Held: Monday, 14 September 2015

Back to Outline 09 - Writing Your Own Procedures, Continued. On to Outline 11 - Documenting Programs and Procedures.

Summary

We consider sipmle textual interactive programs, programs that read input from the user and produce output.

Related Pages

• Reading: Simple Output in Scheme
• Reading: Simple Input in Scheme
• Lab: Input, Output, and Side Effects
• EBoard 10 (Source 10)

Overview

• Simple output in Scheme.
• Using output to trace program behavior.
• Reading lines of text with read-line.
• Reading Scheme values with read.
• Side effects.

Administrivia

• New partners!
  • Grab a card from the stack.
  • Identify the location of the new computer.
  • Drop the card in the jar.
  • When both of you are at the computer, introduce yourselves.
• Review sessions:
  • Wednesday, 8pm (mentor)
  • Thursday, 10am (Sam)
  • Thursday, 8pm (mentor)
• If you have questions you'd like me to answer, you can write them on blank index cards and hand them to the mentors. I'll send out answers tomorrow.

Upcoming Work

• Read for Tuesday: Documenting Procedures
• Assignment 3 due Tuesday @ 10:30 p.m.
  • Your partner is the person who you worked with Wednesday and Friday.
• Exam 1 to be distributed Wednesday.
• Lab writeup (due before class Wednesday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 10 (Your Names)
  • Exercise:

Extra Credit Opportunities

Academic

• Tuesday, noon, CS Table, JRC 224C. Hacking Cars.
• CS Extras, Wednesday, Jonathan Wellons '04 from Google.
• Convo Thursday at 11 am in JRC 101: Mike Latham

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 1:30 and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker.
• Singers concert on Family weekend! Sept. 26, 2pm., probably JRC 101
• Orchestra concert on Family weekend! Sept. 26, 2pm., probably Sebring-Lewis

Quiz 2 Returned

• Generally good. Congratulations!

Problem 1

We expected something like the following.

(define cube (section expt <> 3))

Some of you tried to use *, but each <> is a separate input.

Problem 2a

(define fave (irgb 250 0 42))
(irgb-complement fave)
> (irgb->string fave)
"250/0/42"

This problem was to remind you that irgb-complement does not change fave. Rather, it computes a new color. That's similar to

(define num 5)
(square num)
> num
5

The behavior of irgb-complement and square contrasts a bit with the kinds of procedures we're playing with today.

Problem 2b

(define fave (irgb 250 0 42))
(define fave2 (irgb-darker (irgb-lighter fave)))
> (irgb->string fave2)
"239/0/42"

An important point from the labs was that irgb-lighter and irgb-darker cap when you reach the extreme values (0 and 255). So the red component of (irgb-lighter fave) is 255. When you subtract 16, we end up with 239.

Output

• Use display to print values.
• Use newline to print a newline.

Using Output to Trace Program Behavior

• By inserting appropriate calls to display and newline, we can better see what our program does.

Input

• Read a line of text with read-line.
• Read the next Scheme value with read.
• Use these to write interactive programs.
• We'll revisit these again in the future as we learn more copmlicated forms of Scheme expressions.

Side Effects

• All of these procedures are procedures with side effects - in addition to returning a value (assuming they return a value) they also change the state of the environment.
• Procedures with side effects tend to complicate our understanding of programs and can sometimes lead to unexpected behavior.