AnswerTests: Answer Tests
In swirl: Learn R, in R

Description Details Predefined Answer Tests Custom Answer Tests See Also

Answer tests are how swirl determines whether a user has answered a question correctly or not. Each question has one or more answer tests associated with it, all of which must be satisfied in order for a user's response to be considered correct. As the instructor, you can specify any combination of our predefined answer tests or create your own custom answer tests to suit your specific needs. This document will explain your options.

For each question that you author as part of a swirl lesson, you must specify exactly one correct answer. This is separate and distinct from the answer tests. This does not have to be the only correct answer, but it must answer the question correctly. If a user skips your question, this is the answer that will be entered on his or her behalf.

If you're using the swirlify authoring tool, the correct answer will be automatically translated into the appropriate answer test for most question types. Questions that require the user to enter a valid command at the R prompt (which we'll refer to as command questions) are the only exception. Since there are often many possible ways to answer a command question, you must determine how you'd like swirl to assess the correctness of a user's response. This is where answer tests come in.

You can specify any number of answer tests. If you use more than one, you must separate them with semicolons. If you do not specify any answer tests for a command question, then the default test will be used. The default test is omnitest(correctExpr='<correct_answer_here>'), which will simply check that the user's expression matches the expression that you provided as a correct answer.

In many cases, the default answer test will provide sufficient vetting of a user's response to a command question. While it is somewhat restrictive in the sense that it requires an exact match of expressions (ignoring whitespace), it is liberating to the course author for two reasons.

It allows for fast prototyping of content. As you're developing content, you may find that determining how to test for correctness distracts you from the message you're trying to communicate.
You don't have to worry about what happens if the user enters an incorrect response, but is allowed to proceed because of an oversight in the answer tests. Since swirl sessions are continuous, accepting an incorrect answer early in a lesson can cause problems later on. By using the default answer test, you eliminate this burden by requiring an exact match of expressions and hence not allowing the user to advance until you are certain they've entered the correct response.

It's important to keep in mind that as your content matures, you can always go back and make your answer testing strategy more elaborate. The main benefit of using tests other than the default is that the user will not be required to enter an expression exactly the way you've specified it. He or she will have more freedom in terms of how they respond to a question, as long as they satisfy the conditions that you see as being most important.

Each of the predefined answer tests listed below has its own help file, where you'll find more detailed explanations and examples.

any_of_exprs: Test that the user's expression matches any of several possible expressions.

calculates_same_value: Test that the user's expression evaluates to a certain value.

expr_creates_var: Test that a new variable has been created.

expr_identical_to: Test that the user has entered a particular expression.

expr_is_a: Test that the expression itself is of a specific class.

expr_uses_func: Test that a particular function has been used.

func_of_newvar_equals: Test the result of a computation such as mean(newVar) applied to a specific (user-named) variable created in a previous question.

omnitest: Test for a correct expression, a correct value, or both.

val_has_length: Test that the value of the expression has a particular length.

val_matches: Test that the user's expression matches a regular expression (regex).

var_is_a: Test that the value of the expression is of a specific class.

Occasionally, you may want to test something that is outside the scope of our predefined answer tests. Fortunately, this is very easy to do. If you are using the swirlify authoring tool, then a file called customTests.R (case-sensitive) is automatically created in the lesson directory. If it's not there already, you can create the file manually.

In this file, you can write your own answer tests. These answer tests are then available to you just the same as any of the standard tests. However, the scope of a custom answer test is limited to the lesson within which you've defined it.

Each custom answer test is simply an R function that follows a few basic rules:

Give the function a distinct name that will help you remember what is does (e.g. creates_matrix_with_n_rows).
The first line of the function body is e <- get("e", parent.frame()), which gives you access to the environment e. Any important information, such as the expression typed by the user, will be available to you through e.
Access the expression entered by the user with e$expr and the value of the expression with e$val. Note that e$expr comes in the form of an unevaluated R expression.
The function returns TRUE if the test is passed and FALSE otherwise. You should be careful that no other value could be returned (e.g. NA, NULL, etc.)