CSC151.02 2014S, Class 34: Iteration ==================================== _Overview_ * Preliminaries. * Admin. * Questions. * Reflecting on procedures and side-effects. * A useful tool for repetition: `for-each`. * Contrasting `map`, `for-each`, `repeat`, and recursion. * Lab. Preliminaries ------------- ### Upcoming Work * Homework for Wednesday: [HW 7: GitHub](../assignments/assignment.07.html) * Today's lab writeup: 4c and 4d. * Reading for tomorrow: Geometric Art through Numeric Recursion * HW 8 is also posted. ### Admin * If our Web server is not working correctly, you should be able to get to today's lab at the URL * Continue your partner assignments from Monday. If you missed class on Monday, work with someone else who missed class on Monday. * CS T shirts soon to be available. Vote if you plan to buy one by sending email to sepc-cs@grinnell.edu. The list of possibilities is at . * You should encourage the SEPC to make a "Curried Lamb-Duh" T-shirt * Extra credit: * Convocation, Wednesday, noon: Elizabeth Kolbert (the Sixth Extinction) * Technology in liberal arts education, lots of talks on Thursday * No CS extra this Thursday. * CS table Friday: The API Performance Contract Robert F. Sproull and Jim Waldo. 2014. The API performance contract. Commun. ACM 57, 3 (March 2014), 45-51. DOI=10.1145/2576792 http://doi.acm.org/10.1145/2576792 http://delivery.acm.org/10.1145/2580000/2576792/p45-sproull.pdf ### Questions Reflecting on procedures and side-effects ----------------------------------------- * A transition in how we think about procedures (not necessarily positive) * It used to be, we called procedures to return a result * And nothing else * Now, procedures have "side effects" - they change "the world" (or themselves, or an image) * When you're doing `(+ (square 5) (square 7))`, it doesn't matter which order we do the square * If you do (turtle-forward! t 10) (turtle-turn! t 10) * Order of operations is much more important! * We didn't tell you, but map does not necessarily do the transformations in order from left to right * (map square (list 1 2 5 7 9)) * map is not a good tool for side-effeting operations, since order matters What is a procedure with a side-effect? * One view: The procedure changes something external (e.g., an image, the implicit position of the turtle, ...) * Another view: We are not assured of getting the same behavior each time we call the procedure on the same parameters Examples * Side effects: turtle-forward!, image-select-ellipse!, ... * No side effects (pure): square, increment, iota, rgb-redder, irgb-new, ... * Badly named: image-show A useful tool for repetition: `for-each` ---------------------------------------- * When you want to apply the same function to one thing and then another and then another (as in map) * But you care about the order * To different values * and we do this more for the side effects than the resulting list * USE for-each (for-each PROCEDURE LIST-OF-PARAMETERS) Contrasting `map`, `for-each`, `repeat`, and recursion ----------------------------------------------------- * Are we doing the work with side effects or not? * With side effects: for-each, repeat, recursion * Without: map, recursion * What does our result look like? * A list of transformed values: map * Nothing: for-each, repeat * Variable: recursion * Do we something different at each step? * No: repeat, (recursion) * Yes: for-each, map, recursion Lab ---