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21:03:26
dale
I'm trying to do something like (let ((n 42)) (loop for n = n then n+1 until (>= n 50))) but n is nil on the first trip through the loop. Is there a good (or "remotely sane, relative to loop") way to have loop initialize its n from the surrounding n binding?
21:09:30
dale
verisimilitude: Sorry, my simplified example was obviously oversimplified. The actual code is a bit more complicated: http://ix.io/1Ezd/lisp
21:10:30
dale
verisimilitude: If I use WITH, how do I reassign n on each iteration after the first? Just slam in a DO (SETQ ...)?
21:11:54
verisimilitude
I agree that a simple renaming is likely easiest, but I'm wondering why you need N = N to start with.
21:12:45
dale
verisimilitude: I could rewrite it with a single FOR clause a la: for marker = (nav-stack-go-newer nav-stack-global-stack n) then (nav-stack-go-newer nav-stack-global-stack direction)
21:13:05
makomo
you'd like to start off with the outer value but then do something else on all the other iterations
21:13:58
makomo
dale: i've had a construct like that as well in some code, but the duplication annoys me
21:14:32
dale
I was surprised that "FOR n = n" seems to be like (let (n) (setq n n) ...) rather than (let ((n n)) ...).
21:14:56
dale
Well, I'm mostly thankful not to have been missing something obvious about loop. Thanks folks.
21:20:20
verisimilitude
The FOR = clause does set to NIL, yes. You should simply change the symbol you're using.
21:25:55
verisimilitude
I swear this is one of those pitfalls of LOOP that I had in mind at one point and had simply forgotten by now.
21:26:41
dale
verisimilitude: See, I was trying to very carefully avoid disclosing that I'm actually using cl-loop in Elisp.
21:27:00
dale
verisimilitude: Writing a little code to help me navigating back and forth through source files in Emacs.
21:27:55
dale
But before I asked here I did test what I believed to be equivalent code in SBCL first. :)
22:06:15
comborico1611
I am trying to STEP through a program. But the inner workings of the REPL continue to be included in STEP output. The last thing I've done is called STEP from within the compiled file. I've tried with both SBCL and Allegro. Any ideas?
22:08:57
moldybits
btw, maybe you could make a minimal testcase showing what you get vs what you'd like
22:16:43
pjb
comborico1611: or you may try cl-stepper: https://groups.google.com/d/msg/comp.lang.lisp/nj3jFxcJYM0/QbzGkAbyAtMJ
0:55:44
asarch
One stupid question: I've write a paint program (cl-paint) and I would like to check at its startup if there is already a running process of itself in order to prevent another "instance" of the application. How would I do that?
0:57:01
White_Flame
but, what many programs do in that situation is write a file somewhere containing the OS process id of the running program
0:58:43
verisimilitude
The example of the process ID, as with most things in UNIX, is fragile and held together by convention rather than technical measures.
0:59:23
White_Flame
you could also hold a socket open or something, but afaik there isn't a standard mechanism
0:59:44
White_Flame
generally, it's realy annoying if you don't let a user do somethign they want to do
1:03:42
asarch
I guess I could warn to the user with a "It seems that there is an already running process of the paint program. If not, delete manually the .cl-paint-pid and then retry"
1:04:09
asarch
However, the stupid user also could delete it manually and start another process, right?
1:30:47
verisimilitude
Firefox does this and, for that reason and many others, it's one of the worst programs I regularly use.
1:33:50
pjb
You have two cases: either your program needs exclusive access to a specific resource, or it does not.
1:34:24
pjb
In the former case, you should just test for the exclusive access to the resource, but you should not forbid multiple instances.
1:34:41
verisimilitude
If it really needs exclusive access, it should use a foolproof mechanism, but that unfortunately depends on the nature of the resource, since the OS isn't going to help at all.
2:05:21
White_Flame
with firefox, it uses a bunch of profile information on the local disk, so it needs to serialize on that
4:29:58
pjb
(loop for voo in '(k1 k2 k3) collect (case voo ((k1 k2) '12) (k3 '3))) #| --> (12 12 3) |#
4:31:06
pjb
cryptopsy: actually I would advise to always wrap the keys in the case clauses in parentheses, even when there is a single one (case voo ((k1 k2) '12) ((k3) '3) ((nil) '0)) for the case where you want to test for nil. Because (case nil (nil 'nope) ((nil) 'yep)) #| --> yep |#
4:33:56
cryptopsy
can i do 'else' in a case? like, if none of the cases matched, then do that thing keyn
4:34:36
pjb
(case 3 (2 'nope) (otherwise 'yep)) #| --> yep |# (case 3 (2 'nope) (t 'yep)) #| --> yep |#
4:36:01
pjb
it's used usually in cond or the otherwise case: (let ((foo 3)) (cond ((eql foo 2) 'nope) (t 'default))) #| --> default |#
4:36:51
pjb
on the paper, 0 and 1 were used in 1959, but in the first implementation they already switched to symbols, NIL and T.
4:37:21
cryptopsy
i understand that its a binary logic, just not why it would be true rather than nil
4:37:39
pjb
That was long before otherwise was considered as a keyword. otherwise would have taken two word to store the characters, and two words to store the pointers of the list, so it would have be a little costly as a keyword…
4:37:44
cryptopsy
'otherwise' more like it would be a false, as in it is false that there was a match
4:38:15
pjb
Have a look at https://www.reddit.com/r/lisp/comments/4mp4oy/lisp_i_and_lisp_15_manuals_source_code/
4:39:22
pjb
antecedent1 -> consequent1 , antecedent2 -> consequent2 , … , true -> default-consequent
4:41:25
pjb
that's where the CAR and CDR came from, and this processor also had a XEC instruction similat to eval, to execute an instruction stored at the given address.
4:42:06
pjb
cryptopsy: you understand now why the cost of the computer was nothing: it was the cost of the electric and air conditioner installation, and electricity bill.
4:43:17
pjb
The first 7090 installation was in November 1959. In 1960, a typical system sold for $2.9 million (equivalent to $18 million in 2018) or could be rented for $63,500 a month (equivalent to $405,000 in 2016).
4:43:26
cryptopsy
there are surface grinders with 100kw motors, when the factor runs those the power bill goes up by millions of dollars for the month
4:47:05
pjb
The basic memory cycle was 2.18 μs, so basically (/ 2.18e-6) #| --> 458715.62 |# less than half a mega instructions per second at most.
4:47:57
cryptopsy
i made this program to try to count whitespace at the beginning of each line, but it doesnt give any output when running in sbcl (stuck?) https://pastebin.com/TLvrS9NH
4:50:33
pjb
defvar (and all other forms with operator having a name prefixed with "DEF") should only be used as a toplevel form.
4:51:16
pjb
The reason is because its effects cannot be known before the expression is compiled. So the indent variable in your expression cannot be the indent variable defined by defvar.
4:51:49
pjb
Also, defvar and defparameter define special variables. Since they're special you should name them with stars *indent* to show that they're special and so you're careful in their use.
4:52:09
pjb
The specialness of those variable is that all their bindings are dynamic bindings, not lexical binding.
4:52:46
pjb
You can use (count-if (lambda (ch) (find ch #(#\space #\tab))) line) to count the spaces in a line.
4:53:25
pjb
(well, technically there are a few operators that preserve the toplevelness: progn and eval-when amongst others).
4:54:33
cryptopsy
i had put the definition there so that it gets reinitialized to 0 for each line read
4:55:05
pjb
cryptopsy: also, defvar and defparameter define global variables. You should avoid global variables.
5:00:14
pjb
The advantage of using :for, is that it doesn't intern the symbolin the current package, so if you later use-package a package that exports a symbol with the same name as one loop keyword, you won't get a collision.
5:01:10
pjb
Also, the advantage is to use := instead of = := means assignment, while = means equality…
5:08:59
pjb
cryptopsy: https://pastebin.com/NBn740xp you can use position-if-not to find the first non-space character.
5:09:39
pjb
However, it the line is empty, it returns NIL, so you may want to wrap (or (position-if-not …) 0) to fallback to 0 on those lines.
5:23:06
beach
Otherwise, you force people to count parentheses, which Common Lisp programmers never do.
5:23:51
cryptopsy
that's why i put them on their own lines, so i can fold blocks and copy and paste them around
5:26:59
beach
So start by using an editor that understands Common Lisp syntax to indent your code, and remove all the whitespace before closing parentheses.
5:58:30
akater
seok: when it makes sense to process elements of a sequence in parallel, for example. E.g. if you integrate over the elements. But it's not a hard rule. Lists are often convenient.
5:58:41
beach
seok: When you want O(1) access to the elements and you don't need to change the number of elements very often
6:00:33
beach
seok: Use an array (of which a vector is a special case) when you want O(1) access to the elements and you don't need to change the number of elements very often.
6:02:46
verisimilitude
In any case, it can usually be rather easy to switch them around, without changing the code.
6:03:20
verisimilitude
VECTORs and LISTs are both considered SEQUENCEs and many functions simply work on those.
6:13:01
jackdaniel
seok: do not use lists to represent arrays, they are literally given as an example what one shouldn't do in one of Norvig's essays
6:15:44
jackdaniel
(lists are very versatile and are useful for many things, representing arrays is *not* one of them)
6:33:27
verisimilitude
VECTORs are SEQUENCEs and ARRAYs, but other ARRAYs aren't SEQUENCEs, yes, seok.
6:34:04
verisimilitude
Is that the essay about the FORTRAN programmer complaining about how slow Lisp was at the time?
6:34:22
verisimilitude
Yet, this programmer also didn't understand how to use Lisp was part of the issue?
6:37:16
jackdaniel
http://www.cs.umd.edu/~nau/cmsc421/norvig-lisp-style.pdf here, it is one of examples of "bad code"
6:39:55
verisimilitude
He also thinks Python is a Lisp, so I'm inclined to be skeptical of his opinions instead of blindly following them.
6:41:11
jackdaniel
either way, I've said what I've wanted to say - advising to represent arrays as lists is harmful
6:41:53
verisimilitude
I can't download this PDF; I'm inclined to believe this server is ignoring Tor connections without rejecting them; I believe I may have had this issue with this server before.
6:46:05
verisimilitude
Oh, I believe I've already been recommended this document and read it before, actually.
7:04:23
aeth
jackdaniel: not just in norvig's essays... it's also in the Worse is Better essay, except with matrices as the example
7:06:08
aeth
As for lists being "convenient"... This is Common Lisp. 90% of what you would want to do with vector-like lists you can also just do with vectors without modifying your code because they're generic for sequences.
7:06:32
jackdaniel
aeth: you are right, thanks for a remainder (I think I would quote it if I had remembered)
8:02:57
pjb
seok: accessing the nth element of a list is an operation that takes O(n) memory accesses (and increment and tests). Accessing the nth element of a vector is an operation that takes O(1) memory accesses (and one addition, and one test).
8:03:28
pjb
seok: but notice that accessing the 1st element takes about the same time; when n=1, O(n) = O(1) !
8:04:36
pjb
seok: on the other hand, extending the size of a vector is an operation that takes O(n) (with n=(length vector)), since we have to copy all the old elements of the vector to the new bigger vector. while extending the size of a list if done at the head, is an operation that takes O(1), since we only need to add a single cons cell.
8:05:03
pjb
ecraven: indeed. O(n) is meaningful only when you consider big and bigger data structures.
8:05:22
pjb
seok: so if you only have a handful of elements, it doesn't matter whether you use a vector or a list.
8:06:43
pjb
seok: now, adding elements in vectors can be done in O(1), using vectors with fill-pointers, at the cost of pre-allocating the maximum number of elements you may have to store.
8:07:34
seok
I am specifically looking to store game object data like coordinates, velocity and such
8:08:21
pjb
seok: there's also the case where you need to insert new elements in the middle of the sequence. Here, list can be O(1) if you already have a reference to the cell in the middle of the list, while with a vector you will have to displace the remaining elements, so it'll be O(n).
8:08:28
seok
Probably would reserve empty indices to add or remove object to avoid changing vector length
8:09:48
pjb
Well, there are sorting algorithm that work on list directly, but AFAIK, for big sequences, if will always be faster to convert to vector, soft the vector, and store back the sorted elements in the list. So I would guess that all sane implementation do that. (but I suspect one or two use a list sorting algorithm).
8:12:31
pjb
Also, I would expect to have to access objects geographically. There are more sophisticated data structures that would help there. For example quad-trees. (or octo-trees in the case of 3D).
8:12:55
pjb
Of course, you can implement quad-trees using vectors (I did that once for a big tree, that was mmap'ed from disk).
8:18:53
shka__
having said all that, it is usually good idea to start with vectors is you simply need a sequence of elements with random access
8:23:01
pjb
seok: that said, if you multiply the size of the vector by a constant > 1 (preferably closer to or bigger than 2) when you need more space, then inserting elements remain O(n) instead of O(n²). http://www.cs.cornell.edu/courses/cs3110/2011fa/supplemental/lec20-amortized/amortized.htm
8:24:38
aeth
If it's for a game, though, it's perfectly fine to have some fixed upper limit in size. Most do somewhere.
8:25:06
aeth
And once you have a limit might as well allocate it all because RAM's cheap and it's probably tiny compared to the textures.