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Value

rascal-0.40.13-BOOT2
rascal-tutor-0.19.9

Synopsis​

Question about the value of a Rascal expression or program.

Syntax​

  • QValue OptName: TypeDescriptor
  • QValue OptName: QSteps Test Listing

Description​

A value question presents a Rascal expression and poses a question about its value.

OptName is an optional name of the question (enclosed between [ and ]). If OptName is missing, the question gets a unique number as name.

The desired type of the expression is given by a Type Descriptor.

The first form presents the value generated for the TypeDescriptor and asks about its value.

The second form allows more preparatory steps and also allows adding a listing to the question. The following steps are defined:

  • prep: Cmd: execute Cmd as preparatory step. Mostly used to import necessary libraries.
  • make: Var = TypeDescriptor: create a new variable and use TypeDescriptor to generate its value.
  • expr: Var = Exp: evaluate the Rascal expression Exp and assign its value to the new variable Var.
  • list: Lines: lines that will be displayed as a listing. The listing may contain a placeholder in the form of <?> and ends where a new step begins.
  • test: Exp₁ == Exp₂: the equality is evaluated as Rascal expression. The outcome determines the success or failure to answer this question.
  • hint: Text: a hint that will be shown when the user enters a wrong answer.

The following restrictions apply:

  • Each step starts at the beginning of a new line.
  • Every Exp or Cmd or listing may contain one or more variable references of the form < Var >.
  • Each variable reference < Var > is first replaced by the value of Var. Var should have received a value in a preceeding make or expr step.
  • The listing, and the expressions in the test may contain at most one placeholder <?>.

Examples​

See the effect of the following value questions in the Questions section below.

Question 1​

The following question can be paraphrased as: I give you a union of two sets of integers, what is its value?

QValue: <A:set[int]> + <B:same[A]>

Question 2​

The following question can be paraphrased as: What is the size of a given list of integers?

QValue:
prep: import List;
test: size(<A:list[int]>) == <?>

Note that the List module is imported as a preparatory step.

Question 3​

The following question can be paraphrased as: I give you a union of integers or strings and an unknown set and the result of the union; what is the value of the unknown set?

QValue:
make: A = set[arb[int,str]]
make: B = same[A]
expr: C = <A> + <B>
hint: <B>
test: <A> + <?> == <C>

Observe that we generate values for A and B and compute the value of C. The value of B is the answer we are looking for, and we replace it by <?> in the posed test. When the student gives a wrong answer, we show the value of B as hint.

Question 4​

The following question can be paraphrased as: Fill in the hole in the definition of funcion find to ensure that it returns all strings that contain "o".

QValue:
desc: Return the strings that contain "o".
list:
text = ["andra", "moi", "ennepe", "Mousa", "polutropon"];
public list[str] find(list[str] text){
  return
    for(s <- text)
      if(/o/ := s)
        <?>;
}
test: find(text) == ["moi", "Mousa", "polutropon"];