through `path.expand` for convenience and may be a URL starting http:// or file://.
`sep` | The separator between columns. Defaults to the first character in the set [`,\t |;:`] that exists on line `autostart` outside quoted (`""`) regions, and separates the rows above `autostart` into a consistent number of fields, too.

`sep2`

The separator within columns. A `list` column will be returned where each cell is a vector of values. This is much faster using less working memory than `strsplit` afterwards or similar techniques. For each column `sep2` can be different and is the first character in the same set above [`,\t |;:`], other than `sep`, that exists inside each field outside quoted regions on line `autostart`. NB: `sep2` is not yet implemented.

`nrows`

The number of rows to read, by default -1 means all. Unlike `read.table`, it doesn't help speed to set this to the number of rows in the file (or an estimate), since the number of rows is automatically determined and is already fast. Only set `nrows` if you require the first 10 rows, for example. 'nrows=0' is a special case that just returns the column names and types; e.g., a dry run for a large file or to quickly check format consistency of a set of files before starting to read any.

`header` | Does the first data line contain column names? Defaults according to whether every non-empty field on the first data line is type character. If so, or TRUE is supplied, any empty column names are given a default name.
`na.strings` | A character vector of strings which are to be interpreted as `NA` values. By default `",,"` for columns read as type character is read as a blank string (`""`) and `",NA,"` is read as `NA`. Typical alternatives might be `na.strings=NULL` (no coercion to NA at all!) or perhaps `na.strings=c("NA","N/A","null")`.
`file` | File path, useful when we want to ensure that no shell commands will be executed. File path can also be provided to `input` argument.
`stringsAsFactors` | Convert all character columns to factors?
`verbose` | Be chatty and report timings?

`autostart`

Any line number within the region of machine readable delimited text, by default 30. If the file is shorter or this line is empty (e.g. short files with trailing blank lines) then the last non empty line (with a non empty line above that) is used. This line and the lines above it are used to auto detect `sep`, `sep2` and the number of fields. It's extremely unlikely that `autostart` should ever need to be changed, we hope.

`skip`

If 0 (default) use the procedure described below starting on line `autostart` to find the first data row. `skip>0` means ignore `autostart` and take line `skip+1` as the first data row (or column names according to header="auto"|TRUE|FALSE as usual). `skip="string"` searches for `"string"` in the file (e.g. a substring of the column names row) and starts on that line (inspired by read.xls in package gdata).

`select` | Vector of column names or numbers to keep, drop the rest.
`drop` | Vector of column names or numbers to drop, keep the rest.

`colClasses`

A character vector of classes (named or unnamed), as read.csv. Or a named list of vectors of column names or numbers, see examples. colClasses in fread is intended for rare overrides, not for routine use. fread will only promote a column to a higher type if colClasses requests it. It won't downgrade a column to a lower type since NAs would result. You have to coerce such columns afterwards yourself, if you really require data loss.

`integer64` | "integer64" (default) reads columns detected as containing integers larger than 2^31 as type `bit64::integer64`. Alternatively, `"double"|"numeric"` reads as `base::read.csv` does; i.e., possibly with loss of precision and if so silently. Or, "character".
`dec` | The decimal separator as in `base::read.csv`. If not "." (default) then usually ",". See details.
`col.names` | A vector of optional names for the variables (columns). The default is to use the header column if present or detected, or if not "V" followed by the column number.
`check.names` | default is `FALSE`. If `TRUE` then the names of the variables in the `data.table` are checked to ensure that they are syntactically valid variable names. If necessary they are adjusted (by `make.names`) so that they are, and also to ensure that there are no duplicates.
`encoding` | default is `"unknown"`. Other possible options are `"UTF-8"` and `"Latin-1"`. Note: it is not used to re-encode the input, rather enables handling of encoded strings in their native encoding.
`quote` | By default (`"\""`), if a field starts with a doublequote, `fread` handles embedded quotes robustly as explained under `Details`. If it fails, then another attempt is made to read the field as is, i.e., as if quotes are disabled. By setting `quote=""`, the field is always read as if quotes are disabled.
`strip.white` | default is `TRUE`. Strips leading and trailing whitespaces of unquoted fields. If `FALSE`, only header trailing spaces are removed.
`fill` | logical (default is `FALSE`). If `TRUE` then in case the rows have unequal length, blank fields are implicitly filled.
`blank.lines.skip` | `logical`, default is `FALSE`. If `TRUE` blank lines in the input are ignored.
`key` | Character vector of one or more column names which is passed to `setkey`. It may be a single comma separated string such as `key="x,y,z"`, or a vector of names such as `key=c("x","y","z")`. Only valid when argument `data.table=TRUE`.
`showProgress` | `TRUE` displays progress on the console using `\r`. It is produced in fread's C code where the very nice (but R level) txtProgressBar and tkProgressBar are not easily available.
`data.table` | TRUE returns a `data.table`. FALSE returns a `data.frame`.

### Details

Once the separator is found on line `autostart`, the number of columns is determined. Then the file is searched backwards from `autostart` until a row is found that doesn't have that number of columns. Thus, the first data row is found and any human readable banners are automatically skipped. This feature can be particularly useful for loading a set of files which may not all have consistently sized banners. Setting `skip>0` overrides this feature by setting `autostart=skip+1` and turning off the search upwards step.

A sample of 1,000 rows is used to determine column types (100 rows from 10 points). The lowest type for each column is chosen from the ordered list: `logical`, `integer`, `integer64`, `double`, `character`. This enables `fread` to allocate exactly the right number of rows, with columns of the right type, up front once. The file may of course still contain data of a higher type in rows outside the sample. In that case, the column types are bumped mid read and the data read on previous rows is coerced. Setting `verbose=TRUE` reports the line and field number of each mid read type bump and how long this type bumping took (if any).

There is no line length limit, not even a very large one. Since we are encouraging `list` columns (i.e. `sep2`) this has the potential to encourage longer line lengths. So the approach of scanning each line into a buffer first and then rescanning that buffer is not used. There are no buffers used in `fread`'s C code at all. The field width limit is limited by R itself: the maximum width of a character string (currenly 2^31-1 bytes, 2GB).

The filename extension (such as .csv) is irrelevant for "auto" `sep` and `sep2`. Separator detection is entirely driven by the file contents. This can be useful when loading a set of different files which may not be named consistently, or may not have the extension .csv despite being csv. Some datasets have been collected over many years, one file per day for example. Sometimes the file name format has changed at some point in the past or even the format of the file itself. So the idea is that you can loop `fread` through a set of files and as long as each file is regular and delimited, `fread` can read them all. Whether they all stack is another matter but at least each one is read quickly without you needing to vary `colClasses` in `read.table` or `read.csv`.

If an empty line is encountered then reading stops there, with warning if any text exists after the empty line such as a footer. The first line of any text discarded is included in the warning message.

Line endings: All known line endings are detected automatically: `\n` (*NIX including Mac), `\r\n` (Windows CRLF), `\r` (old Mac) and `\n\r` (just in case). There is no need to convert input files first. `fread` running on any architecture will read a file from any architecture. Both `\r` and `\n` may be embedded in character strings (including column names) provided the field is quoted.

Decimal separator and locale: `fread(...,dec=",")` should just work. `fread` uses C function `strtod` to read numeric data; e.g., `1.23` or `1,23`. `strtod` retrieves the decimal separator (`.` or `,` usually) from the locale of the R session rather than as an argument passed to the `strtod` function. So for `fread(...,dec=",")` to work, `fread` changes this (and only this) R session's locale temporarily to a locale which provides the desired decimal separator.

On Windows, "French_France.1252" is tried which should be available as standard (any locale with comma decimal separator would suffice) and on unix "fr_FR.utf8" (you may need to install this locale on unix). `fread()` is very careful to set the locale back again afterwards, even if the function fails with an error. The choice of locale is determined by `options()$datatable.fread.dec.locale`. This may be a vector of locale names and if so they will be tried in turn until the desired `dec` is obtained; thus allowing more than two different decimal separators to be selected. This is a new feature in v1.9.6 and is experimental. In case of problems, turn it off with `options(datatable.fread.dec.experiment=FALSE)`.

Quotes:

When `quote` is a single character,

- Spaces and other whitespace (other than `sep` and `\n`) may appear in unquoted character fields, e.g., `...,2,Joe Bloggs,3.14,...`.

- When `character` columns are quoted, they must start and end with that quoting character immediately followed by `sep` or `\n`, e.g., `...,2,"Joe Bloggs",3.14,...`.

  In essence quoting character fields are required only if `sep` or `\n` appears in the string value. Quoting may be used to signify that numeric data should be read as text. Unescaped quotes may be present in a quoted field, e.g., `...,2,"Joe, "Bloggs"",3.14,...`, as well as escaped quotes, e.g., `...,2,"Joe \",Bloggs\"",3.14,...`.

  If an embedded quote is followed by the separator inside a quoted field, the embedded quotes up to that point in that field must be balanced; e.g. `...,2,"www.blah?x="one",y="two"",3.14,...`.

  On those fields that do not satisfy these conditions, e.g., fields with unbalanced quotes, `fread` re-attempts that field as if it isn't quoted. This is quite useful in reading files that contains fields with unbalanced quotes as well, automatically.

To read fields as is instead, use `quote = ""`.

### Value

A `data.table` by default. A `data.frame` when argument `data.table=FALSE`; e.g. `options(datatable.fread.datatable=FALSE)`.

### References

Background :
https://cran.r-project.org/doc/manuals/R-data.html
http://stackoverflow.com/questions/1727772/quickly-reading-very-large-tables-as-dataframes-in-r
http://www.biostat.jhsph.edu/~rpeng/docs/R-large-tables.html
https://stat.ethz.ch/pipermail/r-help/2007-August/138315.html
http://www.cerebralmastication.com/2009/11/loading-big-data-into-r/
http://stackoverflow.com/questions/9061736/faster-than-scan-with-rcpp
http://stackoverflow.com/questions/415515/how-can-i-read-and-manipulate-csv-file-data-in-c
http://stackoverflow.com/questions/9352887/strategies-for-reading-in-csv-files-in-pieces
http://stackoverflow.com/questions/11782084/reading-in-large-text-files-in-r
http://stackoverflow.com/questions/45972/mmap-vs-reading-blocks
http://stackoverflow.com/questions/258091/when-should-i-use-mmap-for-file-access
http://stackoverflow.com/a/9818473/403310
http://stackoverflow.com/questions/9608950/reading-huge-files-using-memory-mapped-files

finagler = "to get or achieve by guile or manipulation" http://dictionary.reference.com/browse/finagler

### See Also

`read.csv`, `url`, `Sys.setlocale`

### Examples

    ## Not run:

    # Demo speedup
    n=1e6
    DT = data.table( a=sample(1:1000,n,replace=TRUE),
                     b=sample(1:1000,n,replace=TRUE),
                     c=rnorm(n),
                     d=sample(c("foo","bar","baz","qux","quux"),n,replace=TRUE),
                     e=rnorm(n),
                     f=sample(1:1000,n,replace=TRUE) )
    DT[2,b:=NA_integer_]
    DT[4,c:=NA_real_]
    DT[3,d:=NA_character_]
    DT[5,d:=""]
    DT[2,e:=+Inf]
    DT[3,e:=-Inf]

    write.table(DT,"test.csv",sep=",",row.names=FALSE,quote=FALSE)
    cat("File size (MB):", round(file.info("test.csv")$size/1024^2),"\n")
    # 50 MB (1e6 rows x 6 columns)

    system.time(DF1 <-read.csv("test.csv",stringsAsFactors=FALSE))
    # 60 sec (first time in fresh R session)

    system.time(DF1 <- read.csv("test.csv",stringsAsFactors=FALSE))
    # 30 sec (immediate repeat is faster, varies)

    system.time(DF2 <- read.table("test.csv",header=TRUE,sep=",",quote="",
        stringsAsFactors=FALSE,comment.char="",nrows=n,
        colClasses=c("integer","integer","numeric",
                     "character","numeric","integer")))
    # 10 sec (consistently). All known tricks and known nrows, see references.

    require(data.table)
    system.time(DT <- fread("test.csv"))
    #  3 sec (faster and friendlier)

    require(sqldf)
    system.time(SQLDF <- read.csv.sql("test.csv",dbname=NULL))
    # 20 sec (friendly too, good defaults)

    require(ff)
    system.time(FFDF <- read.csv.ffdf(file="test.csv",nrows=n))
    # 20 sec (friendly too, good defaults)

    identical(DF1,DF2)
    all.equal(as.data.table(DF1), DT)
    identical(DF1,within(SQLDF,{b<-as.integer(b);c<-as.numeric(c)}))
    identical(DF1,within(as.data.frame(FFDF),d<-as.character(d)))

    # Scaling up ...
    l = vector("list",10)
    for (i in 1:10) l[[i]] = DT
    DTbig = rbindlist(l)
    tables()
    write.table(DTbig,"testbig.csv",sep=",",row.names=FALSE,quote=FALSE)
    # 500MB (10 million rows x 6 columns)

    system.time(DF <- read.table("testbig.csv",header=TRUE,sep=",",
        quote="",stringsAsFactors=FALSE,comment.char="",nrows=1e7,
        colClasses=c("integer","integer","numeric",
                     "character","numeric","integer")))
    # 100-200 sec (varies)

    system.time(DT <- fread("testbig.csv"))
    # 30-40 sec

    all(mapply(all.equal, DF, DT))

    # Real data example (Airline data)
    # http://stat-computing.org/dataexpo/2009/the-data.html

    download.file("http://stat-computing.org/dataexpo/2009/2008.csv.bz2",
                  destfile="2008.csv.bz2")
    # 109MB (compressed)

    system("bunzip2 2008.csv.bz2")
    # 658MB (7,009,728 rows x 29 columns)

    colClasses = sapply(read.csv("2008.csv",nrows=100),class)
    # 4 character, 24 integer, 1 logical. Incorrect.

    colClasses = sapply(read.csv("2008.csv",nrows=200),class)
    # 5 character, 24 integer. Correct. Might have missed data only using 100 rows
    # since read.table assumes colClasses is correct.

    system.time(DF <- read.table("2008.csv", header=TRUE, sep=",",
        quote="",stringsAsFactors=FALSE,comment.char="",nrows=7009730,
        colClasses=colClasses)
    # 360 secs

    system.time(DT <- fread("2008.csv"))
    #  40 secs

    table(sapply(DT,class))
    # 5 character and 24 integer columns. Correct without needing to worry about colClasses
    # issue above.

    # Reads URLs directly :
    fread("http://www.stats.ox.ac.uk/pub/datasets/csb/ch11b.dat")

    ## End(Not run)

    # Reads text input directly :
    fread("A,B\n1,2\n3,4")

    # Reads pasted input directly :
    fread("A,B
    1,2
    3,4
    ")

    # Finds the first data line automatically :
    fread("
    This is perhaps a banner line or two or ten.
    A,B
    1,2
    3,4
    ")

    # Detects whether column names are present automatically :
    fread("
    1,2
    3,4
    ")

    # Numerical precision :

    DT = fread("A\n1.010203040506070809010203040506\n")   # silent loss of precision
    DT[,sprintf("%.15E",A)]   # stored accurately as far as double precision allows

    DT = fread("A\n1.46761e-313\n")   # detailed warning about ERANGE; read as 'numeric'
    DT[,sprintf("%.15E",A)]   # beyond what double precision can store accurately to 15 digits

    # For greater accuracy use colClasses to read as character, then package Rmpfr.

    # colClasses
    data = "A,B,C,D\n1,3,5,7\n2,4,6,8\n"
    fread(data, colClasses=c(B="character",C="character",D="character"))  # as read.csv
    fread(data, colClasses=list(character=c("B","C","D")))    # saves typing
    fread(data, colClasses=list(character=2:4))     # same using column numbers

    # drop
    fread(data, colClasses=c("B"="NULL","C"="NULL"))   # as read.csv
    fread(data, colClasses=list(NULL=c("B","C")))      #
    fread(data, drop=c("B","C"))      # same but less typing, easier to read
    fread(data, drop=2:3)             # same using column numbers

    # select
    # (in read.csv you need to work out which to drop)
    fread(data, select=c("A","D"))    # less typing, easier to read
    fread(data, select=c(1,4))        # same using column numbers

    # skip blank lines
    fread("a,b\n1,a\n2,b\n\n\n3,c\n", blank.lines.skip=TRUE)
    # fill
    fread("a,b\n1,a\n2\n3,c\n", fill=TRUE)
    fread("a,b\n\n1,a\n2\n\n3,c\n\n", fill=TRUE)

    # fill with skip blank lines
    fread("a,b\n\n1,a\n2\n\n3,c\n\n", fill=TRUE, blank.lines.skip=TRUE)

    # check.names usage
    fread("a b,a b\n1,2\n")
    fread("a b,a b\n1,2\n", check.names=TRUE) # no duplicates + syntactically valid names

[Package data.table version 1.10.4-1 Index]