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Lecture 19—Tail Recursion 1

Before Lecture

Before this lecture you should:

• Do an especially careful review of the two problems from lecture 18.
• Before this lecture, please watch all the Tail Recursion videos and the Worklist Accumulators part 1 video.

During Lecture

The starters for this lecture are:

• m10-reverse-starter.rkt
• m10-tr-trees-starter.rkt

After Lecture

Tail recursion is different. We arrange for the recursion to just keep calling, calling, calling until it finds a result. Notice that in the sequence? example we did in lecture 18, we could easily have made that code tail recursive, by writing the recusive call using if instead of and:

    
       (if (= (first lon) (+ 1 prev)) ;exploit (use)
           (sequence? (rest lon)
                      (first lon))
           false)

That puts the recursive call to sequence? in tail position and makes the function tail recursive. So tail recursive functions on lists are fairly straightforward.

On trees on the other hand, tail recursion requires that we introduce a worklist, because when we go down one branch of a tree there are other branches that we need to come back to later.

Be sure to finish the m10-tr-trees-starter.rkt file as soon as you can. That will help you lock in how to work with tail recursive tree templates, just like you once locked in how to work with recursive list templates.

The solutions for this lecture are:

• m10-reverse-solution.rkt
• m10-tr-trees-solution.rkt

Be sure to complete the before lecture section of Lecture 20 before that lecture.