Darren's Weekly Nugget 03/30/2009

[Darren]

Here's a quick tip when dealing with path data types.  Sometimes we have to deal with paths that have a value of <Not A Path>.  A value of <Not A Path> can be formed in many different ways...one way might be if you're trying to build a path with the Build Path function, and you accidentally wire an empty string into the "name or relative path" input.  If you want to test how your code handles a value of <Not A Path>, it's not as simple as typing "<Not A Path>" in a path constant or control.  You must operate click the little path symbol on the left side of the control/constant and change its value from "Valid Path" to "Not a Path":

If you try to type the text "<Not A Path>" into the path control/constant, LabVIEW treats that value as a relative path to a folder/file named "<Not A Path>", which is *probably* not what you were wanting to check.  The Not A Number/Path/Refnum? function is the easiest way to programmatically tell if a path value is truly <Not A Path>.

Message Edited by Darren on 03-30-2009 06:12 PM

[D*]

That's good a tip, and reminds me of an interesting caveat that the Path to String and String to Path functions are not completely symmetric.  

For example, say you have a path with a value of <Not A Path>, the Not A Number/Path/Refnum? returns true.  BUT, when you convert this path to string, and then convert it back to the path type, the Not A Number/Path/Refnum? function no loner returns True!  This intuitively symmetric conversion does not preserve the <Not A Path> value.  As Darren pointed out, LabVIEW is now treating this as a folder/file named "<Not A Path>".

So if one anticipates that a path may have been converted from strings, it may be wise to check both Not A Number/Path/Refnum? and for the text "<Not A Path>".

[LabBEAN]

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 Darren said:

You must operate click the little path symbol on the left side of the control/constant and change its value from "Valid Path" to "Not a Path"

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As an integrator, we've gone almost exclusively to system controls to keep things from, dare I say, "looking like LabVIEW".  Any way to do what you've suggested with the System Path Control?  I can get the <Not A Path> type into the control, but not by copying / pasting.  Run the attached VI.  Notice that when you click in the first path control, it highlights "<Not A Path>".

-Jason

Message Edited by LabBEAN on 03-31-2009 02:05 PM

[Darren]

The little path symbol isn't present on the system path control, so I guess the only way to get a true <Not A Path> as the value of your control is to use the Not A Path Constant and wire it to a local variable of the control.

[parthabe]

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LabBEAN wrote:

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I can get the <Not A Path> type into the control, but not by copying / pasting. 

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I tried this, but both yielded the same results; I got False in both the cases.

The only way I could make the System path control to accept the <Not A Path> value is by placing ite constant on the BD & then changing it into a control.

[parthabe]

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Darren wrote:
The little path symbol isn't present on the system path control, so I guess the only way to get a true <Not A Path> as the value of your control is to use the Not A Path Constant and wire it to a local variable of the control.

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Still easier way is to place a <Not A Path> constant on the BD & change it into a Control.

[dan_u]

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D* wrote:

So if one anticipates that a path may have been converted from strings, it may be wise to check both Not A Number/Path/Refnum? and for the text "<Not A Path>".

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Don't forget to also check for an empty path.

[LabBEAN]

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parthabe wrote:

 I tried this, but both yielded the same results; I got False in both the cases.

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I downloaded the VI I posted above on another machine and the top evaluates True.  To get the <Not a Path> type into the top control (versus the string in the bottom control), I replaced the System control with Modern, set <Not a Path>, and repalced the Modern with a System -- an alternate method that doesn't involve diagram mods.

-Jason