Notes and hints on Programming Project 3

Corrections and additions to the original assignment handed out in class:

• (2) count is a builtin function. Some versions of common lisp won't let you redefine this. Ours will, but it may cause further problems. To get around this, call the function count1 , as done in the test data.
• (10) you should write both programs, one each for parts (a) and (b), but you only need to name one of them average and have it work.
• (12) should be (all_nths n lol) - the assignment left out the n.
• (14) the return value should be a list of the columns of the matrix, not rows, as stated (that would just give you the same lol as the input). Check the test data return values for more information.
• Functional Programming: The purpose of the lisp material in the course (and this assignment) is to teach functional programming and recursion. Common lisp actually has functions to set the values of variables and even a do loop. Don't use them! You can do everything you need using only function calls to helper functions if you need to update values to variables. In particular, points will be taken off for using setq, set or setf inside your programs. You also shoudl avoid using the built-in functions that calculate the same or similar fucntions as the ones described here (e.g., nth, count, length). You may, howeve ruse any of the recursive functions or primitives we discussed in class, such as member.
• Error Checking: in general you can assume for these programs that data will be of the correct type (positive integer, list, list of lists). You do have to check the base cases, though. For example, you always need to check to see if a list is empty before taking the car or cdr. For list of lists, this includes both nil (a list of zero lists), and something of the form (nil nil ... nil), e.g., a list of empty lists.

other notes

1. The original test data had a couple errors, make sure you get a copy after 3/11. In particular the return from
   (nth_item 2 '(a b c d)) should be B not BC
2. The comment symbol in lisp is ;
   lisp will ignore anything after a semicolon on a line.
3. To return the symbol *error* (or any symbol for that matter), use the quote special form, described in class: either '*error* or (quote *error*)
4. The emacs editor is very useful for interactively writing and debugging lisp programs:
   • For one thing, it will let you run lisp inside the editor (type to emacs M-x run-lisp ), where you can edit what you type in to lisp, and whenever you hit return, it will evaluate the s-expression just to the left of your cursor.
   • Even more useful: you can edit your definitions in one buffer (the file you will be saving), and you can send a single s-expression (e.g., a defun) to the lisp listener by typing M-C-x (that's escape followed by and x together). This saves you having to load in a whole file each time you make just one change.
   • Emacs has a built in tutorial and extensive on-line help.
   • here are some pointers to short emacs help locations:
     • Short emacs guide: terminal version
     • Short emacs guide: x version
     • Another list of commands
     • A tutorial with commands
     • A general intro with commands