Here is a selection of code snippets that I find helpful or amusing.

;; Common Lisp
; Hamming numbers are those with no prime factors other than 2, 3 or 5.
; Therefore 10, 15, and 30 are Hamming numbers, but 21 is not. How can
; you efficiently implement (hamming n) in lisp?
; It seems natural to do a recursive solution- the hamming numbers are 1
; and 2, 3, or 5 times some other hamming number. Autrijus Tang solved
; it in Haskell this way:
;
; main = print (take 1000 hamming)
; hamming = 1 : map (2*) hamming ~~ map (3*) hamming ~~ map (5*) hamming
; where
; xxs@(x:xs) ~~ yys@(y:ys) -- To merge two streams:
; | x==y = (x : xs~~ys) -- if the heads are common, take that
; | xy = (y : xxs~~ys) -- and proceed to merge the rest
; note that his way (computing successive values) is just about the only
; one which will be O(n). Any approach that determines whether each
; number is a Hamming, as opposed to just generating them, will be at
; best O(n log n), I think.
(defun n-hammings (twos threes fives tail n out)
(if (= n 0)
out
(let* ((2l (* 2 (car twos)))
(3l (* 3 (car threes)))
(5l (* 5 (car fives)))
(next (min 2l 3l 5l)))
(progn (setf (cdr tail) (cons next ()))
(let ((out2 (if (= next 2l) (cdr twos) twos))
(out3 (if (= next 3l) (cdr threes) threes))
(out5 (if (= next 5l) (cdr fives) fives)))
(n-hammings out2 out3 out5 (cdr tail) (1- n) out))))))
(defun n-hamming-nums (n)
(let ((res (list 1)))
(n-hammings res res res res n res)
(nth n res)))
;; scheme
(define (n-hammings twos threes fives tail n out)
(if (= n 0)
out
(let* ((2l (* 2 (car twos)))
(3l (* 3 (car threes)))
(5l (* 5 (car fives)))
(next (min 2l 3l 5l)))
(set-cdr! tail (cons next ()))
(let* ((out2 (if (= next 2l) (cdr twos) twos))
(out3 (if (= next 3l) (cdr threes) threes))
(out5 (if (= next 5l) (cdr fives) fives)))
(n-hammings out2 out3 out5 (cdr tail) (-- n) out)))))
(define (n-hamming-nums n)
(let ((res (list 1)))
(n-hammings res res res res n res)
(list-ref res n)))
; the only differences in CL and scheme here are that you need to define
; -- , and that set-cdr! instead of setf is required. I prefer the
; greater level of abstraction of setf, but it is not important.
; relative to haskell, this takes 2x as much code, in this case
; because I had to manually simulate lazy lists. Interestingly, I
; posted this example to CLL, and no one was able to produce a
; shorter version that was also O(n) (search hamming on CLL to see
; the discussion).
Scheme:
; This is perhaps the most general bit of code I have ever written. It
; will do a very fast filtered accumulation between two endpoints, given
; functions for getting from one number to the next, a function to
; evaluate a term, and a filter. It can, for instance, tell you the sum
; of the square of all the primes between 10 and 100,000 within a few
; seconds. One of the downsides of learning scheme is that there are
; things that are incredibly powerful and general, but are almost never
; truly necessary. This is such a thing. Interestingly, it does not
; make sense to implement this in Common Lisp, as tail call optimization
; is not mandated by the CL spec.
; f is the function that accumulates a result, nullv is the null value,
; term is the function that determines how to compute a term value from
; an input value, a and b are limits, acc is an accumulator, and term and
; filter are self-explanatory.
(define (facc-tr f nullv term filter a next b acc)
(if (> a b)
acc
(facc-tr f nullv term filter (next a) next b (if (filter a) (f acc (term a)) acc))))
; example usages-
; sum the squares of all odd numbers between 1 and 100000 inclusive:
(facc-tr + 0 sq odd? 1 ++ 100000 0)
; take the product of the squares of all the primes between 1 and 10000:
(facc-tr * 1 sq prime? 1 ++ 10000 1)
; This procedure calculates a class of iterative procedures in
; logarithmic time. The sort of procedure eligible for this sort of
; treatment is one for which there is a way to double the step size
; of the increment. A surprisingly large number of such procedures
; exist. Exponentiation and Fibonacci numbers are the two examples
; I have tested. This is another example of a bit of code I hope to
; actually need someday. In the mean time, it was an interesting
; exercise.
; in this function, (square transform) "squares" the process, while
; (step x) applies the process once. Examples of domains in which
; this strategy of iterative improvement is applicable include
; exponentiation,
(define (iter-exp-proc step square transform n acc)
(cond ((= n 0) acc)
((even? n) (iter-exp-proc step square (square transform) (/ n 2) acc))
(else (iter-exp-proc step square transform (- n 1) (step transform acc)))))
; this returns a list of the factors of a number.
(define (factors n)
(let* ((end (sqrt n)))
(define (factors-acc n index bot top)
(cond ((> index end) (append (reverse bot) top))
((= n (sq index)) (append (reverse bot) (cons index top)))
((= 0 (modulo n index)) (factors-acc n (+ 1 index) (cons index bot) (cons (/ n index) top)))
(else (factors-acc n (+ 1 index) bot top))))
(factors-acc n 1 ()())))
; Here are two different ways to determine primality of a number. The
; first is always correct, but takes O(sqrt n) time. It is just the
; Seive of Eratosthenes. The second is probabalistic, and takes a lot
; more code, but takes O(log n) time. Note that if the naive expmod
; is used, this will be dog-slow, but if a daisho'd version of expmod
; is used, it will be considerably faster-O(log n) instead of O(sqrt n)
(define (prime? n)
(let* ((top (inexact->exact (floor (sqrt n)))))
(define (fast-prime? n a)
(cond ((> a top) #t)
((= 0 (modulo n a)) #f)
(else (fast-prime? n (+ a 2)))))
(cond ((< n 2) #f)
((= n 2) #t)
((even? n) #f)
(else (fast-prime? n 3)))))
(define (probably-prime? n times)
(cond ((= times 0) #t)
((fermat-test? n) (probably-prime? n (- times 1)))
(else #f)))
(define (fermat-test? n)
(define (try-it a)
(= (bexpmod a n n) a))
(try-it (+ 1 (random (min maxrandom (- n 1))))))
(define (expmod base exp mod)
(modulo (expt base exp) mod))
(define (bexpmod base exp m)
(cond ((= exp 0) 1)
((even? exp)
(remainder (sq (bexpmod base (/ exp 2) m))
m))
(else (remainder (* base (bexpmod base (- exp 1) m))
m))))
(define (efermat-test? n)
(define (efermat-test-int? n i)
(define (try-it a)
(= (bexpmod a n n) a))
(cond ((= i 1) #t)
((try-it i) (efermat-test-int? n (-- i)))
(else #f)))
(efermat-test-int? n (- n 1)))
; It's always nice to be able to name things what you want to: I find
; this to be one of the reasons I like scheme. This function computes
; factorials, and it has exactly the name I would use if I were writing
; on paper. Note that this is tail-recursive, and therefore runs in
; constant memory.
(define (! n)
(define (fact-tr n a)
(if (< n 2)
a
(fact-tr (- n 1) (* a n))))
(fact-tr n 1))
Perhaps this is my perl accent showing, but I really like the ..
operator. Here it is in scheme.
(define (.. l h . s)
(let* ((step (if (null? s) 1 (car s))))
(if (> h l)
(range l h step)
(reverse (range h l step)))))
; The 8 queens problem is a classic, and one I enjoyed writing. I
; am not aware of an effecient algorithm for finding legal placements.
(define (acclist f init lst)
(cond ((null? lst) init)
((atom? lst) (f lst))
(else (f (car lst) (acclist f init (cdr lst))))))
(define (flatmap f l)
(acclist append () (map f l)))
(define (queens board-size)
(define (queen-cols k)
(if (= k 0)
(list empty-board)
(filter
(lambda (positions) (safe? k positions))
(flatmap
(lambda (rest-of-queens)
(map (lambda (new-row)
(adjoin-position new-row k rest-of-queens))
(.. 1 board-size)))
(queen-cols (- k 1))))))
(queen-cols board-size))