Hidden webR code cells

Hidden Evaluation without Output

In this example, we create a hidden setup code cell within the document by using the special comment of #| context: setup. The setup code cell executes code in the background and does not display the code or its output.

```{webr-r}
#| context: setup
meaning_of_life = 42
```

Thus, we have pre-loaded the meaning_of_life variable. So, if we run the next code cell, then we will see the value of meaning_of_life being displayed as 42 instead of an error.

Caution

Be advised that the contents of the hidden code cell is displayed if the web page’s source is viewed.

Hidden Loading of a Dataset

Outside of just specifying a single variable, we can use the setup hidden code cell to pre-load and wrangle an entire data set. This allows for students to directly interact with a loaded data set.

```{webr-r}
#| context: hidden

# Download a data set
download.file(
  'https://raw.githubusercontent.com/coatless/raw-data/main/penguins.csv',
  'penguins.csv'
)

# Read data
df = read.csv("penguins.csv")
```

Note

If the setup code needs the present of R packages, we suggest specifying the required packages in the document’s YAML. This option communicates to the end user that the webpage is not yet ready to explore through a clear status update at the top. For example, we could add dplyr and ggplot2 using:

---
webr:
  packages: ['ggplot2', 'dplyr']
---

Hidden Solution Checking of Student Work

Warning

Be advised that any solution written into a webR hidden code cell can be obtained by viewing the document’s HTML source. Please avoid using this option for formal assessment (exams, quizzes, homework, …).

Lastly, the webR document can be used to check student answers. We can make available an answer key and a comparison function within the document.

For instance, the solution data frame might look a bit like:

```{webr-r}
#| context: setup
answer_frame <- data.frame(
  problem = c("1a", "1b", "2"),
  answer = c(10, 2, 3/16),
  tol = c(0.001, 0, 1/32)
)
```

Next, we can define an internal check function like so:

```{webr-r}
#| context: setup
check <- function(problem, answer) {
  aframe <- answer_frame
  if(!problem %in% aframe$problem) stop(paste0("Please enter a valid problem. (", paste0(aframe$problem, collapse = ","), ")"))

  solution <- aframe[which(aframe$problem == problem), "answer"]

  ifelse(
    all.equal(answer, solution, tolerance = 0.001) == TRUE,
              "Correct! Well done.",
              "Incorrect! Good attempt. Let's try again?")
}
```

Students can then compare their answers to the answer key by using the check() function.

Consider the question:

What is 9 + 1?