## Overview

Not all exercises have exactly one right answer: there could be a few different ways to arrive at the same result, or the exercise prompt could be open-ended and ask the student to pick from a selection of possibilities. In either case, and with minimal changes to your learnr tutorial, gradethis can accommodate exercises with multiple solutions!

To signal that your exercise has multiple solutions, write all of the solutions in the exercise’s -solution chunk, separating the solutions with code headers — a comment followed by at least four dashes.

# code header ----

You can think of the code headers as labels for each solution variant. Here’s an example -solution chunk for an exercise with two possible solutions (the full exercise is presented below).

{r average-solution}
# mean ----
mean(fibonacci)

# median ----
median(fibonacci)


There are two types of exercises with multiple solutions:

1. Multiple solutions, same result. When the solutions describe multiple ways of reaching the same final result, gradethis provides code feedback hints based on the closest solution to the user’s code.

In most cases, you’ll only need to write out the solutions in the -solution chunk, and gradethis will take care of the rest. Learn more in Multiple solutions, same result below.

2. Multiple solutions, different results. Other exercises have more than one acceptable result. In these exercises, multiple solutions can be used both for grading students’ results and for providing code feedback.

In addition to writing out the solutions in the -solution chunk, you may also need to modify your grading code slightly. Learn more in Multiple solutions, different results below.

In both cases, multiple solutions are written in the same way in the -solution chunk, and minimal changes are needed in your grading code. For advanced grading requirements, gradethis adds two objects into the grading environment: .solution_code_all and .solution_all. We’ll cover both straight-forward and advanced uses in the examples that follow.

## Multiple solutions, same result

If all your solutions have the same result, you might not need to do anything special in your grading code, as we’ll see in the next example.

### Example

In this example, students are asked to convert a numeric value to a whole number. The most likely solution is to use the round() function, but a creative student might call floor() or as.integer().

Notice that we include all three variations in the round-solution chunk below, but we’ve placed the answer we consider “most correct” as the last solution in the chunk. The last solution in the chunk will be the default solution used by gradethis when the student’s code is not a good match for any of the solutions.

{r setup}
library(learnr)


Convert 5.2 to a reasonable whole number.

{r round, exercise = TRUE}
____(5.2)


{r round-solution}
# floor ----
floor(5.2)

# as.integer ----
as.integer(5.2)

# round ----
round(5.2)


{r round-check}
# provide code hints if submission is wrong
fail_if_code_feedback()

# pass if the value is correct
pass_if_equal()

# or fallback to a failing grade
fail(hint = TRUE)
})


If a student enters any of the correct answers to arrive at the value 5, they receive positive feedback and encouragement.

as.integer(5.2)

If a student gives an incorrect answer using round(), the code feedback hint provided by fail(hint = TRUE) will nudge them towards the correct round() solution:

round(5.2, digits = 1)

And if a student gives an incorrect answer but their text is closer to one of the other solutions, the code feedback will nudge the student toward the correct solution.

flor(5.2)
as.numeric(5.2)

### Providing different messages for different solutions

Grading functions that use solution code — like fail_if_code_feedback(), code_feedback() or fail(hint = TRUE) — will automatically give feedback based on the closest solution to the student’s input. In some cases, you may want to accept multiple solutions, but give different feedback depending on the solution the student gave.

To enable advanced custom grading, gradethis makes all of the solutions to an exercise available to grading code as .solution_code_all, a named list containing the code for each solution.

You can use code like the following template to give a special message (<message>) for user input that matches a particular solution identified by the label used for the code heading above the solution (<solution_label>):

correctly_used_solution <-
is.null(code_feedback(.user_code, .solution_code_all[["<solution_label>"]]))

pass_if(
correctly_used_solution,
message = "<message>"
)

Some things of note in this template:

1. You can obtain the code for a specific solution using the label in its code heading: e.g. .solution_code_all[["floor"]] in our example would return "floor(5.2)".

2. You need to explicitly ask for code_feedback() for the solution of interest. Otherwise, by default, code_feedback() will first match the user’s code with the most option among the set of solutions.

3. code_feedback() returns NULL when user_code matches solution_code. Testing that the feedback is.null() is a robust way to test that the student submitted code that matches a solution because it ignores minor differences like whitespace and pipe usage.

#### Example

Using the template above, we can provide some feedback specifically to students who use the floor() solution.

{r round-check}
correctly_used_floor <-
is.null(code_feedback(.user_code, .solution_code_all[["floor"]]))

pass_if(
correctly_used_floor,
"Correct, but remember that floor() always rounds down.
If you want to round to the nearest whole number, round() is usually safer."
)
pass_if_equal()
fail(hint = TRUE)
})

floor(5.2)

## Multiple solutions, different results

If your exercise has multiple acceptable results, you’ll need to slightly adapt your code to ensure students’ inputs are tested against all acceptable results.

To compare the result of student input to each solution, you should use .solution_all as the first argument in pass_if_equal(). Multiple solution results are saved in an environment called .solution_all. (Because .solution_all is an environment, it requires some special handling. Check Working with .solution_all for more details.)

Within pass_if_equal() messages, you can use the object .solution_label, which corresponds to the code header of the solution that the student’s code matched.

Functions that grade students’ code can still be used without any changes.

### Example

{r setup}
library(learnr)
fibonacci <- c(0, 1, 1, 2, 3, 5, 8, 13, 21, 34)


Use mean() or median() to find the average of fibonacci, a vector containing the first 10 numbers in the Fibonacci sequence.

{r average, exercise = TRUE}
____(fibonacci)


{r average-solution}
# mean ----
mean(fibonacci)

# median ----
median(fibonacci)


{r average-check}
pass_if_equal(.solution_all, "You solved it with {.solution_label}()!")
fail(hint = TRUE)
})


If a student gives either correct solution, pass_if_equal() will give them a passing grade:

mean(fibonacci)
median(fibonacci)

If the student’s input doesn’t match any of the solutions, grading will proceed as normal:

sum(fibonacci)

### Working with all solution results

.solution_all is an environment, not a list. This carries certain advantages. Exercises can be graded faster because each solution can be evaluated as needed rather than all up front.

But it also comes with drawbacks. The most important are:

1. environments can’t be subset with numbers, only names, and
2. all elements of an environment need to have a unique name.

If you need to access the results of specific solutions, make sure you give a unique label to every solution.

# mean ----
mean(fibonacci)

# median ----
median(fibonacci)

To reference a specific solution result in grading code, use the solution label to access the result in the .solution_all:

grade_this({
pass_if_equal(
.solution_all[["mean"]],
"That's right! Do you think the mean is skewed?"
)
pass_if_equal(.solution_all)
fail_if_code_feedback()
fail()
})

If you don’t give your solutions unique names, gradethis will generate unique names for you. For example, these solutions

# mean ----
mean(fibonacci)

# mean ----
mean(fibonacci, na.rm = TRUE)

# median ----
median(fibonacci)

# median ----
median(fibonacci, na.rm = TRUE)

have these names:

#> [1] "mean"     "mean_1"   "median"   "median_1"

This means that if you need to access specific solution results in your grading code, it’s almost always better to give your solutions unique labels. On the other hand, if your passing message makes use of .solution_label, you may want to duplicate the solution labels.

Recall that the -check chunk for the average exercise was:

{r average-check}
pass_if_equal(.solution_all, "You solved it with {.solution_label}()!")

By including the mean() and median() solution variants with na.rm = TRUE we get an appropriate passing message even though the solution labels are duplicated:
median(fibonacci, na.rm = TRUE)