- 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 ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
