刚刚看到两句话，感觉不错，分享给大家：

1.用户的问题。就是我们的问题

2.做一些用户想要的东西

(define add1

  (lambda (x)

    (+ 1 x)))

(define sub1

  (lambda (x)

    (- x 1)))

(add1 67)

(sub1 5)

(sub1 0)

(zero? 22)

(define addup

  (lambda (tup)

    (cond

      ((null?

tup) 0)

      (else (+ (car tup) (addup (cdr tup)))))))

(addup '(3 4 5))

(define *

  (lambda (x y)

    (cond

      ((eq? 1 x) y)

      (else (+ y (* (sub1 x) y))))))

(* 3 9)

(define tup+

  (lambda (tup1 tup2)

    (cond

      ((null?

tup1) '())

      (else (cons (+ (car tup1) (car tup2)) (tup+ (cdr tup1) (cdr tup2)))))))

(tup+ '(2 3 4 5) '(5 4 3 2))

(define >

  (lambda (x y)

    (cond

    ((zero? x) #f)

    ((zero? y) #t)

    (else (> (sub1 x) (sub1 y))))))

(> 4 3)

(> 3 3)

(> 3 4)

(define <

  (lambda (x y)

    (cond

      ((zero?

y) #f)

       ((zero? x) #t)

       (else (< (sub1 x) (sub1 y))))))

(< 3 3)

(< 4 3)

(< 3 4)

(define =

  (lambda (x y)

    (cond

      ((> x y) #f)

      ((< x y) #f)

      (else #t))))

(= 3 4)

(= 3 3)

(= 4 3)

(define expt

  (lambda (x y)

    (cond

    ((zero?

y) 1)

    (else (* x (expt x (sub1 y)))))))

(expt 5 3)

(define /

  (lambda (x y)

    (cond

      ((< x y) 0)

      (else (add1 (/ (- x y) y))))))

(/ 10 2)

(define length

  (lambda (lat)

    (cond

      ((null? lat) 0)

      (else (add1 (length (cdr lat)))))))

(length '(good hello))

(define pick

  (lambda (n lat)

    (cond

      ((eq? n 1) (car lat))

      (else (pick (sub1 n) (cdr lat))))))

(pick 3 '(hotdogs with hot mustard))

(define no-nums

  (lambda (list)

    (cond

      ((null? list) '())

      ((number? (car list)) (no-nums (cdr list)))

      (else (cons (car list) (no-nums (cdr list)))))))

(no-nums '(5 pears 6 prunes 9 dates))

(define all-nums

  (lambda (lat)

    (cond

      ((null? lat) '())

      ((number? (car lat)) (cons (car lat) (all-nums (cdr lat))))

      (else (all-nums (cdr lat))))))

(all-nums '(99 abc 33 mm 9 gg fuck))

(define eqlat?

  (lambda (a1 a2)

    (cond

      ((and (null? a1) (null? a2)) #t)

      ((and (number? (car a1)) (number? (car a2)))

       (cond

         ((= (car a1) (car a2)) (eqlat?

(cdr a1) (cdr a2)))

         (else #f)))

      ((eq?

(car a1) (car a2)) (eqlat?

(cdr a1) (cdr a2)))

      (else #f))))

(eqlat? '(a b c 33) '(a b c 33))

(define occur

  (lambda (a lat)

    (cond

      ((null? lat) 0)

      ((number? a)

       (cond

         ((and (number? (car lat)) (= (car lat) a)) (add1 (occur a (cdr lat))))

         (else (occur a (cdr lat)))))

      ((number? (car lat)) (occur a (cdr lat)))

      ((eq?

a (car lat)) (add1 (occur a (cdr lat))))

      (else (occur a (cdr lat))))))

(occur 'a '(a b c d e a 3 4 a))

(occur 3 '(b cd e 3 4 3 ed 3))

(define one?

  (lambda (n)

    (= n 1)))

(one? 3)