045-chunk-hook.md

Three arguments in chunk hooks

A chunk hook has three arguments: before, options and envir. We show how they work through some simple examples.

The before argument

It is a logical argument: before == TRUE executes code before a chunk.

library(knitr)
knit_hooks$set(foo1 = function(before, options, envir) {
    if (before) {
        "_I appear before a chunk!_\n\n"
    } else {
        "\n\n_I am after a chunk..._"
    }
})

Test the foo1 hook:

I appear before a chunk!

1 + 1

## [1] 2

I am after a chunk...

The options argument

It contains all the chunk options (include global options) for the current chunk.

knit_hooks$set(foo2 = function(before, options, envir) {
    if (!before) {
        z = capture.output(str(options[c("eval", "dev", "results", "bar1", "bar2", 
            "bar3")]))
        z = paste("    ", z, sep = "", collapse = "\n")
        paste("Some chunk options in the above chunk are:\n\n", z, sep = "")
    }
})

Test the foo2 hook:

1 + 1

## [1] 2

Some chunk options in the above chunk are:

List of 6
 $ eval   : logi TRUE
 $ dev    : chr "png"
 $ results: chr "markup"
 $ bar1   : logi TRUE
 $ bar2   : chr "asdf"
 $ bar3   : num 3.14

The envir argument

It is the environment of the current chunk.

knit_hooks$set(foo3 = function(before, options, envir) {
    if (!before) {
        paste("Objects available in the above chunk:", paste("`", ls(envir), 
            "`", sep = "", collapse = ", "))
    }
})

Test the foo3 hook:

x2 = 1 + 1
y3 = rnorm(10)

Objects available in the above chunk: x2, y3

Another example:

knit_hooks$set(foo4 = function(before, options, envir) {
    if (!before && exists("z5", envir = envir)) {
        sprintf("**Ha! I see z5 = %.3f!**", envir$z5)
    }
})

Test foo4:

pi

## [1] 3.142

This above chunk is quiet because z5 does not exist yet.

z5 = 2 * pi

Ha! I see z5 = 6.283!