read_chunk: Read chunks from an external script

Description

Chunks can be put in an external script, and this function reads chunks into the current knitr session; read_demo() is a convenience function to read a demo script from a package.

Usage

read_chunk(path, lines = readLines(path, warn = FALSE), labels = NULL, from = NULL, 
    to = NULL, from.offset = 0L, to.offset = 0L)
read_demo(topic, package = NULL, ...)

Arguments

path

the path to the R script

lines

a character vector of the code lines (by default read from path)

labels

a character vector of chunk labels (default NULL)

from, to

a numeric vector specifying the starting/ending line numbers of code chunks, or a character vector; see Details

from.offset, to.offset

an offset to be added to from/to

topic, package

name of the demo and the package see demo

...

arguments to be passed to read_chunk

Value

As a side effect, code chunks are read into the current session so that future chunks can (re)use the code by chunk label references.

Details

There are two approaches to read external code into the current session: (1) Use a special separator of the from

## ---- chunk-label} (at least four
dashes before the chunk label) in the script; (2) Manually specify the
labels, starting and ending positions of code chunks in the script.
The second approach will be used only when labels is not NULL.
For this approach, if from is NULL, the starting position is 1;
if to is NULL, each of its element takes the next element of
from minus 1, and the last element of to will be the length of
lines (e.g. when from = c(1, 3, 8) and the script has 10 lines
in total, to will be c(2, 7, 10)). Alternatively, from
and to can be character vectors as regular expressions to specify the
positions; when their length is 1, the single regular expression will be
matched against the lines vector, otherwise each element of
from/to is matched against lines and the match is
supposed to be unique so that the numeric positions returned from
grep() will be of the same length of from/to. Note
labels always has to match the length of from and to.

This function can only be used in a chunk which is not cached (chunk option cache = FALSE), and the code is read and stored in the current session without being executed (to actually run the code, you have to use a chunk with a corresponding label). ## put this in foo.R and read_chunk('foo.R')

## ---- my-label ---- 1 + 1 lm(y ~ x, data = data.frame(x = 1:10, y = rnorm(10)))

## later you can use <>= to reference this chunk

## the 2nd approach code = c("#@a", "1+1", "#@b", "#@a", "rnorm(10)", "#@b") read_chunk(lines = code, labels = "foo") # put all code into one chunk named foo read_chunk(lines = code, labels = "foo", from = 2, to = 2) # line 2 into chunk foo read_chunk(lines = code, labels = c("foo", "bar"), from = c(1, 4), to = c(3, 6)) # automatically figure out 'to' read_chunk(lines = code, labels = c("foo", "bar"), from = c(1, 4)) read_chunk(lines = code, labels = c("foo", "bar"), from = "^#@a", to = "^#@b") read_chunk(lines = code, labels = c("foo", "bar"), from = "^#@a", to = "^#@b", from.offset = 1, to.offset = -1)

## later you can use, e.g., <>= knitr:::knit_code$get() # use this to check chunks in the current session knitr:::knit_code$restore() # clean up the session

[object Object] http://yihui.name/knitr/demo/externalization/