12.1 Redact source code

12.1 Redact source code

Sometimes we may not want to fully display our source code in the report. For example, you may have a password in a certain line of code. We mentioned in Section 11.7 that you can use the chunk option echo to select which expressions in the R code to display (e.g., show the second expression via echo = 2). In this section, we provide a more flexible method that does not require you to specify the indices of expressions.

The basic idea is that you add a special comment to the code (e.g., # SECRET!!). When this comment is detected in a line of code, you omit that line. Below is a full example using the source hook:

---
title: Using the `source` hook to hide certain lines of code
---

First, we set up a `source` hook to exclude the lines of code
that contain the string `# SECRET!!` at the end.

```{r, include=FALSE}
local({
  hook_source <- knitr::knit_hooks$get(‘source’)
  knitr::knit_hooks$set(source = function(x, options) {
    x <- x[!grepl(‘# SECRET!!$’, x)]
    hook_source(x, options)
  })
})

Now we can test the new hook. When you knit this document, you will not see the lines with the special comment # SECRET!!.

1 + 1  # normal code to be displayed

## please use your real username and password
auth <- httr::authenticate(“user”, ”passwd“)
auth <- httr::authenticate(“yihui”, ”horsebattery“)  # SECRET!!
httr::GET(“http://httpbin.org/basic-auth/user/passwd”, auth)

The key part in the above `source` hook is this line, which matches the trailing comment `# SECRET!!` in the source code vector `x` via `grepl()` and exclude the matches:

x <- x[!grepl(“# SECRET!!$”, x)]

Precisely speaking, the above hook will exclude whole _expressions_ containing the trailing comment `# SECRET!!`, instead of individual lines, because `x` is actually a vector of R expressions. For example, for the code chunk below:

1 + 1 if (TRUE) { # SECRET!! 1:10 }

The value of `x` in the `source` hook is:

c(“1 + 1”, ”if (TRUE) { # SECRET!!\n 1:10\n}“)

If you want to hide lines instead of expressions of R code, you will have to split `x` into individual lines. You may consider using the function `xfun::split_lines()`. The body of the hook function will be:

x <- xfun::split_lines(x) # split into individual lines x <- x[!grepl(“# SECRET!!$”, x)] x <- paste(x, collapse = “\n”) # combine into a single string hook_source(x, options) ```

This example shows you how to manipulate the source code string, and grepl() is certainly not the only choice of string manipulation. In Section 12.2, we will show another example.