I am reading sicp, there's a problem (practice 1.29), I write a scheme function to solve the the question, but it seems that the recursive call of the function get the wrong answer. Really strange to me. The code is following:
(define simpson
(lambda (f a b n)
(let ((h (/ (- b a) n))
(k 0))
(letrec
((sum (lambda (term start next end)
(if (> start end)
0
(+ (term start)
(sum term (next start) next end)))))
(next (lambda (x)
(let ()
(set! k (+ k 1))
(+ x h))))
(term (lambda (x)
(cond
((= k 0) (f a))
((= k n) (f b))
((even? k) (* 2
(f x)))
(else (* 4
(f x)))))))
(sum term a next b)))))
I didn't get the right answer.
For example, if I try to call the simpson function like this:
(simpson (lambda (x) x) 0 1 4)
I expected to get the 6, but it returned 10 to me, I am not sure where the error is.It seems to me that the function "sum" defined inside of Simpson function is not right.
If I rewrite the sum function inside of simpson using the iteration instead of recursive, I get the right answer.
You need to multiply the sum with h/3:
(* 1/3 h (sum term a next b))
Related
If I have a recursive function like this:
(define (double-n-times x n)
(if (= n 0)
x
(double-n-times (* 2 x) (- n 1))))
How can I make a lambda version of it and never give it a name? ... like if i want to inline it somewhere. Is that possible? (I mean in this case I could use fold - so maybe the example isn't that great) - Is there some kind of symbol or placeholder for "self" that I haven't been able to find? Or do you just have to give it a name.
The Y-Combinator in Racket is:
(lambda (f)
((lambda (h) (h h))
(lambda (g) (f (lambda args (apply (g g) args))))))
This function can take any anonymous function and apply it on themselves recursively.
Let us define your function's part. double-n-times-part written only with lambdas:
(lambda (f)
(lambda (x n)
(if (= n 0) x (f (* 2 x) (- n 1))))))
where f we could name as we want - so we could also call it double-n-part.
If we apply the Y-Combinator on this, we get:
((lambda (f)
((lambda (h) (h h))
(lambda (g) (f (lambda args (apply (g g) args))))))
(lambda (f)
(lambda (x n)
(if (= n 0) x (f (* 2 x) (- n 1))))))
This spits out a function which takes the arguments x and n and applies the inner function of the second definiton on them.
So now, without any named functions - only using lambda expressions - you can apply on your arguments - let's say x=3 and n=4:
(((lambda (f)
((lambda (h) (h h))
(lambda (g) (f (lambda args (apply (g g) args))))))
(lambda (f)
(lambda (x n)
(if (= n 0) x (f (* 2 x) (- n 1))))))
3 4)
;;=> 48 ; as expected (3 * 2 * 2 * 2 * 2)
This is more convenient to read.
But we could also define the Y combinator without apply and args when we allow only monadic functions (functions with one arguments) instead of variadic ones. Then it looks like this (and we have to give the arguments one after another like this):
((((lambda (f)
((lambda (h) (h h))
(lambda (g) (f (lambda (x) ((g g) x))))))
(lambda (f)
(lambda (x)
(lambda (n)
(if (= n 0) x ((f (* 2 x)) (- n 1)))))))
3) 4)
;;=> 48
The answer to your question is yes, by using macros. But before I talk about that, I have to ask this first: do you ask because you are just curious? Or do you ask because there are some issues, like you don't want to pollute the namespace with names?
If you don't want to pollute the namespace with names, you can simply use local constructs like named let, letrec, or even Y combinator. Alternatively, you can wrap define inside (let () ...).
(let ()
(define (double-n-times x n)
(if (= n 0)
x
(double-n-times (* 2 x) (- n 1))))
(double-n-times 10 10))
;; double-n-times is not in scope here
For the actual answer: here's a macro rlam that is similar to lambda, but it allows you to use self to refer to itself:
#lang racket
(require syntax/parse/define)
(define-syntax-parse-rule (rlam args body ...+)
#:with self (datum->syntax this-syntax 'self)
(letrec ([self (λ args body ...)])
self))
;; compute factorial of 10
((rlam (x)
(if (= 0 x)
1
(* x (self (sub1 x))))) 10) ;=> 3628800
Yes. Being a placeholder for a name is what lambda function's parameters are there for:
(define (double-n-times x n)
(if (= n 0)
x
(double-n-times (* 2 x) (- n 1))))
=
(define double-n-times (lambda (x n)
(if (= n 0)
x
(double-n-times (* 2 x) (- n 1)))))
=
(define double-n-times (lambda (self) ;; received here
(lambda (x n)
(if (= n 0)
x
(self (* 2 x) (- n 1)))))) ;; and used, here
but what is this "self" parameter? It is the lambda function itself :
= ;; this one's in error...
(define double-n-times ((lambda (u) ;; call self with self
(u u)) ;; to receive self as an argument
(lambda (self)
(lambda (x n)
(if (= n 0)
x
(self (* 2 x) (- n 1)))))))
;; ...can you see where and why?
= ;; this one isn't:
(define double-n-times ((lambda (u) (u u))
(lambda (self)
(lambda (x n)
(if (= n 0)
x
((self self) (* 2 x) (- n 1)))))))
;; need to call self with self to actually get that
;; (lambda (x n) ... ) thing to be applied to the values!
And now it works: (double-n-times 1.5 2) returns 6.0.
This is already fine and dandy, but we had to write ((self self) ... ...) there to express the binary recursive call. Can we do better? Can we write the lambda function with the regular (self ... ...) call syntax as before? Let's see. Is it
= ;; erroneous
(define double-n-times ((lambda (u) (u u))
(lambda (self)
(lambda (x n)
(lambda (rec body) (self self)
(if (= n 0)
x
(rec (* 2 x) (- n 1))))))))
(no) Or is it
= ;; also erroneous...
(define double-n-times ((lambda (u) (u u))
(lambda (self)
(lambda (x n)
((lambda (rec body) body)
(self self)
(if (= n 0)
x
(rec (* 2 x) (- n 1)))))))) ;; ...can you see why?
(still no) Or is it perhaps
= ;; still erroneous...
(define double-n-times ((lambda (u) (u u))
(lambda (self)
((lambda (rec)
(lambda (x n)
(if (= n 0)
x
(rec (* 2 x) (- n 1)))))
(self self) ))))
(no yet again ... in an interesting way) Or is it actually
=
(define double-n-times ((lambda (u) (u u))
(lambda (self)
((lambda (rec)
(lambda (x n)
(if (= n 0)
x
(rec (* 2 x) (- n 1)))))
(lambda (a b) ((self self) a b)) ))))
(yes!) such that it can be abstracted and separated into
(define (Y2 g) ((lambda (u) (u u))
(lambda (self)
(g
(lambda (a b) ((self self) a b))))))
(define double-n-times (Y2
(lambda (rec) ;; declare the rec call name
(lambda (x n)
(if (= n 0)
x
(rec (* 2 x) (- n 1))))))) ;; and use it to make the call
and there we have it, the Y combinator for binary functions under strict evaluation strategy of Scheme.
Thus we first close over our binary lambda function with our chosen recursive call name, then use the Y2 combinator to transform this "rec spec" nested lambdas into a plain callable binary lambda function (i.e. such that expects two arguments).
Or course the name rec itself is of no importance as long as it does not interfere with the other names in our code. In particular the above could also be written as
(define double-n-times ;; globally visible name
(Y2
(lambda (double-n-times) ;; separate binding,
(lambda (x n) ;; invisible from
(if (= n 0) ;; the outside
x
(double-n-times (* 2 x) (- n 1))))))) ;; original code, unchanged
defining exactly the same function as the result.
This way we didn't have to change our original code at all, just close it over with another lambda parameter with the same name as the name of our intended recursive call, double-n-times, thus making this binding anonymous, i.e. making that name unobservable from the outside; and then passing that through the Y2 combinator.
Of course Scheme already has recursive bindings, and we can achieve the same effect by using letrec:
(define double-n-times ;; globally visible name
(letrec ((double-n-times ;; internal recursive binding:
(lambda (x n) ;; its value, (lambda (x n) ...)
(if (= n 0)
x
(double-n-times (* 2 x) (- n 1))))))
double-n-times)) ;; internal binding's value
Again the internal and the global names are independent of each other.
I'm new to CHICKEN and Scheme. In my quest to understanding tail recursion, I wrote:
(define (recsum x) (recsum-tail x 0))
(define (recsum-tail x accum)
(if (= x 0)
accum
(recsum-tail (- x 1) (+ x accum))))
This does what I expect it to. However, this seems a little repetitive; having an optional argument should make this neater. So I tried:
(define (recsum x . y)
(let ((accum (car y)))
(if (= x 0)
accum
(recsum (- x 1) (+ x accum)))))
However, in CHICKEN (and maybe in other scheme implementations), car cannot be used against ():
Error: (car) bad argument type: ()
Is there another way to implement optional function arguments, specifically in CHICKEN 5?
I think you're looking for a named let, not for optional procedure arguments. It's a simple way to define a helper procedure with (possibly) extra parameters that you can initialize as required:
(define (recsum x)
(let recsum-tail ((x x) (accum 0))
(if (= x 0)
accum
(recsum-tail (- x 1) (+ x accum)))))
Of course, we can also implement it with varargs - but I don't think this looks as elegant:
(define (recsum x . y)
(let ((accum (if (null? y) 0 (car y))))
(if (= x 0)
accum
(recsum (- x 1) (+ x accum)))))
Either way, it works as expected:
(recsum 10)
=> 55
Chicken has optional arguments. You can do it like this:
(define (sum n #!optional (acc 0))
(if (= n 0)
acc
(sum (- n 1) (+ acc n))))
However I will vote against using this as it is non standard Scheme. Chicken say they support SRFI-89: Optional positional and named parameters, but it seems it's an earlier version and the egg needs to be redone. Anyway when it is re-applied this should work:
;;chicken-install srfi-89 # install the egg
(use srfi-89) ; imports the egg
(define (sum n (acc 0))
(if (= n 0)
acc
(sum (- n 1) (+ acc n))))
Also your idea of using rest arguments work. However keep in mind that the procedure then will build a pair on the heap for each iteration:
(define (sum n . acc-lst)
(define acc
(if (null? acc-lst)
0
(car acc-lst)))
(if (= n 0)
acc
(sum (- n 1) (+ acc n))))
All of these leak internal information. Sometimes it's part of the public contract to have an optional parameter, but in this case it is to avoid writing a few more lines. Usually you don't want someone to pass a second argument and you should keep the internals private. The better way would be to use named let and keep the public contract as is.
(define (sum n)
(let loop ((n n) (acc 0))
(if (= n 0)
acc
(loop (- n 1) (+ acc n))))
Here is what I have done so far:
(define sumOdd
(lambda(n)
(cond((> n 0)1)
((odd? n) (* (sumOdd n (-(* 2 n) 1)
output would look something like this:
(sumOdd 1) ==> 1
(sumOdd 4) ==> 1 + 3 + 5 + 7 ==> 16
(sumOdd 5) ==> 1 + 3 + 5 + 7 + 9 ==> 25
This is what I am trying to get it to do: find the sum of the first N odd positive integers
I can not think of a way to only add the odd numbers.
To elaborate further on the sum-odds problem, you might solve it in terms of more abstract procedures that in combination accumulates the desired answer. This isn't necessarily the easiest solution, but it is interesting and captures some more general patterns that are common when processing list structures:
; the list of integers from n to m
(define (make-numbers n m)
(if (= n m) (list n) ; the sequence m..m is (m)
(cons n ; accumulate n to
(make-numbers (+ n 1) m)))) ; the sequence n+1..m
; the list of items satisfying predicate
(define (filter pred lst)
(if (null? lst) '() ; nothing filtered is nothing
(if (pred (car lst)) ; (car lst) is satisfactory
(cons (car lst) ; accumulate item (car lst)
(filter pred (cdr lst))) ; to the filtering of rest
(filter pred (cdr lst))))) ; skip item (car lst)
; the result of combining list items with procedure
(define (build-value proc base lst)
(if (null? lst) base ; building nothing is the base
(proc (car lst) ; apply procedure to (car lst)
(build-value proc base (cdr lst))))) ; and to the building of rest
; the sum of n first odds
(define (sum-odds n)
(if (negative? n) #f ; negatives aren't defined
(build-value + ; build values with +
0 ; build with 0 in base case
(filter odd? ; filter out even numbers
(make-numbers 1 n))))) ; make numbers 1..n
Hope this answer was interesting and not too confusing.
Let's think about a couple of cases:
1) What should (sumOdd 5) return? Well, it should return 5 + 3 + 1 = 9.
2) What should (sumOdd 6) return? Well, that also returns 5 + 3 + 1 = 9.
Now, we can write this algorithm a lot of ways, but here's one way I've decided to think about it:
We're going to write a recursive function, starting at n, and counting down. If n is odd, we want to add n to our running total, and then count down by 2. Why am I counting down by 2? Because if n is odd, n - 2 is also odd. Otherwise, if n is even, I do not want to add anything. I want to make sure that I keep recursing, however, so that I get to an odd number. How do I get to the next odd number, counting down from an even number? I subtract 1. And I do this, counting down until n is <= 0. I do not want to add anything to my running total then, so I return 0. Here is what that algorithm looks like:
(define sumOdd
(lambda (n)
(cond ((<= n 0) 0)
((odd? n) (+ n (sumOdd (- n 2))))
(else (sumOdd (- n 1))))))
If it helps you, here is a more explicit example of a slightly different algorithm:
(define sumOdd
(lambda (n)
(cond ((<= n 0) 0)
((odd? n) (+ n (sumOdd (- n 1))))
((even? n) (+ 0 (sumOdd (- n 1))))))) ; note that (even? n) can be replaced by `else' (if its not odd, it is even), and that (+ 0 ..) can also be left out
EDIT:
I see that the problem has changed just a bit. To sum the first N positive odd integers, there are a couple of options.
First option: Math!
(define sumOdd (lambda (n) (* n n)))
Second option: Recursion. There are lots of ways to accomplish this. You could generate a list of 2*n and use the procedures above, for example.
You need to have 2 variables, one which keep counter of how many odd numbers are still to be added and another to hold the current odd number which gets increment by 2 after being used in addition:
(define (sum-odd n)
(define (proc current start)
(if (= current 0)
0
(+ start (proc (- current 1) (+ start 2)) )))
(proc n 1))
Here is a nice tail recursive implementation:
(define (sumOdd n)
(let summing ((total 0) (count 0) (next 1))
(cond ((= count n) total)
((odd? next) (summing (+ total next)
(+ count 1)
(+ next 1)))
(else (summing total count (+ next 1))))))
Even shorter tail-recursive version:
(define (sumOdd n)
(let loop ((sum 0) (n n) (val 1))
(if (= n 0)
sum
(loop (+ sum val) (- n 1) (+ val 2)))))
Inspired this post .
I trying to implement a fibonacci series with nested lambda -
(( (lambda (x) (x x)) ;; evaluate x on x
((lambda (fibo-gen)) ;; fibo-gen get another func as arg
(lambda (N it second first)
(cond ;; here the body of the above func ..
((= N 1) 1)
((= N 1) 1)
((= N it) (+ second first))
(else (fibo-gen (+ it 1) (+ second first) (second)))
)
)
)
)
5 1 1 1)
It's prompts r5rs:body: no expression in body in: (r5rs:body)
By my examination each function has a "body" here , so what I did wrong ?
Note that the implementation I trying to do here is iterative mode which avoid re-calculate previous series ..
Edit :
Another mode which also works -
(( (lambda (x) (x x)) ;; evaluate x on x
(lambda (fibo-gen) ;; fibo-gen body use another lambda ..
(lambda (N it second first)
(cond ;; here the body of the above func ..
((= N 1) 1)
((= N 2) 1)
((= N it) second)
(else ((fibo-gen fibo-gen) N (+ it 1) (+ second first) second))
)
)
)
)
5 1 1 1)
=> 8
Well, this is quite a contrived way to calculate fibonacci, but nevertheless possible:
(((lambda (x) (x x))
(lambda (fib-gen)
(lambda (it second first)
(if (zero? it)
first
((fib-gen fib-gen) (sub1 it) (+ first second) second)))))
10 1 0) ; here n = 10
=> 55
If you're aiming for a general way for writing a recursive function without using define, first implement the Y-Combinator:
(define (Y X)
((lambda (proc) (proc proc))
(lambda (proc)
(X (lambda args
(apply (proc proc) args))))))
With this, you can write anonymous recursive procedures with a variable number of arguments, for example:
((Y
(lambda (fib-gen)
(lambda (it second first)
(if (zero? it)
first
(fib-gen (sub1 it) (+ first second) second)))))
10 1 0) ; here n = 10
=> 55
(lambda (fibo-gen))
in the second line has no body.
I have the following 2 functions that I wish to combine into one:
(defun fib (n)
(if (= n 0) 0 (fib-r n 0 1)))
(defun fib-r (n a b)
(if (= n 1) b (fib-r (- n 1) b (+ a b))))
I would like to have just one function, so I tried something like this:
(defun fib (n)
(let ((f0 (lambda (n) (if (= n 0) 0 (funcall f1 n 0 1))))
(f1 (lambda (a b n) (if (= n 1) b (funcall f1 (- n 1) b (+ a b))))))
(funcall f0 n)))
however this is not working. The exact error is *** - IF: variable F1 has no value
I'm a beginner as far as LISP goes, so I'd appreciate a clear answer to the following question: how do you write a recursive lambda function in lisp?
Thanks.
LET conceptually binds the variables at the same time, using the same enclosing environment to evaluate the expressions. Use LABELS instead, that also binds the symbols f0 and f1 in the function namespace:
(defun fib (n)
(labels ((f0 (n) (if (= n 0) 0 (f1 n 0 1)))
(f1 (a b n) (if (= n 1) b (f1 (- n 1) b (+ a b)))))
(f0 n)))
You can use Graham's alambda as an alternative to labels:
(defun fib (n)
(funcall (alambda (n a b)
(cond ((= n 0) 0)
((= n 1) b)
(t (self (- n 1) b (+ a b)))))
n 0 1))
Or... you could look at the problem a bit differently: Use Norvig's defun-memo macro (automatic memoization), and a non-tail-recursive version of fib, to define a fib function that doesn't even need a helper function, more directly expresses the mathematical description of the fib sequence, and (I think) is at least as efficient as the tail recursive version, and after multiple calls, becomes even more efficient than the tail-recursive version.
(defun-memo fib (n)
(cond ((= n 0) 0)
((= n 1) 1)
(t (+ (fib (- n 1))
(fib (- n 2))))))
You can try something like this as well
(defun fib-r (n &optional (a 0) (b 1) )
(cond
((= n 0) 0)
((= n 1) b)
(T (fib-r (- n 1) b (+ a b)))))
Pros: You don't have to build a wrapper function. Cond constructt takes care of if-then-elseif scenarios. You call this on REPL as (fib-r 10) => 55
Cons: If user supplies values to a and b, and if these values are not 0 and 1, you wont get correct answer