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World: r3wp

[Core] Discuss core issues

Maxim
21-Dec-2006
[6502]
compose strips the outer block.
Dirk
21-Dec-2006
[6503]
the block from (vals) ?
Maxim
21-Dec-2006
[6504]
reduce simply evaluates each value in the block and inserts the result
Dirk
21-Dec-2006
[6505]
i see
Maxim
21-Dec-2006
[6506]
yep
Dirk
21-Dec-2006
[6507]
this is not even mentioned in the tutorial i read... do more operators 
behave like that?
Maxim
21-Dec-2006
[6508x4]
a common trick is to do this to unify a series as a block! which 
might also accept a string! :

val: compose [(val)]


this way, if val was originally a block, it stays that way, but if 
it was a string, its then inserted within one.  Note that the above 
actually creates a new block... so that the original val (if it was 
a block) and the new one are not the same
nope, its a special case for compose... and is one of its differentiating 
features.
also note that compose has a /deep refinement  which is very handy
ex:


a-big-view-block:  [ button "press me"  [print rejoin ["You have 
pressed " (val)]]]

val: "the button"

view layout compose/deep a-big-view-block
Dirk
21-Dec-2006
[6512x2]
neat (and important)
im using rebol on sparc/solaris (for automation). not even perl installed 
on these boxes .-)
Maxim
21-Dec-2006
[6514]
note that in the above, if you didn't compose the block it would 
still work, since val would be evaluated on the fly... BUT if you 
have several windows opened at the same time and each new window 
displayed a different caption, then the above is necessary... otherwise 
changing the value of val will change ALL windows at the same time 
 :-)   which is a common error we all make at some point.
Dirk
21-Dec-2006
[6515]
so the callback of the button will be bound to the value of 'val' 
at compose/deep time .. ? ok.
Maxim
21-Dec-2006
[6516x3]
it will replace the (val) by its value at specific time
another very good use of compose:

time: now
my-object: make object! compose [
	name: "me"
	time: (time)
]

without the compose, my-object/time will be none
actually it will raise an error since time within the context of 
the object, is not yet set.
Dirk
21-Dec-2006
[6519x2]
hm, isn't:

my-object: make object! [
  name: "me"
  time: now
]

equivalent?
ah, i see.
Maxim
21-Dec-2006
[6521]
yes... obviously, but sometimes, the value you want to set within 
your object is already used within the code that is populating it... 
it was just an example  :-)
Dirk
21-Dec-2006
[6522]
yeah, i see the difference. but i think the behaviour is a bit weird
Maxim
21-Dec-2006
[6523]
why?
Dirk
21-Dec-2006
[6524x3]
i would expect that time is identical to now after

time: now

but it's not.
forget it.

time: now
my-object: make object! [
  name: "me"
  mtime: time
]

behaves the same ...
time: time was the problem
Maxim
21-Dec-2006
[6527x2]
yes exactly.  that's what the compose allows.
obviously one can say well, just use other words... but... its not 
always pretty... actually it rarely is.
Dirk
21-Dec-2006
[6529]
one last question:

a: [ "a" "b" ]
how to write/append %to.txt a
so that
a b <- with whitespace inbetween 
gets written?
Maxim
21-Dec-2006
[6530x2]
btw, Ladislav has another more flexible function similar to compose... 
 its called 'BUILD   ( http://www.fm.vslib.cz/~ladislav/rebol/)
write/append %to.txt form a
Dirk
21-Dec-2006
[6532]
woot! thx
Maxim
21-Dec-2006
[6533]
glad I can help  :-)
Dirk
21-Dec-2006
[6534]
should have read strings, not blocks in the reference ....

Thanx for you help Maxim. Have to drive home now ...
Anton
26-Dec-2006
[6535]
Dirk, check out  compose/ONLY, eg:
>> blk: [1 2 3] compose/only [hello (blk)]
== [hello [1 2 3]]
Anton
28-Dec-2006
[6536x2]
I want to speed up access to a block of objects (unassociated) for 
a search algorithm.
Should I use LIST! or HASH! ?

It's a growing list of visited objects, and I'm searching it each 
time to see if the currently visited object has already been visited.
It looks like I should use HASH!, as it is designed to make lookup 
faster.
LIST! is designed for faster modifications.
Henrik
28-Dec-2006
[6538]
I wonder what the conversion time is between BLOCK!, LIST! and HASH! 
there must be some kind of penalty there.
Tomc
29-Dec-2006
[6539]
perhaps the objects could just contain a  visited field
Anton
29-Dec-2006
[6540]
No, I'm searching the system object; I can't modify the objects.
Tomc
29-Dec-2006
[6541x2]
ah
what are you keying off  of?
Anton
29-Dec-2006
[6543]
Search term is a word, but I want to support strings as well.
Gregg
29-Dec-2006
[6544]
If lookup speed is primary, use hash!, if you're doing a lot of inserts 
and removes, use list! There is overhead to has items for lookup, 
but the speedup is enormous on lookups..
Maxim
29-Dec-2006
[6545]
also remmember that lists are at their tails after inserts and append... 
this is usefull, but quite frustrating to discover when you don't 
know about it.
Anton
29-Dec-2006
[6546x2]
A quick test looks like using a hash didn't help in my case, the 
major work must be somewhere else.
Yes, I am aware of the index position confusion with list!.
Geomol
31-Dec-2006
[6548]
Is it possible to turn off: ** Math Error: Math or number overflow 
?

My gcc compiler here warn me of integer overflow, but I can run the 
C program, and bits are lost. But that can be ok in some cases, for 
example to calculate noise. I'm wondering, if I can do that in a 
fast way with REBOL. Example:
46341 * 46341

will produce an integer overflow. I just want the result with the 
high order bits lost.
Robert
31-Dec-2006
[6549]
Good question. I would like to avoid this error as well. A overflow 
would be OK in most cases.
Sunanda
31-Dec-2006
[6550]
Youd could bypass it with something like:
(1. * 46341 * 46341) // (2 ** 31)
Robert
31-Dec-2006
[6551]
How do you do this when you have a lot of claculation that use user 
input?