Re: [Chicken-users] Problem with (fold)
On Tue, Mar 11, 2014 at 03:47:53PM +0100, Daniel Carrera wrote: > On 11 March 2014 15:41, Peter Bex wrote: > > > To avoid such mistakes, it's helpful to use mnemonic names: > > > > (fold (lambda (item result) (+ (* result 10) item)) 0 '(1 2 3)) > > > > > Thanks. I was mentally reading from left to right, so I ended up assuming > that it was (result item). That would make more sense, I guess, and is the ordering used by other functional languages. There's a built-in version of fold which obeys this ordering, called foldl: http://api.call-cc.org/doc/library/foldl I think SRFI-1's FOLD uses the oher way around is because of the notion that the procedure argument is called a CONStructor, and "cons" accepts the item first and the list on which to cons second, making "reverse" trivial to implement like this: (fold cons '() '(1 2 3 4)) => (4 3 2 1) I find it's easy enough to remember if you keep this in mind. Cheers, Peter -- http://www.more-magic.net ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
Re: [Chicken-users] Problem with (fold)
On 11 March 2014 15:41, Peter Bex wrote: > To avoid such mistakes, it's helpful to use mnemonic names: > > (fold (lambda (item result) (+ (* result 10) item)) 0 '(1 2 3)) > > Thanks. I was mentally reading from left to right, so I ended up assuming that it was (result item). Cheers, Daniel. -- When an engineer says that something can't be done, it's a code phrase that means it's not fun to do. ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
Re: [Chicken-users] Problem with (fold)
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
Re: [Chicken-users] Problem with (fold)
I think I've answered my own question: I have to switch the "a" and the "b". I guess I was confused as to how parameters are sent into the lambda. Cheers, Daniel. On 11 March 2014 15:30, 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? > > Cheers, > Daniel. > -- > When an engineer says that something can't be done, it's a code phrase > that means it's not fun to do. > -- When an engineer says that something can't be done, it's a code phrase that means it's not fun to do. ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
[Chicken-users] Problem with (fold)
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? Cheers, Daniel. -- When an engineer says that something can't be done, it's a code phrase that means it's not fun to do. ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
