How would I loop this list in scheme?
(define test-document '(
((h e l l o))
((t h i s)(i s)(t e s t))
))
What I tried it only showed the first column.
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What do you mean by "loop this list"?R Sahu– R Sahu2014-04-20 02:55:49 +00:00Commented Apr 20, 2014 at 2:55
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Being able to access each word in the list in order.Philip Rego– Philip Rego2014-04-20 03:07:02 +00:00Commented Apr 20, 2014 at 3:07
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3 Answers
car and cdr family of functions are your friends to navigate lists. Here are some examples.
(define test-document '(
((h e l l o))
((t h i s)(i s)(t e s t))
))
(car test-document) ;; `((h e l l o))
(caar test-document) ;; `(h e l l o)
(cadr test-document) ;; `((t h i s) (i s) (t e s t))
(car (cadr test-document) ;; `(t h i s)
(cadr (cadr test-document) ;; `(i s)
(caddr (cadr test-document) ;; `(test )
Define a function that will walk the list and call a function for each item that is not a list.
(define (walk-list lst fun)
(if (not (list? lst))
(fun lst)
(if (not (null? lst))
(begin
(walk-list (car lst) fun)
(walk-list (cdr lst) fun)))))
Call it to print each item.
(walk-list test-document print)
3 Comments
Philip Rego
But what if you don't know the size of the document.
R Sahu
@PhilipRego I added some more content to my answer to illustrate how you can walk a list and do something with the items in it.
Philip Rego
How do I get it to print the words without parethesis. it gets the words as a list but i cant do anything with it cause it has the dumb pareteses around them (define (walk-list lst fun) (if (not (equal? (car lst) '())) (fun lst) (if (not (null? lst)) (begin (walk-list (car lst) fun) (walk-list (cdr lst) fun))))) (walk-list test-document display)
What you have is a list of lists of lists:
(define test-document '(((h e l l o)) ((t h i s) (i s) (t e s t))))
To loop over its elements you must create a loop of a loop of a loop. To do so we can use map and curry as follows:
(map (curry map (curry map
(compose string->symbol string-upcase symbol->string)))
test-document)
This produces the following output:
(((H E L L O)) ((T H I S) (I S) (T E S T)))
If your Scheme interpreter doesn't have a built-in curry function then you can define one as follows:
(define (curry func . args)
(lambda x (apply func (append args x))))
Hope this helped.
2 Comments
Philip Rego
It says curry is undefined
Aadit M Shah
Which scheme interpreter are you using? The
curry function is available in mzscheme. Anyway I updated my answer with an implementation of curry.Were you thinking of something like this?
(define (walk-list lst)
(define (sub-walk lst)
(if (null? lst)
'()
(let ((x (car lst)))
(if (list? x)
(cons (sub-walk x) (sub-walk (cdr lst)))
(apply string-append (map symbol->string lst))))))
(flatten (sub-walk lst)))
then
(walk-list test-document)
=> '("hello" "this" "is" "test")
which you can process using the usual suspects (map, filter, ...).
If your Scheme has no flatten procedure, you can use this one:
(define (flatten lst)
(reverse
(let loop ((lst lst) (res null))
(if (null? lst)
res
(let ((c (car lst)))
(loop (cdr lst) (if (pair? c) (loop c res) (cons c res))))))))
2 Comments
Philip Rego
I don't have the function flatten
uselpa
That's why I put one in my answer!