Typecast - What does "*(type *) &x" mean?

[JackDunaway]

Earlier today I was showing the LabVIEW Typecast function to a Perl developer, and I got to thinking to myself: "What does the punctuation (operators?) on the output of that function mean?"

Can anyone answer the question: What does "*(type *) &x" mean?

[Darin]

&x is the address of x

(type *) is a coercion to pointer to type

* is dereferencing value at x treated as type.

[vugie]

There are few others descriptions of terminals understandable only for text programmers (if not for C-programmers strictly):

x-y*floor(x/y) - reminder

r*e^(i*theta) - complex number

s?t:f - select

NaN - not a number

[asbo]

  On 3/15/2012 at 7:59 AM, vugie said:

s?t:f - select

Trivia: this is referred to as a ternary operation.

[crelf]

  On 3/15/2012 at 7:59 AM, vugie said:

s?t:f - select

  On 3/15/2012 at 2:41 PM, asbo said:

Trivia: this is referred to as a ternary operation.

More trivia: ...and is the same format for a Boolean selection in TestStand expressions.

[asbo]

  On 3/15/2012 at 3:42 PM, crelf said:

More trivia: ...and is the same format for a Boolean selection in TestStand expressions.

That's because TestStand expressions are modeled after text-based languages like C.

[JackDunaway]

  On 3/15/2012 at 5:56 AM, Darin said:

&x is the address of x

(type *) is a coercion to pointer to type

* is dereferencing value at x treated as type.

Thanks! That syntax was teetering just above my level of C syntax comprehension (rather, tolerance? )

  On 3/15/2012 at 7:59 AM, vugie said:

There are few others descriptions of terminals understandable only for text programmers (if not for C-programmers strictly):

x-y*floor(x/y) - reminder

r*e^(i*theta) - complex number

s?t:f - select

NaN - not a number

Another notable carry-over is the use of "[]" to signify "array". You can find this C syntax many of the primitive icons themselves, not just the terminal names.

(And just for the record: generally, it makes sense to reuse these conventions in LabVIEW rather than invent new analogies. The typecast example is one of the few instances where I might consider breaking this convention, just because the syntax is confusing)

[crelf]

  On 3/15/2012 at 5:05 PM, JackDunaway said:

And just for the record: generally, it makes sense to reuse these conventions in LabVIEW rather than invent new analogies.

Absolutely! Another reason I like to use the OpenG comment, rather than other arbitrary free text labels.

[asbo]

  On 3/15/2012 at 5:31 PM, crelf said:

Absolutely! Another reason I like to use the OpenG comment, rather than other arbitrary free text labels.

Ah, this is actually one I don't like. The syntactic significance of /* ... */ is moot in LabVIEW - what else could a free label be used for on the block diagram but to comment? I particularly dislike the color contrast chosen for the OpenG comment. I see no value-add except having more colors on your BD.

I whole-heartedly embraced the square-bracket array notation (but rarely find cases where I think its necessary) and think that the typecast terminals are only misleading in that they use the & and * operators in a language that's almost exclusively by-value. I remember the first time I read that terminal, I got a little warm and fuzzy feeling for knowing exactly what it meant, though. Ah, C, I miss you sometimes...

[crelf]

  On 3/15/2012 at 6:09 PM, asbo said:

Ah, this is actually one I don't like.

Well then you're a stupid poo bum doodie head.

  On 3/15/2012 at 6:09 PM, asbo said:

The syntactic significance of /* ... */ is moot in LabVIEW - what else could a free label be used for on the block diagram but to comment?

Lots of things - like custom classes of tags that cen be read by scripting nodes (eg: run a report that scans free text labels that are a particular color or start with a particular tag to show all those "fixme" notes).

  On 3/15/2012 at 6:09 PM, asbo said:

I particularly dislike the color contrast chosen for the OpenG comment. I see no value-add except having more colors on your BD.

It wasn't as much chosen for OpenG, it's an already accepted standard in multiple programming languages - to make it something else is unintuative.

[asbo]

  On 3/15/2012 at 6:24 PM, crelf said:

Well then you're a stupid poo bum doodie head.

Elegantly put. I redact my comments.

  On 3/15/2012 at 6:24 PM, crelf said:

Lots of things - like custom classes of tags that cen be read by scripting nodes (eg: run a report that scans free text labels that are a particular color or start with a particular tag to show all those "fixme" notes).

It wasn't as much chosen for OpenG, it's an already accepted standard in multiple programming languages - to make it something else is unintuative.

This sounds like a weak case to use scripting to dig through property nodes. Personally, sticking a "TODO:", "DEBUG:", or "FIXME:" just as easily facilitates finding these comments without requiring that the free label be of any specific format. I recognize that it is a standard, but argue that it's simply one that does not apply.

Keep an eye on your svn logs, I'm going to start changing all of your comments.

[crelf]

  On 3/15/2012 at 6:38 PM, asbo said:

Elegantly put. I redact my comments.

Damn right. I choose to stop reading your post at this point.

[hooovahh]

  On 3/15/2012 at 7:18 PM, crelf said:

Damn right. I choose to stop reading your post at this point.

I hate it when mom and dad fight...

[crelf]

  On 3/15/2012 at 7:49 PM, hooovahh said:

I hate it when mom and dad fight...

I call "dad".

[asbo]

[jgcode]

This thread should be pinned. Hilarious!

[crelf]

  On 3/15/2012 at 3:51 PM, asbo said:

That's because TestStand expressions are modeled after text-based languages like C.

PS: wikipedia page updated to include TestStand.

[asbo]

  On 3/16/2012 at 5:26 PM, crelf said:

PS: wikipedia page updated to include TestStand.

I added some formatting and expanded it a bit. I was going to come up with an example of using completely different functions in each sub-expression, but got lazy.

[Aristos Queue]

  On 3/15/2012 at 5:40 AM, JackDunaway said:

Can anyone answer the question: What does "*(type *) &x" mean?

It means a LabVIEW developer momentarily forgot that his/her users were not C++ programmers, the tech writer was on vacation, and this was written so long ago that when I asked if we could change it, the answer I got was, "No, that's the label of the terminal, which means someone might be using it in scripting code, and you'll break their code if you change it now."

  On 3/15/2012 at 7:59 AM, vugie said:

NaN - not a number

That one doesn't count. That's an IEEE standard definition of the numerical value of Not A Number. That's correct as it stands, not a C++-ism that should have been scrubbed back in the dawn of LabVIEW.

  On 3/15/2012 at 7:49 PM, hooovahh said:

I hate it when mom and dad fight...

At least he's not drinking tonight. "Two beds and a coffee machine..."

*

* Hah! Take that stupid editor. Try to inline my media files when I don't want you to... grrr...

Edited March 16, 2012 by Aristos Queue

[crelf]

  On 3/16/2012 at 6:07 PM, asbo said:

I added some formatting and expanded it a bit. I was going to come up with an example of using completely different functions in each sub-expression, but got lazy.

Yeah, I think keeping the terms the same as the previous example works. Also, I added "()" for clarity.