extract {grr}

Extract/return parts of objects

Description

Alternative to built-in Extract or [. Allows for extraction operations that are ambivalent to the data type of the object. For example, extract(x,i) will work on lists, vectors, data frames, matrices, etc.

Usage

extract(x, i = NULL, j = NULL)

Arguments

x

object from which to extract elements

i, j

indices specifying elements to extract. Can be numeric, character, or logical vectors.

Details

Extraction is 2-100x faster on data frames than with the built in operation - but does not preserve row names.

Examples

#Typically about twice as fast on normal subselections
orders<-data.frame(orderNum=1:1e5,
 sku=sample(1e3, 1e5, TRUE),
 customer=sample(1e4,1e5,TRUE))
a<-sample(1e5,1e4)
system.time(b<-orders[a,])
system.time(c<-extract(orders,a))
rownames(b)<-NULL
rownames(c)<-NULL
identical(b,c)

#Speedup increases to 50-100x with oversampling 
a<-sample(1e5,1e6,TRUE)
system.time(b<-orders[a,])
system.time(c<-extract(orders,a))
rownames(b)<-NULL
rownames(c)<-NULL
identical(b,c)

#Can create function calls that work for multiple data types
alist<-as.list(1:50)
avector<-1:50
extract(alist,1:5)
extract(avector,1:5)
extract(orders,1:5)#'

## Not run: 
orders<-data.frame(orderNum=as.character(sample(1e5, 1e6, TRUE)),
 sku=sample(1e3, 1e6, TRUE),
 customer=sample(1e4,1e6,TRUE))
system.time(a<-sample(1e6,1e7,TRUE))
system.time(b<-orders[a,])
system.time(c<-extract(orders,a))

## End(Not run)