--- title: Eboard 21 Other forms of list recursion number: 21 section: eboards held: 2018-03-09 link: true --- CSC 151.02, 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. * Hi, I'm SamR * 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.) ### Upcoming work * Flash Cards due Wednesday. * Lab writeup for class 21: Exercise 5. Due before class Monday. (Or whenever TK has things do.) * Reading for Monday * Numeric Recursion * Assignment 6 due Tuesday. * Preliminary reports suggest that TK and I succeeded in making a shorter assignment. * I've rearranged the problems. TK will do so soon, too. Check the problem numbers before submitting. * Evidence suggests that problem 6 or 7 (permutations) is the most difficult. ### Extra credit (Academic/Artistic) * 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. * More info wherever TK keeps the info online. ### Extra credit (Peer) * More info wherever TK keeps the info online. ### Extra credit (Misc) * More info wherever TK keeps the info online. ### 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. * Men's Tennis, Sunday at 9am and 2pm in the Field House. * Host one or more prospective students. ### Friday PSA * Take care of yourselves. * Moderation in all things, even CS homework. * Consent is absolutely, positively, necessary. ### Questions Quiz ---- * Have fun! * If you finish early, take a few minutes to sit quietly and medidate. You have too few options to do so in your lives. What we hope you took from the reading -------------------------------------- In many cases, you will find that when you write recursive procedures, you're solving similar procedures to those you solved before. For example, `product` looks a lot like `sum`. To make yourself a more efficient programmer, identify patterns and keep them somewhere. Example: Sometimes we want to look through a list of values and make sure that all of them meet some predicate. I want to write `all-number?` in order to check the preconditions for `sum` here's a pattern for "everything in a list needs to meet the same predicate" ``` (define all-PRED? (lambda (lst) (or (null? lst) (and (PRED? (car lst)) (all-PRED? (cdr lst)))))) ``` To write `all-number?`, I just copy, paste, and change. ``` (define all-number? (lambda (lst) (or (null? lst) (and (number? (car lst)) (all-number? (cdr lst)))))) ``` Our goal for today is to get you thinking about some of the patterns you use (and, we hope, recording them somewhere). _You just said that copy-paste-change is a bad idea. Why?_ * Some patterns will be wrong. * But you may not realize that until you've copied and pasted and changed some number of times. You'll have to go back and fix a lot of things. * A better strategy is to have the computer do the work of copy/paste/change * You're going to learn how to do that after break. * But you've already been doing it a bit. Lab --- _Is it okay if `smallest` returns an inexact number when the smallest is exact, but the list contains a larger inexact number?_ > Yes. _What is our writeup?_ > Please writeup exercise 5. Debrief ------- ### Determining if a list has exactly one element. * Do *not* use `(= (length lst) 1)` to determine if a list has one element. * Instead, use `(null? (cdr lst))`. * Alternately, use `(and (not (null? lst)) (null? (cdr lst)))` ### Choosing return values * Think about the *type* your procedure returns. * The value you get in your base case should be the same type. * E.g., since `smallest` returns a real number, `null` is a bad thing to return in the base case.