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Factorial

rascal-0.40.17

Synopsis​

Compute the factorial function.

Examples​

The factorial of a number n is defined as n * (n-1) * (n-2) * ... * 1.

Here is the Rascal version:

@synopsis{fac1 demonstrates the ternary conditional and recursion}
int fac1(int n) = n <= 0 ? 1 : n * fac1(n - 1);     ❶  

@synopsis{fac2 demonstrates overloading and dynamic dispatch with pattern matching}
int fac2(0) = 1;      ❷  
default int fac2(int n) = n * fac2(n - 1);     ❸  

@synopsis{fac3 demonstrates structured programming and recursion}
int fac3(int n)  {     ❹  
  if (n == 0) 
     return 1;
  return n * fac3(n - 1);
}
  • ❶ fac1 is defined using a conditional expression to distinguish cases.
  • ❷ fac2 distinguishes cases using pattern-based dispatch (Rascal Functions). Here the case for 0 is defined.
  • ❸ Here all other cases for fac2 are defined (as indicated by the default keyword).
  • ❹ fac3 shows a more imperative implementation of factorial.

Here is how to use fac:

rascal>fac1(47);
int: 258623241511168180642964355153611979969197632389120000000000

NOTE: Indeed, Rascal supports arbitrary length numbers.

Here is an example of fac2:

rascal>fac2(47);
int: 258623241511168180642964355153611979969197632389120000000000

Benefits​

  • This simple demo shows how (recursive) Rascal functions work in three different ways.

Pitfalls​

  • Typically writing recursive functions can be avoided using Visit and Descendant, so this example isn't very exemplary of typical Rascal code.