I am currently learning racket and am having a hard time understanding how to program in a functional language. I am trying to have the function first-item match the first element of my list to either a number or a character, add that token to a result list, and then act on the rest of the list. Currently on the last call of (first-item(rest L)) it sends an empty list and then my let statement fails because it can't work on the empty list. How do I add an exit clause or have my function end on the empty list?
(define(first-item L)
(let ([item (first L)])
(cond
[(regexp-match #rx"[-()+*]" (make-string 1 item)) (first-item (rest L))]
[(regexp-match #px"[0-9]" (make-string 1 item)) (first-item (rest L))]
)
)
)
You need to have your function handle L being empty, whatever that action is.
(define (first-item L)
(if (empty? L)
#| exit |#
#| recursive call |#))
Related
I'm having trouble figuring out how to go about creating a function that can take a series of the same function as arguments with the last argument as an operand. For example:
(func sqrt sqrt sqrt 390625)
The call above should return 5 as (sqrt 390625) > (sqrt 625) > (sqrt 25) > 5
I'm having trouble figuring out the exact way I should write this as any way I have tried has given me errors or achieved an infinite loop.
This the code is have so far:
(define func
(lambda L
(cond ( (equal? (length L) 2) ((car L) (cadr L)) ) ;; If the list consists of only 2 elements, carry out the function (element 1) onto the operand (element 2)
( #t (apply (car L) (func (cdr L))) ) ;; otherwise, apply the function (1st element) onto the rest of the list
)
)
)
The first condition works, for example returning 5 if i call (func sqrt 25), however the recursive call is throwing errors.
I would appreciate any help with this.
The OP doesn't provide a definition for chain, so that part is unclear, but I think that a fundamental problem here is that there is no recursive call to func; further, apply isn't used in the right position.
Instead of using (equal (length L) 2) as a base case, it might be nicer to make recursive calls as long as the first element in the input is a procedure, or otherwise just return the element:
#lang racket
(define multi-call
(lambda args
(let ((arg (car args)))
(if (procedure? arg)
(arg (apply multi-call (cdr args)))
arg))))
Here, when arg is a procedure, then it is applied to the result of calling multi-call recursively on the remaining arguments. Note that multi-call takes an arbitrary number of arguments, wrapping them in the list args. The reduction step provides (cdr args), which is a list of the remaining arguments. This means that apply should be used to call multi-call on those remaining arguments because multi-call expects an arbitrary number of arguments, not a list of arguments.
multi-call.rkt> (multi-call sqrt sqrt sqrt 390625)
5
I am trying to evaluate each atom of a list and see if it's equal to the number provided and remove if its not but I am running into a slight problem.
I wrote the following code:
(defun equal1(V L)
(cond((= (length L) 0))
(T (cond( (not(= V (car(equal1 V (cdr L))))) (cdr L) )))
)
)
(equal1 5 '(1 2 3 4 5))
I obtain the following error
Error: Cannot take CAR of T.
If I add (write "hello") for the action if true, the following error is obtained:
Error: Cannot take CAR of "hello".
I'm still quite new to LISP and was wondering what exactly is going on and how could I fix this so I could evaluate each atom properly and remove it if its not, thus the cdr L for the action.
car and cdr are accessors of objects of type cons. Since t and "hello" are not cons you get an error message.
To fix it you need to know what types your function returns and not car unless you know that it's a cons
EDIT
First off ident and clean up the code.. The nested cond are uneccesary since cond is a if-elseif-else structure by default:
(defun remove-number (number list)
(cond ((= (length list) 0)
t)
((not (= number (car (remove-number number (cdr list)))))
(cdr list))))
(t
nil)))
I want you to notice I've added the default behaviour of returning t when a consequent is not given as we know = returns either t or nil so it returns t when the length is 0 in this case.
I've added the default case where none of the two previous predicates were truthy and it defaults to returning nil.
I've named it according to the functions used. = can only be used for numeric arguments and thus this will never work on symbols, strings, etc. You need to use equal if you were after values that look the same.
Looking at this now we can see that the functions return value is not very easy to reason about. We know that t, nil and list or any part of the tail of list are possible and thus doing car might not work or in the case of (car nil) it may not produce a number.
A better approach to doing this would be:
check if the list is empty, then return nil
check if the first element has the same numeric value as number, then recurse with rest of the list (skipping the element)
default case should make cons a list with the first element and the result fo the recursion with the rest of the list.
The code would look something like this:
(defun remove-number (number list)
(cond ((endp list) '())
((= (car list) number) (remove-number ...))
(t (cons ...))))
There are a couple of things you could do to improve this function.
Firstly, let's indent it properly
(defun equal1 (V L)
(cond
((= (length L) 0))
(T (cond
((not (= V (car (equal1 V (cdr L))))) (cdr L))))))
Rather than saying (= (length l) 0), you can use (zerop (length l)). A minor sylistic point. Worse is that branch returns no value. If the list L is empty what should we return?
The issue with the function is in the T branch of the first cond.
What we want to do is
remove any list item that is the same value as V
keep any item that is not = to V
The function should return a list.
The expression
(cond
((not (= V (car (equal1 V (cdr L))))) (cdr L)))
is trying (I think) to deal with both conditions 1 and 2. However it's clearly not working.
We have to recall that items are in a list and the result of the equal function needs to be a list. In the expression above the result of the function will be a boolean and hence the result of the function call will be boolean.
The function needs to step along each element of the list and when it sees a matching value, skip it, otherwise use the cons function to build the filtered output list.
Here is a skeleton to help you out. Notice we don't need the embedded cond and just have 3 conditions to deal with - list empty, filter a value out, or continue to build the list.
(defun equal-2 (v l)
(cond
((zerop (length L)) nil)
((= v (car l)) <something goes here>) ;skip or filter the value
(t (cons (car l) <something goes here>)))) ;build the output list
Of course, this being Common Lisp, there is a built-in function that does this. You can look into remove-if...
I want my function to print each item in the list and sublist without quotes and return the number of items. The output of the list also needs to be in order, but my function is printing in reverse. I'm not sure why, is there any reasons why? Any suggestions to how I can recursively count the number of items and return that number? In addition why is the last item printed is supposed to be 9.99 instead of 100.999?
Edit: Thanks for the help so far. Just last question: Is there a way to make any output like DAY to be in lower case (day), or is that something that can't be done?
My function:
(defun all-print (inlist)
(cond
((not (listp inlist))
(format t "Error, arg must be a list, returning nil")
())
((null inlist) 0)
((listp (car inlist))
(ffn (append (car inlist)(cdr inlist))))
(t
(format t "~a " (car inlist) (ffn (cdr inlist))))))
My output example:
CL-USER 1 > (all-print (list 5 "night" 3 (list 9 -10) (quote day) -5.9 (* 100.999)))
100.999 -5.9 DAY -10 9 3 night 5
NIL
What it's suppose to output example:
5 night 3 9 -10 day -5.9 9.99 ;print
8 ;returns
It looks like all-print is supposed to be called ffn, since it looks like those are supposed to be recursive calls. In the rest of this answer, I'm just going to use ffn since it's shorter.
Why the output is in reverse
At present, your final cond clause makes the recursive call before doing any printing, because your recursive call is an argument to format:
(format t "~a " (car inlist) (ffn (cdr inlist)))
; ------------ -----------------
; 3rd 4th
All the arguments to format, including the 4th in this case, are evaluated before format is called. The 4th argument here will print the rest of the list, and then format will finally print the first element of the list. Your last cond clause should do the printing, and then make the recursive call:
(cond
…
(t
(format t "~a " (car inlist))
(ffn (cdr inlist))))
Why you get 100.999 rather than 9.99
You're getting 100.999 in your output rather than 9.99 (or something close to it) because the value of (* 100.999) is simply the value of 100.999. I'm guessing that you wanted (* 10 0.999) (note the space between 10 and 0.99). That still won't be quite 9.99 because of floating point arithmetic, though, but it will be close.
How to get the number of elements printed
uselpa's answer provides a good solution here. If you're supposed to return the number of elements printed, then every return value from this function should be a number. You have four cases,
not a list — returning nil is not a great idea. If this can't return a number (e.g., 0), then signal a real error (e.g., with (error "~A is not a list" inlist).
inlist is empty — return 0 (you already do)
(car inlist) is a list — here you make a recursive call to ffn. Since the contract says that it will return a count, you're fine. This is one of the reasons that it's so important in the first case (not a list) that you don't return a non-number; the contract depends on every call that returns returning an number.
In the final case, you print one item, and then make a recursive call to ffn. That recursive call returns the number of remaining elements that are printed, and since you just printed one, you need to add one to it. Thus the final cond clause should actually be something like the following. (Adding one to something is so common that Common Lisp has a 1+ function.)
(cond
…
(t
(format t "~a " (car inlist))
(1+ (ffn (cdr inlist))))) ; equivalent to (+ 1 (ffn (cdr inlist)))
A more efficient solution
We've addressed the issues with your original code, but we can also ask whether there are better approaches to the problem.
Don't append
Notice that when you have input like ((a b c) d e f), you create the list (a b c d e f) and recurse on it. However, you could equivalently recurse on (a b c) and on (d e f), and add the results together. This would avoid creating a new list with append.
Don't check argument types
You're checking that the input is a list, but there's really not much need to do that. If the input isn't a list, then using list processing functions on it will signal a similar error.
A new version
This is somewhat similar to uselpa's answer, but I've made some different choices about how to handle certain things. I use a local function process-element to handle elements from each input list. If the element is a list, then we pass it to print-all recursively, and return the result of the recursive call. Otherwise we return one and print the value. (I used (prog1 1 …) to emphasize that we're returning one, and printing is just a side effect. The main part of print-all is a typical recursion now.
(defun print-all (list)
(flet ((process-element (x)
(if (listp x)
(print-all x)
(prog1 1
(format t "~A " x)))))
(if (endp list)
0
(+ (process-element (first list))
(print-all (rest list))))))
Of course, now that we've pulled out the auxiliary function, the iteration is a bit clearer, and we see that it's actually a case for reduce. You might even choose to do away with the local function, and just use a lambda function:
(defun print-all (list)
(reduce '+ list
:key (lambda (x)
(if (listp x)
(print-all x)
(prog1 1
(format t "~A " x))))))
Here's my suggestion on how to write this function:
(defun all-print (lst)
(if (null lst)
0 ; empty list => length is 0
(let ((c (car lst))) ; bind first element to c
(if (listp c) ; if it's a list
(+ (all-print c) (all-print (cdr lst))) ; recurse down + process the rest of the list
(progn ; else
(format t "~a " c) ; not a list -> print item, then
(1+ (all-print (cdr lst)))))))) ; add 1 and process the rest of the list
then
? (all-print (list 5 "night" 3 (list 9 -10) (quote day) -5.9 (* 100.999)))
5 night 3 9 -10 DAY -5.9 100.999
8
I want to make a function that checks if an element is a member of a list. The list can contain other lists.
This is what I came with so far:
(defun subl(l)
(if (numberp l)
(if (= l 10)
(princ "Found"))
(mapcar 'subl l)))
Now the number I am searching for is hard-coded and it is 10. I would like to write it somehow so the function takes another parameter(the number I am searching for) and returns true or 1 when it finds it. The main problem is that I can't see a way to control mapcar. mapcar executes subl on each element of l, if l si a list. But how can I controll the returned values of each call?
I would like to check the return value of each subl call and if one of it is true or 1 to return true or 1 till the last recursive call. So in the end subl returns true or one if the element is contained in the list or nil otherwise.
Any idea?
This procedure below should process as you have described;
(defun member-nested (el l)"whether el is a member of l, el can be atom or cons,
l can be list of atoms or not"
(cond
((null l) nil)
((equal el (car l)) t)
((consp (car l)) (or (member-nested el (car l))
(member-nested el (cdr l))))
(t (member-nested el (cdr l)))))
mapcar is a very generic primitive to map a function over a list. You can use one of the built-in combinators which are much more closely suited with what you're trying to do. Look into the member function.
Your function seems to play the role of main function and helper at the same time. That makes your code a lot more difficult to understand than it has to be..
So imagine you split the two:
;; a predicate to check if an element is 10
(defun number10p (l)
(and (numberp l)
(= l 10)))
;; the utility function to search for 10 amongst elements
(defun sublistp (haystack)
(mapcar #'number10p haystack)))
But here when you do (sublistp '(5 10 15 20)) you'll get (nil t nil nil) back. Thats because mapcar makes a list of every result. For me it seems you are describing some since it stops at the first true value.
(defun sublistp (haystack)
(some #'number10p haystack)))
(sublistp '(5 10 15 20)) ; ==> t
Now to make it work for any data type we change the predicate and make it as a local function where we have the argument we are searching for:
(defun sublistp (needle haystack)
(flet ((needlep (x)
(equal x needle)))
(some #'needlep haystack)))
(sublistp '(a b) '(a b c (a b) d e f)) ; ==> t
You can also do this with an anonymous predicate like this:
(defun sublistp (needle haystack)
(some #'(lambda (x)
(equal x needle))
haystack))
An implementation of this is the member function, except it returns the match as truth value. That's ok since anything but nil is true in CL:
(member 10 '(5 10 15 20)) ; ==> (10 15 20)
EDIT
You commented on a different answer that you are required to use mapcar in that case use it together with append to get a list of all matches and check if the list has greater than 0 elements:
(defun sublistp (needle haystack)
(flet ((needle-check (x)
(if (equal x needle) '(t) nil)))
(< 0 (length
(apply #'append
(mapcar #'needle-check haystack))))))
How it works is that for each match you get a list of one element and for every non match you get an empty list. When appending the lists you'll get the empty list when there is not match. For all other results you have a match. This is not a very efficient implementation.
The function is supposed to be tail-recursive and count from 1 to the specified number. I think I'm fairly close. Here's what I have:
(define (countup l)
(if (= 1 l)
(list l)
(list
(countup (- l 1))
l
)
)
)
However, this obviously returns a list with nested lists. I've attempted to use the append function instead of the second list to no avail. Any guidance?
Here's an incorrect solution:
(define (countup n)
(define (help i)
(if (<= i n)
(cons i (help (+ i 1)))
'()))
(help 1))
This solution:
uses a helper function
recurses over the numbers from 1 to n, cons-ing them onto an ever-growing list
Why is this wrong? It's not really tail-recursive, because it creates a big long line of cons calls which can't be evaluated immediately. This would cause a stack overflow for large enough values of n.
Here's a better way to approach this problem:
(define (countup n)
(define (help i nums)
(if (> i 0)
(help (- i 1)
(cons i nums))
nums)))
(help n '()))
Things to note:
this solution is better because the calls to cons can be evaluated immediately, so this function is a candidate for tail-recursion optimization (TCO), in which case stack space won't be a problem.
help recurses over the numbers backwards, thus avoiding the need to use append, which can be quite expensive
You should use an auxiliar function for implementing a tail-recursive solution for this problem (a "loop" function), and use an extra parameter for accumulating the answer. Something like this:
(define (countup n)
(loop n '()))
(define (loop i acc)
(if (zero? i)
acc
(loop (sub1 i) (cons i acc))))
Alternatively, you could use a named let. Either way, the solution is tail-recursive and a parameter is used for accumulating values, notice that the recursion advances backwards, starting at n and counting back to 0, consing each value in turn at the beginning of the list:
(define (countup n)
(let loop ((i n)
(acc '()))
(if (zero? i)
acc
(loop (sub1 i) (cons i acc)))))
Here a working version of your code that returns a list in the proper order (I replaced l by n):
(define (countup n)
(if (= 1 n)
(list n)
(append (countup (- n 1)) (list n))))
Sadly, there is a problem with this piece of code: it is not tail-recursive. The reason is that the recursive call to countup is not in a tail position. It is not in tail position because I'm doing an append of the result of (countup (- l 1)), so the tail call is append (or list when n = 1) and not countup. This means this piece of code is a normal recusrive function but to a tail-recursive function.
Check this link from Wikipedia for a better example on why it is not tail-recusrive.
To make it tail-recursive, you would need to have an accumulator responsible of accumulating the counted values. This way, you would be able to put the recursive function call in a tail position. See the difference in the link I gave you.
Don't hesitate to reply if you need further details.
Assuming this is for a learning exercise and you want this kind of behaviour:
(countup 5) => (list 1 2 3 4 5)
Here's a hint - in a tail-recursive function, the call in tail position should be to itself (unless it is the edge case).
Since countup doesn't take a list of numbers, you will need an accumulator function that takes a number and a list, and returns a list.
Here is a template:
;; countup : number -> (listof number)
(define (countup l)
;; countup-acc : number, (listof number) -> (listof number)
(define (countup-acc c ls)
(if ...
...
(countup-acc ... ...)))
(countup-acc l null))
In the inner call to countup-acc, you will need to alter the argument that is checked for in the edge case to get it closer to that edge case, and you will need to alter the other argument to get it closer to what you want to return in the end.