read_chunk
Read chunks from an external script
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
Path to the R script.
- lines
Character vector of lines of code. By default, this is read from
path.- labels
Character vector of chunk labels (default
NULL).- from, to
Numeric vector specifying the starting/ending line numbers of code chunks, or a character vector; see Details.
- from.offset, to.offset
Offsets to be added to
from/to.- topic, package
Name of the demo and the package. See
demo.- ...
Arguments passed to
read_chunk.
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.
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.
Note
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).
References
Examples
# NOT RUN {
## 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 <<my-label>>= 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., <<foo>>=
knitr:::knit_code$get() # use this to check chunks in the current session
knitr:::knit_code$restore() # clean up the session
# }