What does "my other car is a cdr" mean?

Asked 8/12, 2009 at 5:26 Answered 8/12, 2009 at 6:5

Can anyone well versed in lisp explain this joke to me? I've done some reading on functional programming languages and know that CAR/CDR mean Contents of Address/Decrement Register but I still don't really understand the humour.

Opuntia answered 8/12, 2009 at 5:26 Comment(5)

start "scheming around" a bit and then you'll understand. hahaha. it's better than a bumper sticker that says "this programmer stops at all garbage collections." – Cuevas 29/9, 2011 at 17:24

Approximately half an hour after this question was linked to from news.ycombinator.com/item?id=14416846 it was put on hold. It's an 8 year old question. Annoyingly, I can't argue against the on-hold status but I don't like the vibe of putting it on hold now. – Atrium 25/5, 2017 at 14:21

@Atrium per the meta discussion, it is closed to prevent new answers from clogging up the review queue. – Erlond 25/5, 2017 at 14:59

Aren't there any other hold reasons? Opinion-based is... wrong – Griseldagriseldis 25/5, 2017 at 15:2

Shouldn't it be Protected instead? – Ellga 25/5, 2017 at 15:18

138

In Lisp, a linked list element is called a CONS. It is a data structure with two elements, called the CAR and the CDR for historical reasons. (Some Common Lisp programmers prefer to refer to them using the FIRST and REST functions, while others like CAR and CDR because they fit well with the precomposed versions such as (CADR x) ≡ (CAR (CDR x)).

The joke is a parody of the bumper stickers you sometimes see on beat-up old cars saying "My other car is a Porsche/BMW/etc."

My response to this joke has always been "My other CAR is a CADR. CDR isn't a CAR at all."

Sip answered 8/12, 2009 at 5:45 Comment(6)

very nice, but not true. Not after (rplacd a (car a)) it manifestly won't. :) Common LISP is not Haskell. But thanks for the explanation. +1. – Rucksack 3/5, 2012 at 16:42

@WillNess Common Lisp is not Common LISP either. – Jetsam 1/12, 2014 at 21:30

I didn't want to explain my joke, but... the point is that the CDR operation isn't a CAR operation; that's a separate issue from whether or not the values are equivalent via RPLACD or whatever. – Sip 4/12, 2014 at 21:30

And also, CDR sort of sounds like the name of a sports car, echoing TVR or GT-R, so one could read the text and not even realise the deeper, LISP-y meaning. – Escapement 25/5, 2017 at 12:55

In case anyone wants to know, CAR stands for Contents of the Address part of Register number, and CDR stands for Contents of the Decrement part of Register number. Thanks, Wikipedia! – Atkins 25/5, 2017 at 13:49

How about "My other car is first". :) – Tit 25/5, 2017 at 14:26

Yes, definitely a geek joke.

The names come from the IBM 704, but that's not the joke.

The joke is (bad) pun on "my other car is a ___." But the in-joke is about recursion.

When you loop/manipulate/select/invoke/more in lisp you use a combination of car (the first element in the list) and cdr (the rest of the list) to juggle functions.

So you've got a car, but your other car is your cdr because you can always get a car from a cdr since the cdr is always (in recursion) more elements. Get it? Laugh yet?

You'll probably have to learn lisp to actually chuckle a bit, or not. Of course, by then, you'll probably find yourself chuckling randomly for no apparent reason because:

Lisp makes you loopy.

Woods answered 8/12, 2009 at 6:5 Comment(1)

And with your last breath, another game began. – Dolliedolloff 20/5, 2014 at 4:30

//Coming from Scheme
Scheme has very few data structures, one of them is a tuple: '(first . second). In this case, car is the first element, and cdr is the second. This construct can be extended to create lists, trees, and other structures.
The joke isn't very funny.

Jillian answered 8/12, 2009 at 5:30 Comment(6)

Oh man... If I saw a bumper sticker with that joke I would totally scratch the word "primary" in the side with a key. – Becoming 8/12, 2009 at 5:34

Wouldn't the tuple be '(first . second)? – Munich 8/12, 2009 at 5:42

@Munich - again, I don't know lisp, but scheme doesn't have such a complex syntax. Even lists are made of pairs. – Jillian 8/12, 2009 at 5:46

Indeed, it would be more accurate to say that the tuple is (first . second). The list '(first second) is made of two tuples, like this: (cons first (cons second null)) – Kaine 8/12, 2009 at 6:2

Kobi: I know Lisp, and I'm not sure what you mean by "complex syntax". Dotted syntax is how you write pairs in Lisp, including Scheme: gnu.org/software/mit-scheme/documentation/mit-scheme-ref/… . The cdr of (first second) is (second), not second. – Munich 8/12, 2009 at 6:3

So, now we get downvoted for being corrected? oh, well. The sun will shine. – Jillian 8/12, 2009 at 10:23

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