CSC 151.01, Class 29: Files in Scheme

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Review of hop
• Lab
• Debrief

News / Etc.

• Sorry about the Website weirdness on Friday. The style sheet that I rely on changed significantly.
• Sorry that I could not be in class on Friday.
• No, grading is not done. Thursday, Friday, Saturday, and half of Sunday were booked. I spent six hours grading, but that’s not enough.
  • Tonight is “Write 301 exam and write 151 project”, so no grading will get done.
• http://pioneers.grinnell.edu/news/2017/11/3/womens-volleyball-kuhn-named-mwc-volleyball-newcomer-of-the-year.aspx
• It’s preregistration week. I encourage you to consider CSC 161. (Weinman and Walker are both responsible graders.)

Upcoming Work

• Writeup for class 28 due Monday at 10:30 p.m.
  • Exercise 5. (No documentation necessary, but appreciated.)
  • To: csc151-01-grader@grinnell.edu
  • Subject: CSC 151.01 Writeup 28 (YOUR NAMES)
• Writeup for class 29 due Wednesday at 10:30 p.m.
  • Exercise 6. No documention necessary.
  • To: csc151-01-grader@grinnell.edu
  • Subject: CSC 151.01 Writeup 29 (YOUR NAMES)
• Read Analyzing algorithms for Wednesday’s class.
  • No, it’s not ready.
• Exam 3 distributed.
  • Prologue due Friday.
  • Exam due Tuesday the 14th.
  • Cover pages due Wednesday the 15th.
  • Epilogues due Wednesday the 15th.

Extra credit (Academic/Artistic)

• Leyla McCalla Trio, Nov. 6, 7:30 p.m. Herrick
• Animated Films, Tuesday, Nov. 7, 11:00 a.m., Faulconer
• CS Table (Computer-Aided Gerrymandering), Tuesday, Nov. 7, noon, Day Dining Room
• Crip Technoscience, Disabled People as Makers and Knowers, Wednesday, Nov. 8, 4:15 p.m., JRC 101.

Extra credit (Peer)

Extra credit (Misc)

• Pioneer Weekend, this coming weekend. Design a business idea (can be a nonprofit / for social good) have fun, get advice, and make money.
  • Sign up by Nov. 8.

Other good things

Questions

Review of higher-order programming

What did we learn on Friday?

• Ways of combining procedures.
• l-s, that procedure that E hates.
  • (l-s proc val) == (section proc val <>)
  • Note: We can write l-s
• We learned apply, which is kind of like map and kind of like reduce, but different.
• We can write procedures that take another procedure as a parameter.
  • Procedure parameters look like normal parameters.
  • We just use them in the “context” of procedures. If f is a parameter, we can write (f 1) or (map f (iota 5) or ….
• We can write procedures that return another proceudre as a result.
  • Most typically, use a second lambda to define/return the new procedure.

Lab

Why am I able to read a file in your account?

I set permissions that way. (Linux lets you set permissions on files in your account so that others can read them. There are a variety of options. You’ll learn how to do so in CSC 161.)

How do I “create a file in the interactions pane”?

(define outport (open-output-file "/home/username/Desktop/myinfo"))

How do I write to that file?

(write stuff outport)

(display stuff outport)

(newline outport)

What should I do when I’m done?

(close-output-port outport)

How should I overwrite an existing file

(define outport (open-output-file "/home/username/Desktop/myinfo" #:exists 'replace))

«««< Updated upstream Writeup: Exercise 6. Reload the lab before doing it. ======= Writeup: Exercise 6

Stashed changes

Debrief

How did you figure out that the sum was correct?

(apply + (map string->number (file->words FILE)))

Open the file in DrRacket. Put (+ at the start. Put ‘)’ at the end. Evaluate.

Open the file in a spreadsheet and use the spreadsheet sum.

An incomplete definition of file-size.

(define file size
  (lambda (file-name)
    (let ([source (open-input-file file-name)])
      (let kernel ([size-so-far 0])
        (let ([nextval (read-char source)])
          (cond
            [(eof-object? nextval) 
             (close-input-port source) 
             size-so-far]
            [(char? nextval) 
             (kernel (+ nextval size-so-far))]
            [else 
             (kernel size-so-far)]))))))

Problem: nextval can be the eof object or a character, and nothing else. So …

(define file size
  (lambda (file-name)
    (let ([source (open-input-file file-name)])
      (let kernel ([size-so-far 0])
        (let ([nextval (read-char source)])
          (cond
            [(eof-object? nextval) 
             (close-input-port source) 
             size-so-far]
            [(char? nextval) 
             (kernel (+ nextval size-so-far))]
            [else 
             (kernel size-so-far)]))))))

Note: By default, you can only read from existing files and write to new files.

(define do-with-all
  (lambda (proc lst)
    (when (not (null? lst))
      (proc (car lst))
      (do-with-all proc (cdr lst)))))