--- title: Eboard 21 Other forms of list recursion number: 21 section: eboards held: 2018-03-09 link: true --- CSC 151.01, Class 21: Other forms of list recursion ==================================================== _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Friday PSA * Questions * Quiz * Lab * Debrief Preliminaries ------------- ### News / Etc. * New partners! * Don't forget that we "Spring Forward" on Sunday morning. (And sorry, I know that means that you'll be extra tired on Monday a.m.) * I appreciate that those of you who are ill are missing class so as to avoid infecting others. Please remember that you owe me an email message in cases like that. ### Upcoming work * [Lab writeup for class 21](../writeups/writeup21): Exercise 5. Due before class Monday. * Reading for Monday * [Numeric Recursion](../readings/numeric-recursion) * [Assignment 6](../assignments/assignment06) due Tuesday. * Preliminary reports suggest that TK and I succeeded in making a shorter assignment. * I've rearranged the problems. Check the problem numbers before submitting. (The one group that submitted can ignore this comment; I'll let the graders know.) * Evidence suggests that problem 7 (permutations) is the most difficult. * If you weren't here on Wednesday, you and your partner have until the end of class today to tell me when you are working on the assignment. * [Flash Cards](../flashcards/flashcards08) due Wednesday. ### Extra credit (Academic/Artistic) * Visit the two exhibits at the Faulconer Gallery. * CS Extra Monday at 4:15 p.m. in 1023: "An Introduction to the Automatic Extraction of Keyphrases". (Snacks at 4pm.) * CS Table Tuesday at noon: Unknown topic. ### Extra credit (Peer) * Play this weekend. Today through Sunday. See the campus memo or elsewhere for details. * Men's Tennis, Sunday at 9am (Central) and 2pm (Augustana) in the Field House. ### Extra credit (Recurring peer) * Listen to KDIC Wednesdays at 6pm - Witty banter with other personalities and/or co-host. Also Indian, Arabic, and Farsi music. (Up to two units of extra credit.) * Peer editing with SS. Talk to SS about the details. Make your English Lit more literate. * Preliminary reports suggest that these appointments are really helpful. ### Extra credit (Misc) * Host one or more prospective students. ### Other good things * GHS presents "The Little Mermaid" Friday, Saturday. * Singers concert, Sunday, 4 p.m., Plymouth United Church of Christ, 4126 Ingersoll Ave., Des Moines. ### Friday PSA * Take care of yourself. * Even though I make jokes about these issues, I'm very serious. * Consent is even more serious (and necessary). ### Questions _How do I decide whether to use direct recursion or helper recursion?_ > You are new to this. And even experienced programmers have difficulty choosing. So the model will generally be "try one approach; if you don't make forward progress, try another." > Sometimes different ways of thinking about the problem. If a "table" of steps makes sense, helper/tail recursion is probably the way to go. _Why use `and` and `or` instead of `if` or `cond`?_ > To feel superior to those who are unable to understand the intracacies (which sam can't spell) of `and` and `or`. > Sometimes it leads to shorter code. > Sometimes it makes more logical sense / corresponds to the English. > Because you're returning #t or #f and it makes you uncomfortable to do so explicitly. (Sam thinks code that explicilty says `#t` or `#f` is generally less elegant.) _Can you do everything with `and` and `or` (and `not`) that you can do with a conditional?_ > I think so. _Are there uncommon forms of recursive procedures?_ > Certainly. > There is a huge universe of possible problems. Some match common patterns. Some do not. > You'll also learn some new common patterns in the coming weeks. _Why does reduce not work on the empty list?_ > If reduce is supposed to repeatedly combine values until you have a single value. It's hard to get one value from nothing. Hence, reduce expects at least one value. _What's the difference between using helpers and not using helpers?_ > Helpers traditionally carry along an extra parameter. In most cases, that lets you build a nice table to explain to yourself what it's doing. _What's the difference between tail recursion and non-tail recursion?_ > Tail recursion: You do *nothing* with the result of the recursive call (other than give it back). > Non-tail recursion: You do something with ther esult of the recursive call (e.g., add to it) > Non-tail recursion requires the computer to build up a list of incomplete computations, which takes additional effort. So non-tail recursion can be less efficient. Quiz ---- * The most fun you'll have all weekend! * Nonetheless, take quizzes in moderation. Lab --- _Is it okay if `smallest` ends up giving us an inexact number even when the smallest number in the list is exact?_ > Yes. _Couldn't we have written `lists-join` with an empty-list base case?_ > Um ... let me think ... possibly. ``` (define lists-join (lambda (lst) (if (null? lst) null (append (car lst) (lists-join (cdr lst)))))) ``` _Is the last problem the same as problem 2 on the homework?_ > Yes. **Writeup: Problem 5** Debrief ------- Our goal is to build a collection of things like the following: ``` (define REDUCE-PROC (lambda (lst) (if (null? (cdr lst)) (car lst) (PROC (car lst) (REDUCE-PROC (cdr lst)))))) ``` Once we have these, our work often becomes more something like "Determine if an appropriate pattern exists. If so, copy that pattern and change a few things." Note to SamR: Go over how students might have solved problem 5 in class on Monday.