I prefer first/rest for readability. When I write (first (second (first x))) though, my old Lisp heart comes back and I switch to (caadar x) or something like that.
On Mar 7, 2014, at 1:40 PM, Daniel Carrera <dcarr...@gmail.com> wrote: > On the contrary, car and cdr are GUARANTEED to be O(1). > > car == return the first element in a pair. > cdr == return the second element in a pair. > > The fact that the cdr of a list returns the rest of the list is simply an > incidental side-effect of the definition of list. > > Cheers, > Daniel. > > > On 7 March 2014 18:28, Deren Dohoda <deren.doh...@gmail.com> wrote: > Does this mean we shouldn't cdr functional lists but only use rest? > > On Mar 7, 2014 12:02 PM, "Matthias Felleisen" <matth...@ccs.neu.edu> wrote: > > In a world of immutable cons-es you can cache the result so that testing > list? becomes an O(1) operation. > > > > > On Mar 7, 2014, at 11:52 AM, Eric Dong <yd2d...@uwaterloo.ca> wrote: > > > Forgive me if I am super terribly wrong. Isn't it the case that an improper > > list is only known to be improper if we walk to the end and find something > > other than an empty? So wouldn't that mean "first" and "rest" take linear > > time since they must make sure the argument is a list? Clearly that doesn't > > happen. What am I missing? > > > > > > On Fri, Mar 7, 2014 at 10:16 AM, Daniel Carrera <dcarr...@gmail.com> wrote: > > Ok. That makes sense. A list is either '() or something plus a list. > > > > Thanks. > > > > Cheers, > > Daniel. > > > > > > On 7 March 2014 14:46, Jon Zeppieri <zeppi...@gmail.com> wrote: > > Oops, sorry about that empty message. I was going to say that your > > definition of a list is close, but it's missing something, A list is > > either: > > > > - the empty list; or > > - a pair, the second element of which is a list > > > > (cons 3 2) is a pair, and sometimes non-list pairs are called > > "improper lists," but they don't satisfy list?. > > > > > > On Fri, Mar 7, 2014 at 8:43 AM, Jon Zeppieri <zeppi...@gmail.com> wrote: > > > On Fri, Mar 7, 2014 at 8:39 AM, Daniel Carrera <dcarr...@gmail.com> wrote: > > >> What is (cons 3 2) ? What is the definition of a list? I thought that a > > >> list > > >> was defined as either '() or a pair. > > >> > > >> Cheers, > > >> Daniel. > > >> > > >> > > >> On 7 March 2014 13:49, Jens Axel Søgaard <jensa...@soegaard.net> wrote: > > >>> > > >>> The value (cons 3 42) is not a list. The function car will extract 3, > > >>> but first will fail. > > >>> > > >>> /Jens Axel > > >>> > > >>> > > >>> 2014-03-07 13:40 GMT+01:00 Daniel Carrera <dcarr...@gmail.com>: > > >>> > Thanks. That's a very useful tip (being able to get at the source > > >>> > code). > > >>> > I > > >>> > am a bit confused by the condition "(and (pair? x) (list? x))". It > > >>> > seems > > >>> > to > > >>> > me that this could just be replaced with "(pair? x)". The "list?" > > >>> > doesn't > > >>> > add anything. Am I wrong? > > >>> > > > >>> > Also, I don't see exactly how "first" and "car" behave different on a > > >>> > non-list. They both raise an error. The errors are just worded > > >>> > differently. > > >>> > > > >>> > On the same file, I found the definition of empty? > > >>> > > > >>> > (define empty? (lambda (l) (null? l))) > > >>> > > > >>> > Wouldn't it be more economical to write "(define empty? null?)" and > > >>> > allow > > >>> > them to be synonyms? > > >>> > > > >>> > Cheers, > > >>> > Daniel. > > >>> > > > >>> > > > >>> > On 7 March 2014 12:16, Jens Axel Søgaard <jensa...@soegaard.net> > > >>> > wrote: > > >>> >> > > >>> >> For lists first/rest works the same as car/cdr. > > >>> >> For non-lists there is a difference: first and rest signals an error. > > >>> >> The names first and rest makes it easier for a human reader of > > >>> >> a piece of code to see that the program works on lists only. > > >>> >> > > >>> >> For the curious, the definition of first is: > > >>> >> > > >>> >> (define (first x) > > >>> >> (if (and (pair? x) (list? x)) > > >>> >> (car x) > > >>> >> (raise-argument-error 'first "(and/c list? (not/c empty?))" x))) > > >>> >> > > >>> >> I found this definition like this: > > >>> >> 1. Entered this program in DrRacket: > > >>> >> #lang racket > > >>> >> first > > >>> >> 2. Clicked the "Check Syntax" button > > >>> >> 3. Right clicked the identifier first and chose "Open defining file" > > >>> >> 4. Chose "first" in the definition-drop-down in the upper left > > >>> >> corner. > > >>> >> > > >>> >> /Jens Axel > > >>> >> > > >>> >> > > >>> >> > > >>> >> > > >>> >> > > >>> >> > > >>> >> > > >>> >> 2014-03-07 11:45 GMT+01:00 Daniel Carrera <dcarr...@gmail.com>: > > >>> >> > Hello, > > >>> >> > > > >>> >> > Is there any difference between `first` and `car`, or between > > >>> >> > `last` > > >>> >> > and > > >>> >> > `cdr`, or between `empty? and null?` ? > > >>> >> > > > >>> >> > I had assumed that these were just synonyms, added by Racket > > >>> >> > because > > >>> >> > they > > >>> >> > might be more memorable to a student. But apparently Racket doesn't > > >>> >> > think > > >>> >> > they are equal: > > >>> >> > > > >>> >> > -> (equal? first car) > > >>> >> > #f > > >>> >> > -> (equal? last cdr) > > >>> >> > #f > > >>> >> > -> (equal? empty? null?) > > >>> >> > #f > > >>> >> > > > >>> >> > > > >>> >> > I suppose that they could be separate functions that happen to do > > >>> >> > the > > >>> >> > same > > >>> >> > thing, but if so, my next question would be why they aren't just > > >>> >> > aliases. As > > >>> >> > in: > > >>> >> > > > >>> >> > -> (define myfirst car) > > >>> >> > -> (equal? myfirst car) > > >>> >> > #t > > >>> >> > > > >>> >> > 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. > > >>> >> > > > >>> >> > ____________________ > > >>> >> > Racket Users list: > > >>> >> > http://lists.racket-lang.org/users > > >>> >> > > > >>> >> > > >>> >> > > >>> >> > > >>> >> -- > > >>> >> -- > > >>> >> Jens Axel Søgaard > > >>> > > > >>> > > > >>> > > > >>> > > > >>> > -- > > >>> > When an engineer says that something can't be done, it's a code phrase > > >>> > that > > >>> > means it's not fun to do. > > >>> > > >>> > > >>> > > >>> -- > > >>> -- > > >>> Jens Axel Søgaard > > >> > > >> > > >> > > >> > > >> -- > > >> When an engineer says that something can't be done, it's a code phrase > > >> that > > >> means it's not fun to do. > > >> > > >> ____________________ > > >> Racket Users list: > > >> http://lists.racket-lang.org/users > > >> > > > > > > > > -- > > When an engineer says that something can't be done, it's a code phrase that > > means it's not fun to do. > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > > > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > > > > > -- > When an engineer says that something can't be done, it's a code phrase that > means it's not fun to do. ____________________ Racket Users list: http://lists.racket-lang.org/users
