CSC 160
Homework 9

Assigned 17 Nov, due 3 Dec

Links to textbooks

• HtDP Chapter 9
• HtDP Chapter 10

Problems on returning lists

• Developa function stutter that takes in a list of objects and returns a list twice as long with each object appearing twice in a row. For example,

  (check-expect (stutter (cons 3 (cons 8 (cons -2 empty)))) 
      (cons 3 (cons 3 (cons 8 (cons 8 (cons -2 (cons -2 empty)))))))

• Develop a function remove-all that takes a string and a list of strings and returns the same list with all occurrences of the given string removed.
  Example:

  (check-expect
      (remove-all "bat" (list "doll" "bat" "bat" "nintendo" "bat" "ball"))
      (list "doll" "nintendo" "ball"))
  

• Develop a function replace that takes two strings (referred to as old and new) and a list of strings, and returns a list of strings with old replaced throughout by new.
  Example:

  (check-expect
      (replace "bat" "ball" (list "doll" "bat" "bat" "nintendo" "bat" "ball"))
      (list "doll" "ball" "ball" "nintendo" "ball" "ball"))
  

• Develop a function named add-vote-for which takes in a string (the name of a candidate) and a list of candidate structures, and returns a list of candidate structures in which that candidate has one more vote (and all the others are unchanged). You may assume that no name appears more than once in the list of candidate structures.
  Note: it's possible that the name doesn't appear in the list at all (e.g. it's a write-in vote, or something like that); if so, the resulting list should have 1 vote for that candidate.

• Develop a function named tally-votes which takes in a list of strings (Voter 1's favorite candidate, Voter 2's favorite candidate, etc.) and produces a list of candidate structures in which each candidate name appears once, with how many votes were cast for that candidate.

• Write an animation of a bunch of balls, each moving around the screen with constant velocity until they bounce off a wall. Pressing the + key should add one more ball, with random initial location (inside the animation window) and random velocity (say, from -10 to +10 in each dimension). Pressing the - key should remove the most-recently-added ball (unless there are none, in which case it should do nothing). Clicking with the mouse inside a ball should remove the ball you clicked on, but leave all the rest of the balls unchanged.

  Hint: you'll need each ball to have a location and a velocity, as in exercise 16.8.4 from homework 7, and you'll need a list of them, as in the animation from homework 8.

• Extra credit: develop a function named sort-candidates which takes in a list of candidate structs and produces a list of the same candidate structs, but with the candidates sorted in decreasing order of number of votes (so the one with the most votes is first in the list, and the one with the fewest votes is last).

• Extra credit: develop a function named permutations which takes in a list of things (strings, or symbols, or numbers, or characters... it doesn't matter) and produces a list of all the possible "scramblings" of that list. For example,

  (check-expect (permutations (list 'a 'b 'c))
    (list (list 'a 'b 'c) (list 'a 'c 'b) (list 'b 'a 'c) (list 'b 'c 'a) (list 'c 'a 'b) (list 'c 'b 'a)))
  

  (Your function may return the answers in a different order, as long as it returns a list of all the possible answers, once each.)

What to turn in and how

All of the problems are programming problems. There is one animation; deal with it as you did in the previous few assignments.

Also turn in a log of how many errors of different kinds you encountered in the assignment, with brief comments describing each one ("mismatched parentheses" is self-explanatory, but more complex errors might need more description). Note that "errors" means not only error messages from DrScheme, but also wrong answers. You may do this using the PSP forms, or simply by keeping track in a text file or on paper and turning it in.

Grading standards

Error log:       /30
(I'm not grading on how many or how few errors you encountered, only on whether you recorded them correctly.)

I sha'n't actually grade all the problems; consider the others "practice". For each function I grade, see the table below, which has columns for each step in the design recipe. You won't turn in a separate skeleton, inventory, and definition, but rather write a skeleton, then add an inventory, then add a body to turn it into a complete definition. However, if you don't get the definition working, comment it out and you'll still get partial credit for a correct skeleton and/or inventory.

General skills:

Total:         /???