On Tue, Mar 11, 2014 at 03:30:56PM +0100, Daniel Carrera wrote: > Hello, > > I'm having a problem with (fold): > > > (use srfi-1) ; List library. > > (fold (lambda (a b) (+ (* a 10) b)) 0 '(1 2 3)) > > I was expecting this to return 123, but it returns 60. I'm confused. In my > mind, at each step I shift the current value to the left (i.e. multiply by > 10) and add the new digit. So the steps should be: > > 1 , 2 --> 10 + 1 = 12 > 12 , 3 --> 120 + 3 = 123 > > What am I missing?
You need to multiply the "memo" by 10, not the item: #;1> (use srfi-1) #;2> (fold (lambda (a b) (print `(+ (* ,a 10) ,b)) (+ (* a 10) b)) 0 '(1 2 3)) (+ (* 1 10) 0) (+ (* 2 10) 10) (+ (* 3 10) 30) 60 #;3> (fold (lambda (a b) (+ (* b 10) a)) 0 '(1 2 3)) 123 To avoid such mistakes, it's helpful to use mnemonic names: (fold (lambda (item result) (+ (* result 10) item)) 0 '(1 2 3)) Cheers, Peter -- http://www.more-magic.net _______________________________________________ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
