--- title: Eboard 32 Deep recursion number: 32 section: eboards held: 2019-04-22 link: true --- CSC 151 2019S, Class (expt 2 5): Deep recursion ================================================ _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Questions * Quick preview * Lab Preliminaries ------------- ### News / Etc. * Mentor sessions Thursday 7-8 p.m., Thursday 8-9 p.m., Sunday 5-6 p.m. * Note change in schedule. * Folks should think about taking CSC 161 in the fall. It's a great class, and you get to work with robots! * Sorry that I remain a bit behind on way too much. I'm working on it. ### Upcoming work * [Project proposal](../project) due tonight! * [Rubric](../project/rubric.txt) available for your perusal. * Friday's quiz: Higher-order procedures, algorithm analysis, trees * Reading for Wednesday: [Algorithms for searching lists and vectors](../readings/searching) * And yes, it's ready. * Flash cards Wednesday: Higher-order procedures, algorithm analysis, trees ### Extra Credit _I would certainly appreciate suggestions of other extra credit activities (preferably via email)._ #### Extra credit (Academic/Artistic) * **New**: CS Table, Tomorrow, "The Cathedral and the Bazaar" * **New**: "Plan your CS major with WGMC", tomorrow at 7pm. All genders welcome. * Dartanyan Brown discussion, 4pm Wednesday April 24, HSSC S3325 * Dartanyan Brown concert, 7:30 pm Wednesday April 24, Sebring-Lewis * PBK Convo, Thursday, 11am: "Antievolutionism in Historical Perspective" * **New**: McKibben lecture, Thursday, 4:15 p.m., JRC 101 #### Extra credit (Peer) * **New**: Track and Field at Grand View on Friday. #### Extra credit (Wellness) #### Extra credit (Wellness, Regular) * **Today** 30 Minutes of Mindfulness at SHACS (SHAW) every Monday 4:15-4:45 * Any organized exercise. (See previous eboards for a list.) * 60 minutes of some solitary self-care activities that are unrelated to academics or work. Your email reflection must explain how the activity contributed to your wellness. * 60 minutes of some shared self-care activity with friends. Your email reflection must explain how the activity contributed to your wellness. #### Extra credit (Misc) * **Tonight** Public speaking workshop - April 22 at 7pm in HSSC S3325, with Kathy Clemons-Beasley '05. ### Other good things ### Questions Someone posted Sam's dining hall price rant. What's that about? > I used to write an essay ('blog post) every day. > I'm a Grinnell parent and a curmudgeon. > I did the math. Dining plans were more expensive than buying individual meals. > > But it appears they were discussing a different rant, one about the "cash value" of a dining hall meal. There are too many for me too keep track of. How do I extract text from a directory? > The files have a pattern. Sam can also work on writing some helper code. > The `(directory-list dir)` procedure produces a list of "paths". (Something like filenames, but not as strings.) > The `(path->string path)` procedure converts a path to a string. > We can filter out the ones of interest. > We can use `file->string` with `map` to extract the contents of each. > We can join them together with `apply` and `string-append`. ```drracket ;;; Procedure: ;;; sandb-files ;;; Parameters: ;;; pattern, a regular expression ;;; Purpose: ;;; Make a list of all the SandB files that match the particular expression. ;;; Produces: ;;; fnames, a list of strings. (define sandb-files (lambda (pattern) (map (section string-append sandb <>) (filter (section regexp-match? pattern <>) (map path->string (directory-list sandb)))))) ;;; Procedure: ;;; sandb-text ;;; Parameters: ;;; pattern, a regular expression ;;; Purpose: ;;; Extract the text from all of the files that match the pattern. ;;; Produces: ;;; text, a string (define sandb-text (lambda (pattern) (apply string-append (map file->string (sandb-files pattern))))) ``` Quick preview ------------- * When I'm doing something with lists, what if I also want to look at the sublists? * E.g., tallying strings. ```drracket (define tally-strings (lambda (lst) (if (null? lst) 0 (if (string? (car lst)) (+ 1 (tally-strings (cdr lst))) (tally-strings (cdr lst)))))) ``` ```drracket > (tally-strings '("a" 2 3 "b" ("d" "e"))) 2 ``` Question: Can we make it count the "d" and the "e"? ```drracket (define deep-tally-strings (lambda (lst) (if (null? lst) 0 (if (string? (car lst)) (+ 1 (deep-tally-strings (cdr lst))) (if (list? (car lst)) (+ (deep-tally-strings (car lst)) (deep-tally-strings (cdr lst))) (deep-tally-strings (cdr lst))))))) ``` Key idea: If the first element of the list is also a list, recurse on both the car and the cdr. Problems with code above: * Nested ifs. Consider using cond. * Three calls to `(deep-tally-strings (cdr lst))`. Makes code harder to read. (Fortunately, only one will be executed.) ```drracket (define deep-tally-strings (lambda (lst) (if (null? lst) 0 (let ([tally-of-rest (deep-tally-strings (cdr lst))]) (cond [(string? (car lst)) (+ 1 tally-of-rest)] [(list? (car lst)) (+ (deep-tally-strings (car lst)) tally-of-rest)] [else tally-of-rest]))))) (define deep-tally-strings (lambda (lst) (if (null? lst) 0 (+ (cond [(string? (car lst)) 1] [(list? (car lst)) (deep-tally-strings (car lst))] [else 0]) (deep-tally-strings (cdr lst)))))) ``` Note: Deep recursion does not generally work well with tail recursion (so-far and remaining). Use direct recursion. Lab --- For `deep-contains?`, assume that the value is not a list or pair. For deep recursion, don't use a helper with so-far and remaining. (It makes things harder.) For `deep-alphabetically-first`, you can assume that none of the lists are empty. For exercise 6, you may find it easier to allow deep-reverse to act on non-lists. Writeup exercise 5.