i have a problem that i just cant work out,
the user enters a list ie
(total-cost
'((anItem 2 0.01)
(item 3 0.10)
(anotherItem 4 4.10)
(item 5 2.51)))
i need to add the number on the end together and then return the result
my current code returns the code after each addition. and also throws a error about unexpected type
(defun total-cost (list)
(loop with sum = 0
for x in list
collect (setf sum (+ sum (last x)))
)
)
Error: (0.01)' is not of the expected typeNUMBER'
Any help is appreciated
Thanks Dale
Using LOOP:
CL-USER 19 > (loop for (nil nil number) in '((anItem 2 0.01)
(item 3 0.10)
(anotherItem 4 4.10)
(item 5 2.51))
sum number)
6.72
REDUCE is another option:
CL-USER 20 > (reduce '+
'((anItem 2 0.01)
(item 3 0.10)
(anotherItem 4 4.10)
(item 5 2.51))
:key 'third)
6.72
Loop has a keyword sum for summing so you don't have to have an explicit variable nor use setf:
(defun total-cost (list)
(loop for x in list sum (third x)))
As Chris said, use (car (last x)) if the number you're looking for is always the last one. Or you can use (third x) as in my example if it's always the third one.
Also, note that the use of collectis wrong if your aim is to return the sum only; your example (corrected) returns
(0.01 0.11 4.21 6.7200003)
whereas mine returns
6.7200003
Note that if you want so escape the rounding errors as much as possible you need to use an exponent marker to make them double-floats for example:
(total-cost '((anItem 2 0.01D0)
(item 3 0.10D0)
(anotherItem 4 4.10D0)
(item 5 2.51D0)))
=> 6.72D0
last returns the last cons cell in the list, not its value. You need to use (car (last x)) instead.
Just in case you want the code to give you a precise result rather then being short:
(defun kahan-sum (floats)
(loop
:with sum := 0.0 :and error := 0.0
:for float :in floats
:for epsilon := (- float error)
:for corrected-sum := (+ sum epsilon) :do
(setf error (- corrected-sum sum epsilon) sum corrected-sum)
:finally (return sum)))
(defun naive-sum (floats) (loop :for float :in floats :sum float))
(let ((floats (loop :repeat 1000 :collect (- (random 1000000.0) 1000000.0))))
(format t "~&naive sum: ~f, kahan sum: ~f" (naive-sum floats) (kahan-sum floats)))
;; naive sum: -498127420.0, kahan sum: -498127600.0
Read more about why it works like this here: http://en.wikipedia.org/wiki/Kahan_summation_algorithm
Coming late to the party... How about a little lisping instead of looping? ;-)
(defun sum-3rd (xs)
(let ((sum 0))
(dolist (x xs sum)
(incf sum (nth 2 x)))))
Related
I am trying to write a function that takes in the length of a list and a maximum number value and returns a list that is the length given with numbers between 1 and the given max randomly.
so far I have
(define (randomlist n max)
(cond
[(= n 0)empty]
[else
(cons (build-list n (random 1 max))
(randomlist max (- n 1)))]))
I get an error when I run this and was wondering if anybody could help me out.
One can also use for/list to combine loop and list formation:
(define (randomlist n mx)
(for/list ((i n))
(add1 (random mx))))
Testing:
(randomlist 5 10)
Output:
'(5 9 10 4 7)
(random numbers, hence output is very likely to be different each time).
There are several bugs in your code:
It's a bad idea to call a parameter max, that clashes with a built-in procedure. So I renamed it to mx.
There's absolutely no reason to use build-list, that's not how we build an output list, just cons one element with the rest.
random receives zero or one parameters, not two. The single-parameter version returns an integer in the range 0..n-1, hence we have to add 1 to the result to be in the range 1..n.
You switched the order of the parameters when recursively calling randomlist.
This should take care of the problems:
(define (randomlist n mx)
(cond
[(= n 0) empty]
[else
(cons (+ 1 (random mx))
(randomlist (- n 1) mx))]))
It works as expected:
(randomlist 5 10)
=> '(10 7 1 4 8) ; results will vary, obviously
I have a list of lists and want to test if all elements are different from each other, i.e. equal should return nil for all combinations of list elements.
E.g.
(defparameter feld '((1 0 0 5 5 0)
(0 0 0 0 0 0)
(1 1 5 5 0 0)
(0 1 0 1 5 5)
(5 5 1 0 1 0)
(1 0 1 0 5 5)))
I thought of using reduce but as far as I understand it only tests the equality of neighbors, as would do a loop construct like:
(loop for i below (length feld)
for j from 1
if (equal (nth i feld) (nth j feld)) return t)
Is there a simple way using a standard construct which I do not see at the moment or do I have to create a recursive function?
The whole data structure represents a "board game" where every list is a line on the board and each element in the inside-lists is a value of this very field. The three numerical values (0, 1 and 5) are something like empty, Symbol A and Symbol B. A valid board cannot have two identical lines. This is why I want to identify those.
Basically, it is like remove-duplicates without removing. In the meantime I was thinking about something like this:
(defun duplicates-p (lst)
(cond ((null lst) '())
((member (car lst) (cdr lst)) t)
(t (duplicates-p (rest lst)))))
Something like this:
(defun unique (lsts &aux (h (make-hash-table :test 'equal)))
(loop :for lst :in lsts
:never (gethash lst h)
:do (setf (gethash lst h) t)))
I am very new to LISP (so forgive me for any dumb mistakes) and the first lab of the year states:
Define a function, STDEV that will compute the standard deviation of a list of numbers (look up formula)
I wrote this code but I don't know why it refuses to work:
(defun stdev (x)
(sqrt (/ (apply '+ (expt (- x (/ (apply '+ x)
(length x)))
2))
(length x))))
(setq a '(1 2 3 4 5))
(STDEV a)
But on runtime it produces the error:
(1 2 3 4 5) is not a number
I believe that I have correctly emulated the standard deviation formula (though I wouldn't put it past myself to make a dumb mistake), but why does my program not like the list of numbers that I give it to evaluate? It is most likely a simple mistake with inputs from this new style of coding but any and all help is greatly appreciated!
Use indentation. I've edited your question:
(defun stdev (x)
(sqrt (/ (apply '+ (expt (- x (/ (apply '+ x)
(length x)))
2))
(length x))))
expt returns a number. You call (apply '+ some-number)?
Also you subtract a number from a list.
Why?
Generally I would recommend to use a Lisp listener (aka REPL) to get to working code:
Compute the mean value:
CL-USER 21 > (let ((l (list 1 2 3 4 5)))
(/ (reduce #'+ l)
(length l)))
3
Subtract the mean value and square using mapcar:
CL-USER 22 > (mapcar (lambda (item)
(expt (- item 3) 2))
(list 1 2 3 4 5))
(4 1 0 1 4)
Compute the variance as the mean value of above:
CL-USER 23 > (let ((l (list 4 1 0 1 4)))
(/ (reduce #'+ l)
(length l)))
2
Take the square root to get the standard deviation:
CL-USER 24 > (sqrt 2)
1.4142135
Then you only need to assemble it into a few functions: average, variance and standard-deviation.
You’re taking - a ..., when a is your list.
Not a complete answer, because this is homework, but: you want to calculate the mean first, you can implement a sum function, which you will need twice, with a fold, and you can apply a helper function or lambda expression to every element of a list using a map.
I need a function that will check if all digits in some number on even positions are even. The least significant digit is on position 1, starting from right to left. The function need to be written in lisp.
Examples:
245 -> true, since 4 is even
238456 -> false, since 5 is odd and 8 and 2 are even
and so on...
Here`s what I got:
(defun check(number fac)
(cond
((= (/ number fac) 0) t)
((= (mod (/ number fac) 2 ) 0) (check number (* 100 fac) ) )
(nil)))
The initial value for fac is 10, we divide the number with 10, extract the second digit, check if it is even, if so proceed and divide number with 1000 to extract the 4-th digit and so on until we get over all digits, than the function returns true, meanwhile if some digit is odd the function should return nil.
But something is wrong and the function return nil all the time , when I call it like (check 22 10) for example.
Any thoughts?
Here is a non recursive solution that checks for the correctness of the parameter:
(defun check(num)
(assert (integerp num))
(loop for i = (truncate num 10) then (truncate i 100) until (zerop i)
always (evenp i)))
Just another variant. Basicly I'm converting number to list (through string though, maybe not the best way), then reverse it, select every second element and check it all for being even.
;; Helper for getting every
(defun get-all-nth (list period)
"Get all NTH element in the list"
(remove-if-not
(let ((iterator 0))
(lambda (x)
(declare (ignore x))
(= 0 (mod (incf iterator) period)))) list))
(defun check-evens (num)
"Checks if all digits in some number on even positions are even.
Goes Rigth-to-left."
(assert (integerp num))
(every #'evenp
(get-all-nth
(reverse
(map 'list #'digit-char-p
(prin1-to-string num))) 2)))
Some test cases:
CL-USER> (check-evens 123)
T
CL-USER> (check-evens 238456)
NIL
CL-USER> (check-evens 238446)
T
CL-USER> (check-evens 23844681)
T
For Project Euler Problem 8, I am told to parse through a 1000 digit number.
This is a brute-force Lisp solution, which basically goes through every 5 consecutive digits and multiplies them from start to finish, and returns the largest one at the end of the loop.
The code:
(defun pep8 ()
(labels ((product-of-5n (n)
(eval (append '(*)
(loop for x from n to (+ n 5)
collect (parse-integer
1000digits-str :start x :end (+ x 1)))))))
(let ((largestproduct 0))
(do ((currentdigit 0 (1+ currentdigit)))
((> currentdigit (- (length 1000digits-str) 6)) (return largestproduct))
(when (> (product-of-5n currentdigit) largestproduct)
(setf largestproduct (product-of-5n currentdigit)))))))
It compiles without any warnings, but upon running it I get:
no non-whitespace characters in string "73167176531330624919225119674426574742355349194934...".
[Condition of type SB-INT:SIMPLE-PARSE-ERROR]
I checked to see if the local function product-of-5n was working by writing it again as a global function:
(defun product-of-5n (n)
(eval (append '(*)
(loop for x from n to (+ n 5)
collect (parse-integer
1000digits-str :start x :end (+ x 1))))))
This compiled without warnings and upon running it, appears to operate perfectly. For example,
CL_USER> (product-of-5n 1) => 882
Which appears to be correct since the first five digits are 7, 3, 1, 6 and 7.
As for 1000digits-str, it was simply compiled with defvar, and with Emacs' longlines-show-hard-newlines, I don't think there are any white-space characters in the string, because that's what SBCL is complaining about, right?
I don't think there are any white-space characters in the string, because that's what SBCL is complaining about, right?
The error-message isn't complaining about the presence of white-space, but about the absence of non-white-space. But it's actually a bit misleading: what the message should say is that there's no non-white-space in the specific substring to be parsed. This is because you ran off the end of the string, so were parsing a zero-length substring.
Also, product-of-5n is not defined quite right. It's just happenstance that (product-of-5n 1) returns the product of the first five digits. Strings are indexed from 0, so (product-of-5n 1) starts with the second character; and the function iterates from n + 0 to n + 5, which is a total of six characters; so (product-of-5n 1) returns 3 × 1 × 6 × 7 × 1 × 7, which happens to be the same as 7 × 3 × 1 × 6 × 7 × 1.
EVAL is not a good idea.
Your loop upper bound is wrong.
Otherwise I tried it with the number string and it works.
It's also Euler 8, not 9.
This is my version:
(defun euler8 (string)
(loop for (a b c d e) on (map 'list #'digit-char-p string)
while e maximize (* a b c d e)))
since I don't know common lisp, I slightly modified your code to fit with elisp. As far as finding bugs go and besides what have been said ((product-of-5n 1) should return 126), the only comment I have is that in (pep8), do length-4 instead of -6 (otherwise you loose last 2 characters). Sorry that I don't know how to fix your parse-error (I used string-to-number instead), but here is the code in case you find it useful:
(defun product-of-5n (n) ;take 5 characters from a string "1000digits-str" starting with nth one and output their product
(let (ox) ;define ox as a local variable
(eval ;evaluate
(append '(*) ;concatenate the multiplication sign to the list of 5 numbers (that are added next)
(dotimes (x 5 ox) ;x goes from 0 to 4 (n is added later to make it go n to n+4), the output is stored in ox
(setq ox (cons ;create a list of 5 numbers and store it in ox
(string-to-number
(substring 1000digits-str (+ x n) (+ (+ x n) 1) ) ;get the (n+x)th character
) ;end convert char to number
ox ) ;end cons
) ;end setq
) ;end dotimes, returns ox outside of do, ox has the list of 5 numbers in it
) ;end append
) ;end eval
) ;end let
)
(defun pep8 () ;print the highest
(let ((currentdigit 0) (largestproduct 0)) ;initialize local variables
(while (< currentdigit (- (length 1000digits-str) 4) ) ;while currentdigit (cd from now on) is less than l(str)-4
;(print (cons "current digit" currentdigit)) ;uncomment to print cd
(when (> (product-of-5n currentdigit) largestproduct) ;when current product is greater than previous largestproduct (lp)
(setq largestproduct (product-of-5n currentdigit)) ;save lp
(print (cons "next good cd" currentdigit)) ;print cd
(print (cons "with corresponding lp" largestproduct)) ;print lp
) ;end when
(setq currentdigit (1+ currentdigit)) ;increment cd
) ;end while
(print (cons "best ever lp" largestproduct) ) ;print best ever lp
) ;end let
)
(setq 1000digits-str "73167176531330624919")
(product-of-5n 1)
(pep9)
which returns (when ran on the first 20 characters)
"73167176531330624919"
126
("next good cd" . 0)
("with corresponding lp" . 882)
("next good cd" . 3)
("with corresponding lp" . 1764)
("best ever lp" . 1764)
I've done this problem some time ago, and there's one thing you are missing in the description of the problem. You need to read consequent as starting at any offset into a sting, not only the offsets divisible by 5. Therefore the solution to the problem will be more like the following:
(defun pe-8 ()
(do ((input (remove #\Newline
"73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450"))
(tries 0 (1+ tries))
(result 0))
((= tries 5) result)
(setq result
(max result
(do ((max 0)
(i 0 (+ 5 i)))
((= i (length input)) max)
(setq max
(do ((j i (1+ j))
(current 1)
int-char)
((= j (+ 5 i)) (max current max))
(setq int-char (- (char-code (aref input j)) 48))
(case int-char
(0 (return max))
(1)
(t (setq current (* current int-char))))))))
input (concatenate 'string (subseq input 1) (subseq input 0 1)))))
It's a tad ugly, but it illustrates the idea.
EDIT sorry, I've confused two of your functions. So that like was incorrect.