Class 40: Multiple-Valued Procedures

Held Friday, April 13, 2001

Summary

Today we consider why and how to return more than one value from a procedure.

Notes

• No reading this weekend.
• Beware, Friday the 13th falls on a Friday this month.
• I added some comments on partition in the reading.
• Are there any final questions on exam 2?
  • I will not be available for questions over the weekend.
  • You may not talk to each other about the exam or about Scheme over the weekend.
  • map! probably shouldn't return anything, but it's fine if it returns the modified vector.
  • I don't think I've received any other new questions, so I may have resolved all of them.

Overview

• Why return multiple values from procedures
• How to return multiple values from procedures
• What to do with those multiple values
• An alternative

Returning Multiple Values

• As you write more and more procedures, you will find that you occassionally write procedures that naturally return more than one value.
  • Although a list contains many values, you should think of procedures that return a list as returning the list as a value.
• For example,
  • When you divide an integer by another integer, you end up with the result and a possible remainder.
  • Similarly, you can split a real number into a whole part and a fractional part.
  • You might want to split a list into two parts: those that meet a predicate and those that don't meet a predicate.
  • You might want to tally more than one thing at once (e.g., A's, B's, and C's in a list of grades).
• In Scheme, you can use values to return more than one value from a procedure.
  • The values returned are literally separate values; they're not a list.
• However, it's not all that useful to return multiple values unless you can do something with them.
• The call-with-values procedure takes two parameters, a zero-parameter procedure, genproc, that returns multiple values and a procedure, proc, of the same number of parameters. It evaluates genproc and applies proc to the result.
• For example,

  > (values 1 2)
  1
  2
  > (call-with-values (lambda () (values 1 2)) +)
  3
  > (call-with-values (lambda () (values 1 2 3 4))
                      (lambda lst (cadr lst)))
  2
  

• Are there alternatives? Certainly. You could simply construct a list whenever you want to return multiple values and then destruct the list when you want to extract values.
  • Many consider the use of lists in this case inelegant.