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"Bald Eagle" <cre### [at] netscape net> wrote:
>
> I see where it's going wrong, and this will give you an idea of why:
>
> #switch(true)
>
> #case(1)
> rgb <0, 0, 0.2>
> #break
>
> #case(
> // no (strcmp(IMG_TYPE,AA), which is the only 'correct' match
> (strcmp(IMG_TYPE,BB))
> | (strcmp(IMG_TYPE,CC))
> )
>
> #switch(true) is a very strange usage, and I'm sure clipka could give some
> source-code explanation of why it works the way it does.
> But basically, as I suspected, it just selects the FIRST #case() regardless of
> what it is.
But here's a very similar example (though without any ORs, which might make all
the difference), and it selects the 2nd #case (green)...
#switch(true)
#case(0)
rgb <0, 0, 0.2> // BLUE
#break
#case(1)
rgb <0,1,0> // GREEN
#break
....
This makes sense to me (well, at the moment, anyway!), as it looks like
#switch(true) is picking the #case with the appropriate 'true' result of (1)
>
> In fact, check out:
>
> #switch(strcmp(IMG_TYPE,BB) | strcmp(IMG_TYPE,CC) )
>
> #case(-10)
> rgb <1, 1, 0>
> #break
>
> #case(-6)
> rgb <1, 1, 1>
> #break
>
> #case(1)
> rgb <0, 0, 0.2>
> #break
>
> #case(1000)
>
> It selects the blue pigment, which corresponds to 1.
*That's* a really unexpected result, IMO; neither of the #switch's strcmp()s
'match' IMG_TYPE (which would be AA), yet #case(1) is chosen as if #switch is
'true.' And looking at it in a different brain-twisting way (one that's
incorrect, no doubt), it seems that either #case(-6) or #case(-10) should be
chosen, but certainly not #case(1)
It all makes my brain hurt :-O
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clipka <ano### [at] anonymousorg> wrote:
>
> Also, there's a much easier way to achieve a logical OR in any `#switch`
> construct: Just place multiple `#case` statements without a `#break` in
> between, like so:
>
> #switch(0)
> #case(strcmp(STRING,CONST1A))
> #case(strcmp(STRING,CONST1B))
> // go here if STRING = CONST1A or CONST1B
> #break
> #case(strcmp(STRING,CONST2A))
> #case(strcmp(STRING,CONST2B))
> // otherwise go here if STRING = CONST2A or CONST2B
> #break
>
I didn't know that using #cases without #breaks was possible :-O Thanks! That's
the construction I will probably use; it's nice and elegant. In fact, so
interesting that a note in the documentation about it would be justified ;-)
BTW, I almost never use #switch(true) in my scenes, usually just #switch(1) or
#switch(0)-- believing that they are the 'logical equivalents' of true/false,
not just the 'number values' 1 and 0. It has always worked for me, but is my
assumption valid, in light of all that's been said?
Another question, just to clarify something: Is there a difference between the
following two examples? (I don't actually code the latter way-- it may not even
be appropriate-- but I've seen various types of non-strcmp() constructions in
the newsgroups that look similar)...
#switch(0)
#case(strcmp(IMG_TYPE,AA))
and
#switch(0)
#case(strcmp(IMG_TYPE,AA)=0)
I ask because the latter doesn't work--no error message of any kind is created,
but it falls through to the #else clause, even though strcmp(IMG_TYPE,AA) is a
'match' between IMG_TYPE and AA.
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"Kenneth" <kdw### [at] gmailcom> wrote:
> But here's a very similar example (though without any ORs, which might make all
> the difference), and it selects the 2nd #case (green)...
>
> #switch(true)
> #case(0)
> rgb <0, 0, 0.2> // BLUE
> #break
> #case(1)
> rgb <0,1,0> // GREEN
> #break
> ....
>
> This makes sense to me (well, at the moment, anyway!), as it looks like
> #switch(true) is picking the #case with the appropriate 'true' result of (1)
Right. "true" "on" "yes" all evaluate to 1 in SDL.
> *That's* a really unexpected result, IMO; neither of the #switch's strcmp()s
> 'match' IMG_TYPE (which would be AA), yet #case(1) is chosen as if #switch is
> 'true.'
Yes, (now that I've gotten some sleep, and looked at the #debug output) I see
that's the case. I probably won't have an opportunity to run any more tests
until tonight.
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"Kenneth" <kdw### [at] gmailcom> wrote:
> #switch(0)
> #case(strcmp(IMG_TYPE,AA))
This should work, as strcmp(IMG_TYPE,AA) _IS_ 0.
> #switch(0)
> #case(strcmp(IMG_TYPE,AA)=0)
>
> I ask because the latter doesn't work--no error message of any kind is created,
> but it falls through to the #else clause, even though strcmp(IMG_TYPE,AA) is a
> 'match' between IMG_TYPE and AA.
Which means that it evaluates to TRUE, or 1, which is NOT 0.
Stop thinking in terms of conditional Boolean word results, and think in 0 or 1.
#if (strcmp(IMG_TYPE,AA)=0)
would give a Boolean result of 1.
I think you're stuck thinking about #switch() and #case() as some sort of
semi-Boolean operator, when it's more of a "pick this value from the following
list" function. Maybe if you think about the whole block like an array [n] and
the #case statements are picking a specific array item (sort of).
{pseudocode}
#if (Case = n)
#local Return = Array [n]
#end
and
#if (Range > Low & Range < High)
#local Return = Array [some appropriate n]
#end
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Am 08.11.2017 um 05:40 schrieb Kenneth:
> I didn't know that using #cases without #breaks was possible :-O Thanks! That's
> the construction I will probably use; it's nice and elegant. In fact, so
> interesting that a note in the documentation about it would be justified ;-)
It's hinted at very briefly in the v3.7 docs.
> BTW, I almost never use #switch(true) in my scenes, usually just #switch(1) or
> #switch(0)-- believing that they are the 'logical equivalents' of true/false,
> not just the 'number values' 1 and 0. It has always worked for me, but is my
> assumption valid, in light of all that's been said?
`#switch(0)` and `#switch(1)` are perfectly equivalent to
`#switch(false)` and `#switch(true)` respectively, but for code clarity
I would recommend the former when you're dealing with actual numbers
(such as the return value of `strcmp()`), and the latter when you're
dealing with logical expressions (such as the result of the `=` operator).
> Another question, just to clarify something: Is there a difference between the
> following two examples? (I don't actually code the latter way-- it may not even
> be appropriate-- but I've seen various types of non-strcmp() constructions in
> the newsgroups that look similar)...
>
> #switch(0)
> #case(strcmp(IMG_TYPE,AA))
>
> and
>
> #switch(0)
> #case(strcmp(IMG_TYPE,AA)=0)
Yes, absolutely!
If IMG_TYPE is equal to AA, then `strcmp(IMG_TYPE,AA)` will yield 0.
Comparing that result with zero will yield true, i.e. some non-zero number.
So with IMG_TYPE=AA, the first case would resolve to:
#switch(0)
#case(0) // representing `zero difference`
// (we'll enter here, because 0 = 0)
while the second case would resolve to:
#switch(0)
#case(1) // representing `true`
// (we'll NOT enter here, because 0 <> 1)
> I ask because the latter doesn't work--no error message of any kind is created,
> but it falls through to the #else clause, even though strcmp(IMG_TYPE,AA) is a
> 'match' between IMG_TYPE and AA.
The latter case would generate an error or at least a warning in most
contemporary languages, as it is now considered good practice to clearly
distinguish between boolean an numeric values. Even in C, where use of
`int` for boolean values has a long-standing tradition, a dedicated
boolean type has been added by now.
However, POV-Ray's rusty SDL is particularly poor at distinguishing
between values representing entirely different things (another example
being colours vs. vectors), so it can't warn you that you might have a
problem there.
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Am 08.11.2017 um 12:28 schrieb Bald Eagle:
> I think you're stuck thinking about #switch() and #case() as some sort of
> semi-Boolean operator, when it's more of a "pick this value from the following
> list" function. Maybe if you think about the whole block like an array [n] and
> the #case statements are picking a specific array item (sort of).
I would pick the analogy of a `#if...#elseif...#end` construct:
#switch(VALUE)
#case(COMPARE1)
// branch 1
#break
#case(COMPARE2)
// branch 2
#break
...
#default
// branch Z
#break
#end
is fully equivalent (presuming all values are numeric) to:
#if(VALUE=COMPARE1)
// branch 1
#elseif(VALUE=COMPARE2)
// branch 2
...
#else
// branch Z
#end
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Am 08.11.2017 um 04:59 schrieb Kenneth:
> "Bald Eagle" <cre### [at] netscapenet> wrote:
>>
>> I see where it's going wrong, and this will give you an idea of why:
>>
>> #switch(true)
>>
>> #case(1)
>> rgb <0, 0, 0.2>
>> #break
>>
>> #case(
>> // no (strcmp(IMG_TYPE,AA), which is the only 'correct' match
>> (strcmp(IMG_TYPE,BB))
>> | (strcmp(IMG_TYPE,CC))
>> )
>>
>> #switch(true) is a very strange usage, and I'm sure clipka could give some
>> source-code explanation of why it works the way it does.
>> But basically, as I suspected, it just selects the FIRST #case() regardless of
>> what it is.
>
> But here's a very similar example (though without any ORs, which might make all
> the difference), and it selects the 2nd #case (green)...
>
> #switch(true)
> #case(0)
> rgb <0, 0, 0.2> // BLUE
> #break
> #case(1)
> rgb <0,1,0> // GREEN
> #break
> .....
>
> This makes sense to me (well, at the moment, anyway!), as it looks like
> #switch(true) is picking the #case with the appropriate 'true' result of (1)
>>
>> In fact, check out:
>>
>> #switch(strcmp(IMG_TYPE,BB) | strcmp(IMG_TYPE,CC) )
>>
>> #case(-10)
>> rgb <1, 1, 0>
>> #break
>>
>> #case(-6)
>> rgb <1, 1, 1>
>> #break
>>
>> #case(1)
>> rgb <0, 0, 0.2>
>> #break
>>
>> #case(1000)
>>
>> It selects the blue pigment, which corresponds to 1.
>
> *That's* a really unexpected result, IMO; neither of the #switch's strcmp()s
> 'match' IMG_TYPE (which would be AA), yet #case(1) is chosen as if #switch is
> 'true.' And looking at it in a different brain-twisting way (one that's
> incorrect, no doubt), it seems that either #case(-6) or #case(-10) should be
> chosen, but certainly not #case(1)
>
> It all makes my brain hurt :-O
The `#switch` statement expression has no special treatment of logical
operators.
The logical-OR operator `A|B` returns `false` (0) if both A and B are
equivalent to `false` (0); otherwise, it returns or `true` (non-0, in
this case 1).
However, the `strcmp(A,B)` function does /not/ return a boolean value,
but a numerical one: Negative if A<B, positive if A>B, and zero if A=B.
It is therefore highly recommended to always and immediately compare the
`strcmp()` result with zero (unless you know exactly what you're doing),
which turns it into a reasonably easy-to-grasp logical expression.
For example, to examine whether A is "larger" than B, you would write
`strcmp(A,B)>0`.
Then again, with v3.7 you should really use `A>B`.
If you really want to use the `strcmp()` result directly as a boolean
value, you should ditch the idea of it as an equality test, and think of
it as a /non-equality/ test instead: The result will be non-zero (and
thus equivalent to `true`) if the two strings /differ/.
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"Kenneth" <kdw### [at] gmailcom> wrote:
> "Bald Eagle" <cre### [at] netscapenet> wrote:
> > In fact, check out:
> >
> > #switch(strcmp(IMG_TYPE,BB) | strcmp(IMG_TYPE,CC) )
> >
> > #case(-10)
> > rgb <1, 1, 0>
> > #break
> >
> > #case(-6)
> > rgb <1, 1, 1>
> > #break
> >
> > #case(1)
> > rgb <0, 0, 0.2>
> > #break
> >
> > #case(1000)
> >
> > It selects the blue pigment, which corresponds to 1.
>
> *That's* a really unexpected result, IMO; neither of the #switch's strcmp()s
> 'match' IMG_TYPE (which would be AA), yet #case(1) is chosen as if #switch is
> 'true.' And looking at it in a different brain-twisting way (one that's
> incorrect, no doubt), it seems that either #case(-6) or #case(-10) should be
> chosen, but certainly not #case(1)
It seems to me that none of the individual cases should be chosen, as neither of
the Boolean expressions evaluate to 1, which means the logical OR should return
zero - so -10, -6, 1, and 1000 should ALL be skipped, and any extant #else
should selected instead.
No POV-Ray on _this_ computer, so I can't test anything for the next 8 hours.
(which is why I'd love a simple editor/parser-no-render combo, or a true
stand-alone POV-Ray ("USB") version)
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Kenneth wrote...
> #switch(0)
> #case(strcmp(IMG_TYPE,AA)=0)
>
> I ask because the latter doesn't work--no error message of any kind is created,
> but it falls through to the #else clause, even though strcmp(IMG_TYPE,AA) is a
> 'match' between IMG_TYPE and AA.
Bald Eagle wrote...
> Which means that it evaluates to TRUE, or 1, which is NOT 0.
> Stop thinking in terms of conditional Boolean word results, and think in 0 or 1.
> #if (strcmp(IMG_TYPE,AA)=0)
> would give a Boolean result of 1.
And Clipka wrote...
> If IMG_TYPE is equal to AA, then `strcmp(IMG_TYPE,AA)` will yield 0.
> Comparing that result with zero will yield true, i.e. some non-zero number.
Ah! OK, this finally makes sense to me! Thanks to you both. (All of this stuff
reminds me of my college days, when I struggled mightily to grasp the
complexities of Boolean operations. Ugh. It made my brain hurt then too.)
Kenneth wrote...
> I didn't know that using #cases without #breaks was possible :-O Thanks!
> ...a note in the documentation about it would be justified ;-)
Clipka...
> It's hinted at very briefly in the v3.7 docs.
Oops, you're right (as I see after re-reading the docs for the 137-th
time...ha...)
"If a clause evaluates true but no #break is specified, the parsing will fall
through to the next #case or #range and that clause conditional is evaluated."
So apparently it also works if a clause evaluates 'false' (the pseudo-OR
behavior.)
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"Bald Eagle" <cre### [at] netscapenet> wrote:
>
> No POV-Ray on _this_ computer, so I can't test anything for the next 8 hours.
> (which is why I'd love a simple editor/parser-no-render combo, or a true
> stand-alone POV-Ray ("USB") version)
I seem to remember that someone in the newsgroups mentioned running POV-Ray from
a USB stick.(?) In modern computers, isn't it treated like 'just another hard
drive'? (Sorry, I've never tried running apps that way, so I don't know.)
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