# List Recursion, Revisited

Summary: We've learned a number of basic techniques for writing recursive functions over lists, including the a pattern of recursion. But these techniques aren't always the clearest or most elegant for every case. Here, we extend your recursion toolbox to include a few more techniques, particularly the identification of special base cases and ways to write recursive predicates.

## Singleton Base Cases

Sometimes the problem that we need an algorithm for doesn't apply to the empty list, even in a vacuous or trivial way, and the base case for a direct recursion instead involves singleton lists -- that is, lists with only one element. For instance, suppose that we want an algorithm that finds the leftmost element of a given non-empty list of drawings. (The list must be non-empty because there is no “leftmost element” of an empty list.)

````>` ```(drawings-leftmost
(list (drawing-rectangle 10 0 20 20)
(drawing-rectangle 3 5 20 20)
(drawing-rectangle 6 2 20 20)
(drawing-rectangle 1 50 20 20)
(drawing-rectangle 8 5 20 20)))```
`(drawing rectangle 0 "" 1 50 20 20)`
```

The assumption that the list is not empty is a precondition for the meaningful use of this procedure, just as a call to Scheme's built-in `quotient` procedure requires that the second argument, the divisor, be non-zero. A precondition is a requirement that must be met in order for your procedure to work correctly. You should form the habit of figuring out when such preconditions are appropriate. With the Six-P technique for documenting procedures, you should also make it a habit to document such preconditions as you write the initial comment for a procedure:

```;;; Procedure:
;;;   drawings-leftmost
;;; Parameters:
;;;   drawings, a list of drawings.
;;; Purpose:
;;;   Find the leftmost drawing in drawings.
;;; Produces:
;;;   leftmost, a drawing.
;;; Preconditions:
;;;   drawings is not empty.
;;;   All the values in drawings are drawings.
;;; Postconditions:
;;;   leftmost is an element of drawings (and is, by implication, a drawing).
;;;   For each drawing, d, in drawings, leftmost either starts in the same
;;;     column as di, or to the left of d.
```

Alternately, we can make the structure of the list part of the specification of the parameters (and therefore an implicit precondition).

```;;; Parameters:
;;;   drawings, a nonempty list of drawings.
```

Whether you specify the precondition in the parameters or the preconditions section is often a matter of personal taste. What is most important is that you specify it somewhere.

Now that we've documented the procedure, let's think about how to implement it. If a list of drawings contains only one element, the answer is trivial -- its only element is its leftmost. Otherwise, we can take the list apart into its car and its cdr, invoke the procedure recursively to find the leftmost of the cdr, and then try to figure out which comes first. How do we figure whether or not one drawing is to the left of another? We compare their left edge, which we can do with `drawing-left`. Let's use a helper procedure to compare the two drawings and return the leftmost. (This is not a recursive helper procedure. Rather, like `rgb-darker`, it is a relatively straightforward procedure that simplifies our recursive definitions.)

```;;; Procedure:
;;;   drawing-leftmost
;;; Parameters:
;;;   drawing1, a drawing
;;;   drawing2, a drawing
;;; Purpose:
;;;   Determine the leftmost of drawing1 and drawing2.
;;; Produces:
;;;   leftmost, a drawing.
;;; Preconditions:
;;;   drawing1 and drawing2 have the correct form for drawings.
;;; Postconditions:
;;;   leftmost is equal to either drawing1 or drawing2.
;;;   leftmost either begins in the same column as both drawing1 and
;;;     drawing2 or begins in the same column as one, and begins to the
;;;     left of the other.
(define drawing-leftmost
(lambda (drawing1 drawing2)
(if (< (drawing-left drawing1) (drawing-left drawing2))
drawing1
drawing2)))
```

We can test whether the given list has a single element by checking whether its cdr is an empty list. The value of the expression `(null? (cdr drawings))` is `#t` if `drawings` has a single element and `#f` if `drawings` has two or more elements. (It gives an error if `drawings` has zero elements.)

Here, then, is the procedure definition:

```(define drawings-leftmost
(lambda (drawings)
(if (null? (cdr drawings))
(car drawings)
(drawing-leftmost (car drawings) (drawings-leftmost (cdr drawings))))))
```

If someone who uses this procedure happens to violate its precondition, applying the procedure to the empty list, the Scheme interpreter notices the error and prints out a diagnostic message:

````>` `(drawings-leftmost null)`
cdr: expects argument of type <pair>; given ()
```

## Singleton Values and Difference

If you think back to the tail-recursive version of `difference`, you may note another time that we had a special singleton case. When we compute `v`1 - `v`2 - `v`3 - `v`k, the base case is not “we have nothing to subtract”, but rather “we have nothing to subtract from `v`1”.

We didn't note the need for a singleton base case until we tried to write a tail-recursive version, but the need was there. Of course, that means that we might consider rewriting the non-tail-recursive version, but that version gave us the wrong answer, anyway.

## Some Common Forms of Recursive Procedures

If you consider the examples you've studied over the past few days, you will see that there is a common form for most of the procedures. The form goes something like this:

```(define recursive-proc
(lambda (val)
(if (base-case-test? val)
(base-case-computation val)
(combine (partof val)
(recursive-proc (simplify val))))))
```

For example, for the `drawings-leftmost` procedure,

• The recursive-proc is `drawings-leftmost`.
• The val is `drawings`, our list of numbers.
• The base-case-test is ```(null? (cdr drawings))```, which checks whether `drawings` has only one element.
• The base-case-computation is `car`, which extracts the one drawing left in `drawings`.
• The partof procedure is also `car`, which extracts the first drawing in `drawings`.
• The simplify procedure is `cdr`, which drops the first element, thereby giving us a simpler (well, smaller) list.
• Finally, the combine procedure is `drawing-leftmost`.

Similarly, consider the first complete version of `sum`.

```;;; Procedure:
;;;   sum
;;; Parameters:
;;;   numbers, a list of numbers.
;;; Purpose:
;;;   Find the sum of the elements of a given list of numbers
;;; Produces:
;;;   total, a number.
;;; Preconditions:
;;;   All the elements of numbers must be numbers.
;;; Postcondition:
;;;   total is the result of adding together all of the elements of numbers.
;;;   If all the values in numbers are exact, total is exact.
;;;   If any values in numbers are inexact, total is inexact.
(define sum
(lambda (numbers)
(if (null? numbers)
0
(+ (car numbers) (sum (cdr numbers))))))
```

In the `sum` procedure,

• The recursive-proc is `sum`.
• The val is again `numbers`, a list of numbers.
• The base-case-test is `(null? numbers)`, which checks if we have no numbers.
• The base-case-computation is `0`. (This computation does not quite match the form above, since we don't apply the 0 to `numbers`. As this example suggests, sometimes the base case does not involve the parameter.)
• The partof procedure is `car`, which extracts the first value in `numbers`.
• The simplify procedure is `cdr`, which drops the the first element.

When you write your own recursive procedures, it's often useful to start with the general structure and then to fill in the pieces. When you are recursing over lists (as you have in our first explorations of recursion), partof is almost always `car` and simplify is almost always `cdr`. There's a bit more to the base-case-test, since we've used both `(null? ___)` and `(null? (cdr? ___))`. We may also find other techniques.

However, when you work with other kinds of information (as you will do soon), you'll have different techniques for extracting a piece of information, for simplifying the argument, and for deciding when you're done.

Note, also, that examples like filtering suggest a similar, but more complex structure for recursive procedures.

```(define recursive-proc
(lambda (val)
(cond
((one-base-case-test?)
(one-base-case-computation val))
((another-base-case-test?)
(another-base-case-computation val))
...
((special-recursive-case-test-1?)
(combine-1 (partof-1 val)
(recursive-proc (simplify-1 val))))
((special-recursive-case-test-2?)
(combine-2 (partof-2 val)
(recursive-proc (simplify-2 val))))
...
(else
(combine (partof val)
(recursive-proc (simplify val)))))))
```

However, in practice you will find that you rarely have more than two base-case tests (mostly one) and rarely more than two recursive cases.

When we write tail-recursive procedures, we simply use the result of the recursive call, and don't combine it with anything. Here's a simple tail recursive pattern.

```(define procedure
(lambda (val)
(procedure-helper initial-result val)))
(define procedure-helper
(lambda (so-far remaining)
(if (base-case-test? remaining)
(final-computation so-far)
(procedure-helper (combine (part-of remaining) so-far)
(simplify remaining)))))
```

## Using And and Or

Of course, these common forms are not the only way to define recursive procedures. In particular, when we define a predicate that uses direct recursion on a given list, the definition is usually a little simpler if we use `and`- and `or`-expressions rather than `if`-expressions. For instance, consider a predicate `rgb-all-dark?` that takes a given list of colors and determines whether all of them are dark. As usual, we consider the cases of the empty list and non-empty lists separately:

• Since the empty list has no elements, it is (as mathematicians say) “vacuously true” that all of its elements are dark -- there is certainly no counterexample that one could use to refute the assertion. So `rgb-all-dark?` should return `#t` when given the empty list.
• For a non-empty list, we separate the car and the cdr. If the list is to count as “rgb-all dark”, the car must clearly be dark, and in addition the cdr must be an all-dark list. We can use a recursive call to determine whether the cdr is all dark, and we can combine the expressions that test the car and cdr conditions with `and` to make sure that they are both satisfied.

Thus, `rgb-all-dark?` should return `#t` when the given list either is empty or has a dark first element and all dark elements after that. This yields the following definition:

```;;; Procedure:
;;;   rgb-all-dark?
;;; Parameters:
;;;   colors, a list of rgb colors.
;;; Purpose:
;;;   Determine whether all of the elements of a list of colors
;;;   represent dark colors.
;;; Produces:
;;;   all-dark?, a Boolean.
;;; Preconditions:
;;;   All the values in the list are rgb colors.
;;; Postconditions:
;;;   all-dark? is #t if all of the elements of values are dark.
;;;   all-dark? is #f if at least one element is not dark.
(define rgb-all-dark?
(lambda (colors)
(or (null? colors)
(and (rgb-dark? (car colors))
(rgb-all-dark? (cdr colors))))))
```

When `colors` is the empty list, `rgb-all-dark?` applies the first test in the `or`-expression, finds that it succeeds, and stops, returning `#t`. In any other case, the first test fails, so `rgb-all-dark?` proceeds to evaluate the first test in the `and`-expression. If the first element of `colors` is not dark, the test fails, so `rgb-all-dark?` stops, returning `#f`. However, if the first element of `colors` is dark, the test succeeds, so `rgb-all-dark?` goes on to the recursive procedure call, which checks whether all of the remaining elements are dark, and returns the result of this recursive call, however it turns out.

Here's a template for that solution.

```(define all-____?
(lambda (lst)
(or (null? lst)
(and (____? (car lst))
(all-____? (cdr lst))))))
```

We can use a similar technique to ask if any value in a list is dark. In this case, if there are no values in the list, we know that no values are dark. Otherwise, we check if the first value is dark. If it is, then some value must be dark.

The complicated part is getting the base case right (particularly if we want to avoid using `if`). The standard technique is to require that the list not be null (using `not` and `and`). If the list is null, `(not (null? lst))` returns `#f`. And, since `(and #f ...)` is `#f`, we get false back for the empty list, just as we wanted.

```;;; Procedure:
;;;   rgb-any-dark?
;;; Parameters:
;;;   colors, a list of rgb colors.
;;; Purpose:
;;;   Determine whether any of the elements of a list of colors
;;;   represent dark colors.
;;; Produces:
;;;   any-dark?, a Boolean.
;;; Preconditions:
;;;   All the values in the list are rgb colors.
;;; Postconditions:
;;;   any-dark? is #t if at least one of the elements of values are dark.
;;;   any-dark? is #f if all of the elements are not dark.
(define rgb-any-dark?
(lambda (colors)
(and (not (null? colors))
(or (rgb-dark? (car colors))
(rgb-any-dark? (cdr colors))))))
```

And, once again, we can generalize.

```(define any-____?
(lambda (lst)
(and (not (null? lst))
(or (____? (car lst))
(any-____? (cdr lst))))))
```

Copyright (c) 2007-9 Janet Davis, Matthew Kluber, Samuel A. Rebelsky, and Jerod Weinman. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)

This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit `http://creativecommons.org/licenses/by-nc/2.5/` or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.