# CS 231 Assignment 3 Key

CS 231 Programming Assignments

Below is a key for assignment 3.
;
; This file provides some simplification of the expressions produced by
; the derivative system.
;

; ** Main simplifier **
; This just calls a single simplifier for each of sum and product, but
; sends some parameters to adapt it.  It also does an extra step for
; products which changes products containing 0 to 0.
(defun simplify (E)
(cond
((is-sum E) (simpl '+ 'collapse-dup-terms 'is-sum
(lambda (x) (cons '+ x)) 0 E))
((is-product E)
(kill-zero (simpl '* 'identity 'is-product
(lambda (x) (cons '* x)) 1 E)))
(t E)
)
)

; ** Zero killer
; If the argument is a product and contains zero, return zero.  Else
; return the argument.
(defun kill-zero (E)
(cond
((not (is-product E)) E)
((null (remove-if (lambda (x) (not (equal 0 x))) E)) E)
(t 0)
)
)

; ** General simplifier
; This works for either sums or products.  It takes the function which
; identifies sub-expressions of the same type, the function which adds the
; correct operator symbol to the front of the list, the identity for the
; operation, and the expression.  It gets the parts (terms or factors),
; simplifies each one, flattens the list (which removes sublists of
; the same type, as (+ 5 (+ 6 7)) to + (+ 5 6 7)), removes the identity
; (which is 0 or 1), then does some cleanup and adds the operator symbol
; back to the list.
(defun simpl (the-operator merge-op is-same-type add-operator identity E)
(let*
(
(parts (cdr E))
(simparts (mapcar 'simplify parts))
(flatparts (flatten is-same-type (funcall merge-op simparts)))
(combined (const-combine the-operator identity flatparts))
)
)
)

; This combines the constants in the list of items (terms or factors),
; moves the combined constant to the front, and omits it if it is the identity.
(defun const-combine (the-operator identity items)
(let*
(
(nonconst (remove-if 'is-constant items))
(constval
(reduce the-operator
(remove-if (lambda (x) (not (is-constant x))) items)
)
)
)
(if  (not (equal identity constval))
(cons constval nonconst)
nonconst
)
)
)

; This function collects all itmes similar to the first into a (* ) sublist.
; If there are no items similar to the first, the list is unchagned.
(defun collect-like-first (items)
(let*
(
(tailitems (cdr items))
(we-like (lambda (x) (equal x headitem)))
(we-dont-like (lambda (x) (not (equal x headitem))))
(like-these (remove-if we-dont-like tailitems))
(dont-like-these (remove-if we-like tailitems))
)
(if (null like-these)
items
(cons
(cons '*
(const-combine '* 1
(flatten 'is-product
(list (+ 1 (length like-these)) headitem)
)
)
)
dont-like-these
)
)
)
)

;
; This function looks in a sum for repeated terms, and changes them into
; a multiplication by a constant.  It ignores constants, as they will be
; taken care of by const-combine.
(defun collapse-dup-terms (terms)
(cond
((null terms) ())
((is-constant (car terms))
(cons (car terms) (collapse-dup-terms (cdr terms)))
)
(t
(let*
((collected (collect-like-first terms)))
(cons (car collected) (collapse-dup-terms (cdr collected)))
)
)
)
)

; Here is the code for flatten.  It takes sublists of the same type and
; merges them into the whole list, as (* (* a b 2) 3 (* 5 2)) to
; (* a b 2 3 5 2)
(defun flatten (is-same items)
(cond
((null items) ())
((funcall is-same (car items))
(append (cdr (car items)) (flatten is-same (cdr items)))  )
(t (cons (car items) (flatten is-same (cdr items))))
)
)

; A clean-up step to avoid stuff like (*) and (+ x).  Its basic function is
; to add the operator back to the front of a list of simplified parts,
; but it checks and avoids producing expressions with zero or one part.