Review zero divide zro as well: the proper framework is: how you define the
functor nvolved to respond propely within the domain of the context you wish
it to operate in.
Wrt infinity 9or ideterminant), the caution and hazards th ROger is rising
are entirely proper.
HOWeVr, if you fully deifne the proper domai and rnge of ll functiors
involved, nd know exactly wht you are doing, it _ca_ cary meanng,
in the same wy that "undfined" can you jut hav to b exactly cautious with
it, simlr o the way that other mathematicians
avoid zero ivide zero, and earlier 8th century Catholic monks tended to
freak out with subtraction of a quantity larger than the base
value alrady in hand: before _negativs_ we recognized and properly
codified.
It dos not mean anytyhing until defined, and is useless until a use is
properly found for it.
Witness the lack of respect for imaginary numbers until engieneering found a
use for them in th the ecpreson,
for example of fluid flows under earthen dams (nd a hole (not a typo) host
of uses thereafter).
BUT:L you MUST be cautious whe initially fuly establishng all of the side
effects involved with a different codification, or,
more correctly:
A different _Mathematics_.
Cheers.
On 6/20/06, Oleg Kobchenko <[EMAIL PROTECTED]> wrote:
- That was a concrete question, so it should have
been put under a separate subject.
- It is related to programming, so it should be in Programming forum
- If anyone has nothing to say concretely on the subject,
related to J language, better stay away from this thread (I don't mean
John).
Variables are called "names" in J: pronoun, provers, etc.
nc NB. To get a name class, there is a "name class" conjunction
4!:0
b=. 1
c=. +
nc ;:'b c' NB. Class is the same you put on right of explicit
definition ":"
0 3
a + 1
|value error: a
| a+1
nc <'a' NB. undefined (unassigned) name returns _1
_1
You can have a "default for name" conjunction which can
substitute avalue unless the name is defined.
a dn 2 + 1
3
b dn 2 + 1
2
If name is a verb the "adverse" (::) will also work
2 d 1
|value error: d
| 2 d 1
2 d ::+ 1
3
Null is assigned in languages with reference types,
as pointers to nothing.
In J contents of names is always a value type.
So to "clear" a name, you simple erase it.
erase <'b'
1
b + 1
|value error: b
| b+1
----- Original Message ----
From: John Randall <[EMAIL PROTECTED]>
To: General forum <[email protected]>
Sent: Tuesday, June 20, 2006 7:42:02 PM
Subject: Re: [Jgeneral] Mathematical Roots of J & more musings
dly wrote:
> By undefined in J
> do you mean a variable to which no value has been assigned?
> an attribute that is defined for some but not all?
> is it associated with something like null which you can test for?
> how does J handle the undefined?
J sometimes assigns definite values to expressions some would consider
indeterminate. For heated debate on this topic, search the archives for
discussions on 0^0 (I do not want to reopen it).
J has a primitive -. (Indeterminate). The Dictionary entry, in full,
reads:
The indeterminate _. results from expressions such as _-_ (infinity minus
infinity) and from expressions (such as 3+_.) in which an indeterminate
argument occurs.
The official advice from Roger is:
However, use of the indeterminate at any time in any circumstance is not
recommend. I consider it a data virus, almost as bad as -0 (minus 0).
Best wishes,
John
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