R: Create an index from a filter statement

ffwhich {ffbase} | R Documentation

## Create an index from a filter statement

### Description

`ffwhich` creates an `ff` integer index vector from a filter expression. The resulting vector can be used to index or subset a ffdf or ff vector.

### Usage

    ffwhich(x, expr, ...)

### Arguments

`x` | `ff` or `ffdf` object
`expr` | R code that evaluates to a logical
`...` | not used

### See Also

ffindexget ffindexset

### Examples

    # create a ff vector
    x <- ff(10:1)
    # make an ff index vector
    idx <- ffwhich(x, x < 5)
    # use it to retrieve values from x
    x[idx][]

    # create a ffdf data.frame
    dat <- ffdf(x1=x, y1=x)
    # create an ff index vector from a filter statement
    idx <- ffwhich(dat, x1 < 5 & y1 > 2)
    # use it to select data from the data.frame
    dat[idx,][,]

[Package ffbase version 0.13.1 Index]