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Scheme function

Discuss Scheme function in the Other Programming Languages forum on Dev Shed.
Scheme function A place for discussing programming languages not covered in specific forums such as Assembler, COBOL, etc. - you get the idea.

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  #1  
Old November 22nd, 2006, 03:15 PM
NetBSD NetBSD is offline
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Scheme function
How can I use foldl to write a function for the running totals of a list, i.e.

(total (list 1 1 1)) => (list 1 2 3)

Any hints?
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  #2  
Old December 19th, 2006, 11:01 PM
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netytan netytan is offline
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Hey Net,

This should do the job; I haven't really tested it as I'm on windows and don't have Scheme installed.

Code:
(define (total lst)
  (reverse
   (foldr (lambda (a b)
            (cons (if (null? b)
                      a
                      (+ a (car b)))
                   b))
          ‘()
          lst)))


Please let me know if you came up with something different .

Take care,

Mark.
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  #3  
Old December 19th, 2006, 11:39 PM
spoon! spoon! is offline
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you mean
Code:
(define (total lst)
  (reverse
   (foldl (lambda (a b)
	    (cons (if (null? a)
		      b
		      (+ b (car a)))
		  a))
	  '()
	  lst)))
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Old December 20th, 2006, 12:12 AM
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netytan netytan is offline
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Quote:
Originally Posted by spoon!
you mean
Code:
(define (total lst)
  (reverse
   (foldl (lambda (a b)
	    (cons (if (null? a)
		      b
		      (+ b (car a)))
		  a))
	  '()
	  lst)))


Hi,

I really did mean foldr - it is easy to mix the two functions up though. Please check out the fold page on wikipedia .

http://en.wikipedia.org/wiki/Fold_%28higher-order_function%29

Mark.
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