The beginning of one of my programs results in an error. This is the problem area. I am trying to define a variable as the result of a recursive function.
(define (test n)
(define (a1func i)
(if (= i 1) 0
(+ (/ 1 i) (a1func (- i 1)))))
(define a1 (a1func (- n 1))))
if you were to give it say (test 10) the error would be:
procedure application: expected procedure, given: #<undefined>; arguments were: 9
I assumed this could be done in Scheme?? ideas?
In pure FP languages computations are done passing parameters to functions, which return some values as a result. You could bind the result of test in the function which called test:
(define (test n)
(define (a1func i)
(if (= i 1) 0
(+ (/ 1 i) (a1func (- i 1)))))
(a1func (- n 1)))
(define (calltest x)
(define (r (test (+ 2 x))))
(- r 4))
Variables usually bound once and cannot be changed. A function must return the value, a result of expression, but (define a1 (a1func(- n 1))) is rather a definition, not an expression, so the correct code would be:
(define (test n)
(define (a1func i)
(if (= i 1) 0
(+(/ 1 i) (a1func(- i 1)))))
(define a1 (a1func(- n 1)))
a1)
And since defining variable and immediate returning it is meaningless, a more correct code would be:
(define (test n)
(define (a1func i)
(if (= i 1) 0
(+(/ 1 i) (a1func(- i 1)))))
(a1func(- n 1)))
If your scheme implementation support lisp macros then you can write this:
(define-macro (test n)
(define (a1func i)
(if (= i 1) 0
(+ (/ 1 i) (a1func (- i 1)))))
`(define a1 ,(a1func (- n 1))))
or using named let
(define-macro (test n)
`(define a1 ,(let a1func ((i (- n 1)))
(if (= i 1)
0
(+ (/ 1 i) (a1func (- i 1)))))))
Related
I am trying to implement a function called funPower, which takes a function f, an integer n and returns the function f^n. For example ((funPower sqrt 2) 16) should return 2, which is (sqrt (sqrt 16)).
This is what I have so far but it is not giving me correct output
(define (funPower f n)
(lambda(x) (if (<= n 1)
(f x)
(f (funPower f (- n 1)) x))))
First, you're missing one more pair of parens.
(define (funPower1 f n)
(lambda (x) (if (<= n 1)
(f x)
;; (f ( funPower1 f (- n 1)) x))))
(f ( ( funPower1 f (- n 1)) x)))) )
;; ^^^ ^^^
because (funPower1 f (- n 1)) returns a function to be called on x, the future argument value, as you show with the example, ((funPower sqrt 2) 16).
Second, it's <= 0, not <= 1, and the function f shouldn't be called at all in such a case:
(define (funPower2 f n)
(lambda (x) (if (<= n 0)
;; (f x) ^^^
x
(f ( ( funPower2 f (- n 1)) x)))) )
Now that it's working, we see that it defers the decisions to the final call time, of ((funPower f n) x). But it really could do all the decisions upfront -- the n is already known.
To achieve that, we need to swap the (lambda and the (funPower, to push the lambda "in". When we do, it'll become an additional argument to such augmented funPower:
(define (funPower3 f n)
(if (<= n 0) (lambda (x)
x )
(funPower3 f (- n 1) (lambda (x) (f x)))) )
Now this is completely out of sync. Where's that third argument?
(define (funPower4 f n fun)
(if (<= n 0) fun
(funPower4 f (- n 1) (lambda (x) (fun (f x)))) ))
That's a little bit better, but what's the fun, originally? Where does it come from? It must always be (lambda (x) x) at first or else it won't be right. The solution is to make it an internal definition and use that, supplying it the correct argument the first time we call it:
(define (funPower5 f n)
(define (loop n fun)
(if (<= n 0) fun
(loop (- n 1)
(lambda (x) (fun (f x))))))
(loop n (lambda (x) x)))
This kind of thing would normally be coded as a named let,
(define (funPower5 f n)
(let loop ((n n)
(fun (lambda (x) x)))
(if (<= n 0) fun
(loop (- n 1)
(lambda (x) (fun (f x)))))))
We could also try creating simpler functions in the simpler cases. For instance, we could return f itself if n is 1:
(define (funPower6 f n)
(cond
((zero? n) .....)
((= n 1) .....)
((< n 0) .....)
(else
(let loop ((n n)
(fun .....))
(if (= n .....) fun
(loop (- n 1)
(lambda (x) (fun (f x)))))))))
Complete it by filling in the blanks.
More substantive further improvement is to use exponentiation by repeated squaring -- both in constructing the resulting function, and to have it used by the function we construct!
try this:
(define funpow
(lambda (f n)
((lambda (s) (s s n (lambda (x) x)))
(lambda (s n o)
(if (zero? n)
o
(s s (- n 1)
(lambda (x)
(o (f x)))))))))
(define sqrt_2 (funpow sqrt 2))
(define pow2_2 (funpow (lambda (x) (* x x)) 2))
(sqrt_2 16)
(pow2_2 2)
I am going through the book Structure and Interpretation of Computer Programming, which uses Scheme, and I just got through a portion on recursion. I wrote a program for exercise 1.11:
A function f is defined by the rule that f(n) = n if n<3 and f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n> 3. Write a procedure that computes f by means of a recursive process. Write a procedure that computes f by means of an iterative process.
I wrote the code at repl.it and when I run the procdure with the input 2, it gives me the error: Error: 2 is not a function [fRecurse, (anon)]. Can somebody explain to me what this means and how I can fix it? Why is it expecting my input to be a function?
Code:
(define (fRecurse n)(
(cond ((< n 3) n)
((>= n 3)
(+ (procRecurse (- n 1))
(* 2 (f (- fRecurse 2)))
(* 3 (f (- fRecurse 3))))))))
(fRecurse 2)
The error is due to an extra pair of parentheses before (cond...). To fix the issue, we simply remove the extra pair of parentheses:
(define (fRecurse n)
(cond ((< n 3) n)
((>= n 3)
(+ (fRecurse (- n 1))
(* 2 (fRecurse (- n 2)))
(* 3 (fRecurse (- n 3)))))))
(fRecurse 2)
And some additonal fixes I made to your example to make it work correctly:
Change f to fRecurse
Change procRecurse to fRecurse
Change (* 2 (f (- fRecurse 2))) to (* 2 (fRecurse (- n 2)))
Change (* 3 (f (- fRecurse 3))) to (* 3 (fRecurse (- n 3)))
See the updated repl.it
I'm a newbie and I didn't understand very well the language. Could anyone please explain to me what this functions do?
First function:
(define (x l)
(cond
((null? l) 0)
((list? (car l))
(+ (x (car l)) (x (cdr l))))
(else (+ 1 (x (cdr l))))
))
Second function:
(define (x l)
(cond
((null? l) 0)
((list? (car l))
(+ (x (car l)) (x (cdr l))))
(else (+ (car l) (x (cdr l)))
))
I do understand the begining but the conditions I didn't understand. Any help?
I will call your second function y.
Writing in pseudocode,
x [] -> 0
x [a . b] -> x a + x b , if list a
x [a . b] -> 1 + x b , else, i.e. if not (list a)
y [] -> 0
y [a . b] -> y a + y b , if list a
y [a . b] -> a + y b , else, i.e. if not (list a)
So for example,
x [2,3] = x [2 . [3]]
= 1 + x [3]
= 1 + x [3 . []]
= 1 + (1 + x [])
= 1 + (1 + 0 )
and
y [2,3] = y [2 . [3]]
= 2 + y [3]
= 2 + y [3 . []]
= 2 + ( 3 + y [])
= 2 + ( 3 + 0 )
See? The first counts something in the argument list, the second sums them up.
Of course both functions could be called with some non-list, but then both would just cause an error trying to get (car l) in the second clause, (list? (car l)).
You might have noticed that the two are almost identical. They both accumulates (fold) over a tree. Both of them will evaluate to 0 on the empty tree and both of them will sum the result of the same procedure on the car and cdr when the car is a list?. The two differ when the car is not a list and in the first it adds 1 for each element in the other it uses the element itself in the addition. It's possible to write the same a little more compact like this:
(define (sum l)
(cond
((null? l) 0) ; null-value
((not (pair? l)) l) ; term
(else (+ (sum (car l)) (sum (cdr l)))))) ; combine
Here is a generalisation:
(define (accumulate-tree tree term combiner null-value)
(let rec ((tree tree))
(cond ((null? tree) null-value)
((not (pair? tree)) (term tree))
(else (combiner (rec (car tree))
(rec (cdr tree)))))))
You can make both of your procedures in terms of accumulate-tree:
(define (count tree)
(accumulate-tree tree (lambda (x) 1) + 0))
(define (sum tree)
(accumulate-tree tree (lambda (x) x) + 0))
Of course you can make a lot more than this with accumulate-tree. It doesn't have to turn into an atomic value.
(define (double tree)
(accumulate-tree tree (lambda (x) (* 2 x)) cons '()))
(double '(1 2 ((3 4) 2 3) 4 5)) ; ==> (2 4 ((6 8) 4 6) 8 10)
Is there way to change - (minus) function to + (plus) function?
My homework is to implement sin calculation on Macluaurin series
sin(x) = x-(x^3/3!)+(x^5/5!) -(x^7/7!)+(x^9/9!)-...
Each article has different sign. This is my Lisp code
(defun sinMac (x series n plusminus)
(cond ((= series 0) 0)
(t (funcall plusminus
(/ (power x n) (factorial n))
(sinMac x (- series 1) (+ n 2) plusminus)))))
Is it possible to change plusminus to exchange sign? if I get '+ function send '- to next recursive call. From that call (got '-) I call '+ and so on...
You could do it with a circular list. Like so:
(defun sin-mac (x series n plus-minus)
(cond ((zerop series) 0)
(t (funcall (car plus-minus)
(/ (power x n) (factorial n))
(sin-mac x (1- series) (+ n 2) (cdr plus-minus))))))
(sin-mac x series 1 '#0=(+ - . #0#))
Or even better, wrap up the initial arguments using labels:
(defun sin-mac (x series)
(labels ((recur (series n plus-minus)
(cond ((zerop series) 0)
(t (funcall (car plus-minus)
(/ (power x n) (factorial n))
(recur (1- series) (+ n 2) (cdr plus-minus)))))))
(recur series 1 '#0=(+ - . #0#))))
If the function is a symbol, this is easy:
(defun next-function (function)
(ecase function
(+ '-)
(- '+)))
(defun sinMac (x series n plusminus)
(cond ((= series 0) 0)
(t (funcall plusminus
(/ (power x n) (factorial n))
(sinMac x
(- series 1)
(+ n 2)
(next-function plusminus))))))
I would not swap the function but just the sign. Using a loop for this also seems clearer to me (and is most likely more efficient, although there is still plenty of opportunity for optimization):
(defun maclaurin-sinus (x n)
"Calculates the sinus of x by the Maclaurin series of n elements."
(loop :for i :below n
:for sign := 1 :then (- sign)
:sum (let ((f (1+ (* 2 i))))
(* sign
(/ (expt x f)
(factorial f))))))
A few optimizations make this about 10 times faster (tested with n = 5):
(defun maclaurin-sinus-optimized (x n)
"Calculates the sinus of x by the Maclaurin series of n elements."
(declare (integer n))
(loop :repeat n
:for j :from 0 :by 2
:for k :from 1 :by 2
:for sign := 1 :then (- sign)
:for e := x :then (* e x x)
:for f := 1 :then (* f j k)
:sum (/ e f sign)))
How can I write a function max-list-function that consumes a list of functions, then produces a function f such that for every x, (f x) produces the maximum value of all the functions g in the list of functions?
For example (max-list-function (lambda (n) (+ n 4)) (lambda (n) (- 15 n))))
produces a function such that (f 2) returns 13 and (f 10) returns 14.
This is to be done with abstract list functions (filter, foldr, map, ...) without recursion.
Try this:
(define (max-list-function flist)
(lambda (n)
(foldr max -inf.0
(map (lambda (f) (f n))
flist))))
Use it like this:
(define f (max-list-function
(list (lambda (n) (+ n 4)) (lambda (n) (- 15 n)))))
(f 2)
> 13.0
(f 10)
> 14.0