CSC151.01 2014F, Class 19: Conditionals ======================================= * New Partners! _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions. * The parts of an algorithm, revisited. * Choosing between two options with `if`. * Optional evaluation with `when`. * Making multiple choices with `cond`. * Comparing approaches. * Lab. Preliminaries ------------- ### Admin * Epilogue for exam 1: <151-2014F-exam1epi> * I've been reflecting a bit on the "vocabulary" you need for our quizzes. On one hand, I think you do much better if you memorize what the procedures are and what they do. On the other, I know that such memorization can be quite difficult. Hence, as an experiment on this Friday's quiz, I will allow you to bring one page of notes to the quiz. (8.5x11 or A4, double sided, hand written) * Office hours (modified due to meetings): * Wednesday 8:00-9:45 * Thursday 8:00-9:45, 2:30-3:15 * Friday 1:45-3:15 (also Walking hours 1:15-1:45) * Review sessions 8pm Wednesday and Thursday. * Let me know by 5pm today if you want to work alone on the next HW. * I've had a student indicate that they will not be able to start HW 5 until late Sunday afternoon and that they want partners for the HW. * Please fill out HW 5 partners form: * To provide you with incentive to take your own notes on each topic we cover: If you send me a summary of what you think are the important takeaways from any reading/lab combination, I will comment on your summary. ### Upcoming Work * [HW 4](../assignments/assignment.04.html) due tonight. * HW 5 to be assigned tomorrow. * Lab writeup: Conditionals, 3c and 4d Due Friday! * Reading for Wednesday * [Anonymous Procedures](../readings/anonymous-procedures-reading.html) ### Fun Things with no Extra Credit Value * GHS Homecoming Parade Thursday at 5:30 pm. (Small town Iowa homecoming is a fascinating experience.) * Friends of Drake Library Book Sale, Friday night and Saturday. At about 11 am on Saturday, you can get a box of books for a buck. (Earlier, it's 50 cents for paperbacks and $1 for hardcovers.) ### Extra Credit Opportunities #### Academic * Campus Town Hall TODAY at noon or 7:30 p.m. * CS extras next Thursday: Alumni * CS table Friday: Serendipity and Computing (with Grinnell's Artist in Residence) * Grinnell Prize events the following week (particularly events related to the prize winners whose project involves Web tools) #### Peer Support * Wednesday night's [Concerts] * Anna Christie, Oct. 9-12 (SB plays Marthy) * Football (???) * Men's Tennis (???) * Women's Tennis (???) * Swimming (???) * Evan's radio show 5pm Friday nights on KDIC * Donna's radio show Sunday midnight on KDIC #### Miscellaneous ### Questions _What's the relationship between a Boolean and a conditional? Where does the Boolean part come in?_ > A Boolean is a truth value. True or False. > Scheme takes a slightly different view, False or "not False" "Not false" includes true, but also 45, "hello", "not false", drawing-unit-circle, ... > Procedures that assess some chacteristic of values are called predicatees (return true/false, or ??/false) The parts of an algorithm, revisited ------------------------------------ * We need to be able to write subroutines/procedures * Parameters * Instructions * `(define NAME lambda (PARAMETERS) BODY)` * We need to be able to name values ("Variables") * `(define NAME VALUE)` * We need to be able to sequence operations, implicitly or explicitly * Approximate: Nesting. Inner expression gets evaluated first. But if the outer expression has multiple inner expressions, they can be evaluated in any order. * Exact: In the body a procedure, if we have one expression, then another, then another, they are evaluated in sequence. (define image-quincunx! (lambda (image color) (image-set-pixel! image 0 0 color) (image-set-pixel! image (+ -1 (image-width image)) 0 color) * Clever use of lambda can also achieve some control of sequence * We need to be able to repeat operations * Fixed number of times * Until a goal is achieved * `map` * `image-transform!` or `image-variant` * We will learn more general approaches next week * We need a set of basic operations and values * Numbers and numeric operations * Images and their operations * Drawings and their operations * We need to be able to make choices `if`, `when`, and `cond` ------------------------ * See the start of lab. Comparing approaches -------------------- * Do you have an alernative? Use `if` or `cond`? If not, use `when` * Do you want to do more than one thing if a test holds? If so, use `when` or `cond`. * Do you have multiple tests to do in sequence? If so, use `cond` (or nested `if` statements). Lab --- Two ways to write `irgb-brighter?` (define irgb-brighter? (lambda (color1 color2) (if (> (rgb-brightness color1) (rgb-brightness color2)) #t #f))) vs. (define irgb-brighter? (lambda (color1 color2) (> (rgb-brightness color1) (rgb-brightness color2)))) Two ways to write `irgb-grey4` with `if`? (define irgb-grey4 (lambda (color) (if (< (irgb-brightness color) 0.25) grey1 (if (and (< (irgb-brightness color) 0.5) (>= (irgb-brightness color) 0.25)) grey2 ...)))) vs. (define irgb-grey4 (lambda (color) (if (< (irgb-brightness color) 0.25) grey1 (if (< (irgb-brightness color) 0.5) grey2 ...))))