StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
primarykey
Id
4435627
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
2
CommunityOwnedDate
CreationDate
2010-12-14T03:34:59.027
FavoriteCount
0
LastActivityDate
2010-12-14T03:34:59.027
LastEditDate
LastEditorUserId
0
OwnerUserId
26227
ParentId
4435391
PostTypeId
2
Score
4
ViewCount
0
LastEditorDisplayName
text
Body
In the most polite sense, your code is a bit off. You're learning Lisp this week, aren't you? That's OK! It's a fun language and can really do some awesome things. So I'm going to walk through the creation of the routine, and take you along the tour. Your basic case is - <pre><code>(defun negate (n) (if (> n 0) (- 0 n))) (map #'negate '(1 2 3 4)) </code></pre> Walking the tree is more complex, but let's walk through the ideas. Essentially, you have three cases to answer: is the current element nil, a list or an atom? <pre><code>(if (not (car seq)) (if (listp (car seq)) ;;Recurse ;;Otherwise negate the current element and append it to the recursed. </code></pre> Let's try a first cut at this: <pre><code>(defun negate-seq (seq) (if (not seq) (return-from negate-seq)) (if (listp (car seq)) (negate-seq seq) (list (negate (car seq)) (negate-seq (cdr seq))))) </code></pre> That's great! Except... <pre><code>(negate-seq '(1 2)) ==> (-1 (-2 NIL)) </code></pre> And... <pre><code> (negate-seq '(1 (1 2 -3))) ==> STACK OVERFLOW! </code></pre> Oh boy. We're in trouble now. First, let's just try a <code>cons</code> instead of a <code>list</code>. That cleans up the weird nested list problem. It's obvious that we're gotten into a loop of infinite recursion. That shouldn't be possible, because we've got the <code>not seq</code> guard. Okay, so let's try an debug. I'm using CLISP, and I can trace arguments with: <pre><code>(trace 'negate-seq) </code></pre> then, <pre><code>(negate-seq '(1 (1 2 -3))) </code></pre> Suddenly I see an explosion of <pre><code>1621. Trace: (NEGATE-SEQ '((1 2 -3))) 1622. Trace: (NEGATE-SEQ '((1 2 -3))) 1623. Trace: (NEGATE-SEQ '((1 2 -3))) 1624. Trace: (NEGATE-SEQ '((1 2 -3))) </code></pre> Crikey, I forgot my cdr and to cons up the list case! Hmmmm. Let's try this: <pre><code>(defun negate-seq (seq) (if (not seq) (return-from negate-seq)) (if (listp (car seq)) (cons (negate-seq (car seq)) (negate-seq (cdr seq))) (cons (negate (car seq)) (negate-seq (cdr seq))))) </code></pre> Recurse for the car, recuse on the car, cons them together, we might be on to something. <pre><code> (negate-seq '(1 (1 2 -3))) => (-1 (-1 -2 NIL) </code></pre> Hmmmm. Let's take a look at the trace. <blockquote> <ol> <li>Trace: (NEGATE-SEQ '(1 (1 2 -3)))</li> <li>Trace: (NEGATE-SEQ '((1 2 -3)))</li> <li>Trace: (NEGATE-SEQ '(1 2 -3))</li> <li>Trace: (NEGATE-SEQ '(2 -3))</li> <li>Trace: (NEGATE-SEQ '(-3))</li> <li>Trace: (NEGATE-SEQ 'NIL)</li> <li>Trace: NEGATE-SEQ ==> NIL</li> <li>Trace: NEGATE-SEQ ==> (NIL)</li> <li>Trace: NEGATE-SEQ ==> (-2 NIL)</li> <li>Trace: NEGATE-SEQ ==> (-1 -2 NIL)</li> <li>Trace: (NEGATE-SEQ 'NIL)</li> <li>Trace: NEGATE-SEQ ==> NIL</li> <li>Trace: NEGATE-SEQ ==> ((-1 -2 NIL))</li> <li>Trace: NEGATE-SEQ ==> (-1 (-1 -2 NIL))</li> </ol> </blockquote> So I recurse until the -3, then.... it falls off? Odd. Ah! I'm continually grabbing the CDR of things. A CDR is always a list. (cdr '(-3)) is nil! Let's see here.... (much rummaging around) Negate returns nil on positive. D'oh. <pre><code>(defun negate (n) (if ( > n 0) (- 0 n) n)) (defun negate-seq (seq) "Written by Paul Nathan" (if (not seq) (return-from negate-seq)) (if (listp (car seq)) (cons (negate-seq (car seq)) (negate-seq (cdr seq))) (cons (negate (car seq)) (negate-seq (cdr seq))))) </code></pre>
Tags
Title
singulars
PostAcceptedAnswerId
1. This table or related slice is empty.
PostParentId
1. POTo write a function in common lisp that will return the negation of a list
 singulars
 PostTypePostTypeId
 PTQuestion
PostTypePostTypeId
1. PTAnswer
UserLastEditorUserId
1. This table or related slice is empty.
UserOwnerUserId
1. USPaul Nathan
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. This table or related slice is empty.
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. This table or related slice is empty.
VotesPostIdCreationDate
1. VO
 singulars
 PostPostId
 PO
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
2. VO
 singulars
 PostPostId
 PO
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
3. VO
 singulars
 PostPostId
 PO
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
CommentsPostId
1. COa) `listp` returns `t` on `nil`, `consp` doesn't; b) use `cond` for `if/else`-s.
 singulars
 PostPostId
 PO
 UserUserId
 USkhachik
2. CO@hkachik: cond for if-else - *why*? The added complexity isn't worth it.
 singulars
 PostPostId
 PO
 UserUserId
 USPaul Nathan

Querying!

Guidance

A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.