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Lecture 18—Accumulators

Before Lecture

There is no pre-work for this lecture. Use this time to review the concepts we have covered previously.

During Lecture

The starters for this lecture are:

• m10-sequencep-starter.rkt
• m10-bstp-starter.rkt

After Lecture

Today we introduced accumulators. Accumulators are used in recursive functions to accumulate information from prior recursive calls. While all accumulators have this basic functionality, it is also useful to think of accumulators as falling into one of three categories:

• Accumulators that directly accumulate the result so far (RSF). In these accumulators the base (or trivial) case of the recursion typically produces the value of the accumulator directly.
• Accumulators that preserve important context from prior recursive calls. These accumulators differ from RSF accumulators in that while they form an important part of the function, and can contribute to the result, they are typically not the direct result of the function.
• (We will see these later.) Accumulators that maintain a worklist of elements that remain to be visited in a tree or graph, or that maintain a tandem worklist of accumulator values to restore when the corresponding element is visited.

In lecture we did one example of using a single accumulator when traversing a list and one of using two accumulators while traversing a tree.

As always, review the lecture by practicing what we did in class. You can also use the videos from the Accumulators module up to but not including Tail Recursion.

The solutions for this lecture are:

• m10-sequencep-solution.rkt
• m10-bstp-solution.rkt

Be sure to complete the before lecture section of Lecture 19 before that lecture.