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Revision 1191 - (download) (as text) (annotate)
Tue Feb 25 04:48:27 2014 UTC (4 years, 3 months ago) by mdowle
File size: 15850 byte(s)
+fread's progress meter is back
+added time for setkey to intro and timings vignette
\title{ Fast and friendly file finagler }
   Similar to \code{read.table} but faster and more convenient. All controls such as \code{sep}, \code{colClasses} and \code{nrows} are automatically detected. \code{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 \code{fasttime} package or standard base functions.

   `fread` is for \emph{regular} delimited files; i.e., where every row has the same number of columns. In future, secondary separator (\code{sep2}) may be specified \emph{within} each column. Such columns will be read as type \code{list} where each cell is itself a vector.
fread(input, sep="auto", sep2="auto", nrows=-1L, header="auto", na.strings="NA",
stringsAsFactors=FALSE, verbose=FALSE, autostart=30L, skip=-1L, select=NULL,
drop=NULL, colClasses=NULL,
integer64=getOption("datatable.integer64"),         # default: "integer64"
showProgress=getOption("datatable.showProgress")    # default: TRUE
  \item{input}{ Either the file name to read (containing no \\n character), a shell command that preprocesses the file (e.g. \code{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 \code{\link[base]{path.expand}} for convenience and may be a URL starting http:// or file://. }
  \item{sep}{ The separator between columns. Defaults to the first character in the set [\code{,\\t |;:}] that exists on line \code{autostart} outside quoted (\code{""}) regions, and separates the rows above \code{autostart} into a consistent number of fields, too. }
  \item{sep2}{ The separator \emph{within} columns. A \code{list} column will be returned where each cell is a vector of values. This is much faster using less working memory than \code{strsplit} afterwards or similar techniques. For each column \code{sep2} can be different and is the first character in the same set above [\code{,\\t |;:}], other than \code{sep}, that exists inside each field outside quoted regions on line \code{autostart}. NB: \code{sep2} is not yet implemented. }
  \item{nrows}{ The number of rows to read, by default -1 means all. Unlike \code{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 \code{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. }
  \item{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. }
  \item{na.strings}{ A character vector of strings to convert to \code{NA_character_}. By default for columns read as type character \code{",,"} is read as a blank string (\code{""}) and \code{",NA,"} is read as \code{NA_character_}. Typical alternatives might be \code{na.strings=NULL} or perhaps \code{na.strings=c("NA","N/A","")}. }
  \item{stringsAsFactors}{ Convert all character columns to factors? }
  \item{verbose}{ Be chatty and report timings? }
  \item{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 \code{sep}, \code{sep2} and the number of fields. It's extremely unlikely that \code{autostart} should ever need to be changed, we hope. }
  \item{skip}{ If -1 (default) use the procedure described below starting on line \code{autostart} to find the first data row. \code{skip>=0} means ignore \code{autostart} and take line \code{skip+1} as the first data row (or column names according to header="auto"|TRUE|FALSE as usual). \code{skip="string"} searches for \code{"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). }
  \item{select}{ Vector of column names or numbers to keep, drop the rest. }
  \item{drop}{ Vector of column names or numbers to drop, keep the rest. }
  \item{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. }
  \item{integer64}{ "integer64" (default) reads columns detected as containing integers larger than 2^31 as type \code{bit64::integer64}. Alternatively, \code{"double"|"numeric"} reads as \code{base::read.csv} does; i.e., possibly with loss of precision and if so silently. Or, "character". }
  \item{showProgress}{ TRUE displays progress on the console using \code{\\r}. It is produced in fread's C code where the very nice (but R level) txtProgressBar and tkProgressBar are not easily available. }

Once the separator is found on line \code{autostart}, the number of columns is determined. Then the file is searched backwards from \code{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 \code{skip>0} overrides this feature by setting \code{autostart=skip+1} and turning off the search upwards step.

The first 5 rows, middle 5 rows and last 5 rows are then read to determine column types. The lowest type for each column is chosen from the ordered list \code{integer}, \code{integer64}, \code{double}, \code{character}. This enables \code{fread} to allocate exactly the right number of rows, with columns of the right type, up front once. The file may of course \emph{still} contain data of a different type in rows other than first, middle and last 5. In that case, the column types are bumped mid read and the data read on previous rows is coerced. Setting \code{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 \code{list} columns (i.e. \code{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 \code{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).

\code{character} columns can be quoted (\code{...,2,"Joe Bloggs",3.14,...}) or not quoted (\code{...,2,Joe Bloggs,3.14,...}). Spaces and other whitepace (other than \code{sep} and \code{\\n}) may appear in an unquoted character field, provided the field doesn't contain \code{sep} itself. Therefore quoting character values is only required if \code{sep} itself appears in the string value. Quoting may also be used to signify that numeric data should be read as text (or that can be achieved by specifying the column type via \code{colClasses}). Field quoting is automatically detected and no arguments are needed to control it.

The filename extension (such as .csv) is irrelevant for "auto" \code{sep} and \code{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 \code{fread} through a set of files and as long as each file is regular and delimited, \code{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 \code{colClasses} in \code{read.table} or \code{read.csv}.

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

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.

    A \code{data.table}.
Background :\cr

finagler = "to get or achieve by guile or manipulation" \url{}
\seealso{ \code{\link[utils]{read.csv}}, \code{\link[base]{url}}
\if{html}{\out{<script type="text/javascript">var sc_project=6237851;var sc_invisible=1;var sc_security="518c93ca";</script><script type="text/javascript" src=""></script><noscript><div class="statcounter"><a title="web statistics" href="" target="_blank"><img class="statcounter" src="" alt="web statistics"></a></div></noscript>}}

# Demo speedup
DT = data.table( a=sample(1:1000,n,replace=TRUE),
                 f=sample(1:1000,n,replace=TRUE) )

cat("File size (MB):", round("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="",
# 10 sec (consistently). All known tricks and known nrows, see references.

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

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

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

all.equal(, DT)

# Scaling up ...
l = vector("list",10)
for (i in 1:10) l[[i]] = DT
DTbig = rbindlist(l)
# 500MB (10 million rows x 6 columns)

system.time(DF <- read.table("testbig.csv",header=TRUE,sep=",",         
# 100-200 sec (varies)

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

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

# Real data example (Airline data)

# 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=",",          
# 360 secs

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

# 5 character and 24 integer columns. Correct without needing to worry about colClasses
# issue above.

# Reads URLs directly :


# Reads text input directly :

# Reads pasted input directly :

# Finds the first data line automatically :
This is perhaps a banner line or two or ten.

# Detects whether column names are present automatically :

# 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

\keyword{ data }
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