captureOutput
Evaluate an R expression and captures the output
Evaluate an R expression and captures the output.
- Keywords
- programming, IO
Usage
captureOutput(expr, file=NULL, append=FALSE, collapse=NULL, envir=parent.frame())Arguments
- expr
The R expression to be evaluated.
- file
A file name or a
connectionto where the output is directed. Alternatively, ifNULLthe output is captured to and returned as acharactervector.- append
If
TRUE, the output is appended to the file or the (unopened) connection, otherwise it overwrites.- collapse
A
characterstring used for collapsing the captured rows. IfNULL, the rows are not collapsed.- envir
The
environmentin which the expression is evaluated.
Details
This method imitates capture.output with the major
difference that it captures strings via a raw connection rather
than via internal strings. The latter becomes exponentially slow
for large outputs [1,2].
Value
References
[1] R-devel thread 'capture.output(): Using a rawConnection() [linear] instead of textConnection() [exponential]?', 2014-02-03. https://stat.ethz.ch/pipermail/r-devel/2014-February/068349.html [2] JottR blog post 'PERFORMANCE: captureOutput() is much faster than capture.output()', 2015-05-26. http://www.jottr.org/2014/05/captureOutput.html
See Also
Internally, eval() is used to evaluate the expression.
and capture.output to capture the output.
Examples
# NOT RUN {
# captureOutput() is much faster than capture.output()
# for large outputs when capturing to a string.
for (n in c(10e3, 20e3, 30e3, 40e3)) {
printf("n=%d\n", n)
x <- rnorm(n)
t0 <- system.time({
bfr0 <- capture.output(print(x))
})
print(t0)
t1 <- system.time({
bfr <- captureOutput(print(x))
})
print(t1)
print(t1/t0)
bfr2n <- captureOutput(print(x), collapse="\n")
bfr2r <- captureOutput(print(x), collapse="\r")
stopifnot(identical(bfr, bfr0))
} # for (n ...)
# }