A custom printing function

The S3 generic function knit_print is the default printing function in knitr. The chunk option render uses this function by default. The main purpose of this S3 generic function is to customize printing of R objects in code chunks. We can fall back to the normal printing behavior by setting the chunk option render = normal_print.

knit_print(x, ...)

normal_print(x, ...)

Arguments

x

An R object to be printed

...

Additional arguments passed to the S3 method. Currently ignored, except two optional arguments options and inline; see the references below.

Value

The value returned from the print method should be a character vector or can be converted to a character value. You can wrap the value in asis_output() so that knitr writes the character value as is in the output.

Details

Users can write custom methods based on this generic function. For example, if we want to print all data frames as tables in the output, we can define a method knit_print.data.frame that turns a data.frame into a table (the implementation may use other R packages or functions, e.g. xtable or kable()).

Note

It is recommended to leave a ... argument in your method, to allow future changes of the knit_print() API without breaking your method.

References

See vignette('knit_print', package = 'knitr').

Examples

library(knitr)
# write tables for data frames
knit_print.data.frame = function(x, ...) {
    res = paste(c("", "", kable(x, output = FALSE)), collapse = "\n")
    asis_output(res)
}
# register the method
registerS3method("knit_print", "data.frame", knit_print.data.frame)
# after you define and register the above method, data frames will be printed
# as tables in knitr, which is different with the default print() behavior