Re: defined

[Daniel J S. <dan...@ie...>]

Ethan A Merritt wrote:
> On Monday 29 May 2006 05:53 pm, Daniel J Sebald wrote:
> 
>>>>gnuplot> print defined("foo")
>>>>141158968
>>>
>>>We don't have a user-visible TBOOLEAN type, 
>>>so any non-zero value represents "TRUE"
>>
>>Right.  The point is that the test and the result are meaningless.  
>>foo was never defined, so if anything the result should be zero.  
> 
> 
> I think you are looking at this the wrong way.
> The defined() operator is to protect you from trying to evaluate
> an expression with an undefined quantity.
> "foo" (with the quotes) is a legal string and hence a legal
> token to place in an expression.  So defined("foo") is TRUE.

OK, I see your perspective now; that the test is if and what the hunk of ascii code means to gnuplot's parser.  What exactly the utility of that is, I don't know.

> 
> This is no stranger than
> 	print defined(42)
> or	print defined(sin(x))
> or for that matter
> 	print (4 || 2)

I think it is stranger.  I mean, 4||2 is part of the language, the math rules.  Nowhere in 4||2 is there any kind of *variable*.  Now reading the documentation, I see:

Subtopic of functions: defined
 `defined(X)` returns 1 if a variable named X has been defined, otherwise
 it returns 0.

specifically "variable named X", key word "variable".

> 
> The problem is not in the syntax of defined(); the problem
> is that you should not be trying to "print" a BOOLEAN.  It would
> be nicer if 'print defined(1)' resulted in printing the string
> TRUE, but I don't see a way to do that. 

I actually think this should be easier; just look in the linked list for a definition matching.  If it is there, the variable is defined (1), otherwise (0).

Here's how Octave behaves:

octave:2> exist("foo")
ans = 0
octave:3> foo = 23
foo = 23
octave:4> exist("foo")
ans = 1
octave:5>

Should everything behave like Octave?  No, but one would think similar in concept, because afterall as in many languages gnuplot has similar concepts like x*y, sin(x)/y, etc.

> 
> If there is a bug lurking anywhere here, it is that 
> 	print defined(0)
> results in 0 rather than 1.
> 

The thing is that "defined()" is currently returning the definition of something, it is not answering the question as to whether something is defined.

I say it is much more than that.  I still don't see anything as to why the defined("foo") vs. defined(foo) should be any less confusing to a marginally familiar user; the programmer's perspective is different from the user's on this I think.  Also, I see little utility in defined("foo") or defined(1) in light of what the gnuplot documentation says.  In the same category as defined(0), there is

gnuplot> print defined(1/0)
               ^
         undefined value

This one is funny because certainly 1/0 has meaning in gnuplot, so why doesn't "defined(1/0)" evaluate to non-zero by it's current interpretation?  Or!  :-)  1/0 means an undefined value, so why doesn't "defined(1/0)" return 0?  Instead, it returns undefined value, which is not the same as

gnuplot> print defined(blah(x))
         undefined function: blah


I'll conclude this like the discussion of a couple weeks ago... Put me in the "if it's a defined variable 1, otherwise 0" camp, for what it's worth.

Dan