Bug report #8457
$length fails silently for polygons
| Status: | Closed | ||
|---|---|---|---|
| Priority: | Normal | ||
| Assignee: | - | ||
| Category: | Vectors | ||
| Affected QGIS version: | master | Regression?: | No |
| Operating System: | Easy fix?: | No | |
| Pull Request or Patch supplied: | No | Resolution: | duplicate |
| Crashes QGIS or corrupts data: | No | Copied to github as #: | 17225 |
Description
I have a polygon. If I label it with the following it works fine. 'polygon' || $area
However, if I change it to the below, using $length, I get absolutely no output, not even the string. 'polygon' || $length
Yes, I know I should use $perimeter. The bug is that QGIS isn't telling me as the user that. Worse, it's not rendering anything that is still valid (the string).
Both the "Output preview" and the on-map labels are completely empty.
windows 7 x64 - QGIS a13cdf8
Related issues
History
#1
Updated by Nathan Woodrow about 11 years ago
$legnth is returning NULL. String + NULL = NULL.
#2
Updated by Jonathan Moules about 11 years ago
Also field calculator does the exact same thing, so it's not labelling specific.
Nathan Woodrow wrote:
$legnth is returning NULL. String + NULL = NULL.
Maybe it should be converted to:
String + NULL = String.
After all, 1 + 0 = 1, and String + '' = String. I'm aware that NULL != 0 or '' but I think the premise holds :-)
#3
Updated by Nathan Woodrow about 11 years ago
We have had this talk before. It sounds great but like you said NULL != 0 or '' so we are not going to treat NULL as such. It's not going to change at the moment, not for 2.0 anyway.
#4
Updated by Nathan Woodrow about 11 years ago
- Category deleted (
Labelling)
#5
Updated by Jonathan Moules about 11 years ago
Ah, Ok.
I think outside of the Operator functions, the user likely isn't going to care about the difference between NULL, 0 and '' and they can be treated as the same. But then I come from a PHP background so... heh.
#6
Updated by Giovanni Manghi about 11 years ago
- Category set to Vectors
- Status changed from Open to Feedback
Can we change this to something like "add a warning when using the $length function with polygons"?
#7
Updated by Jonathan Moules about 11 years ago
I'm not sure it should be renamed to that as it's not unique to length.
If I try "$x" and "$y" with a polygon they also fail (though I'd argue that's a problem with the calculator - I expected it to return the centroid, but that's neither here nor there for this). $geometry is also blank (no help so not sure what it's supposed to do).
I imagine that area\\length\\perimeter all become blank when used against a POINT feature.
So the issue is more generic than a single function.
#8
Updated by Giovanni Manghi about 11 years ago
we can rename/rephrase the ticket anyway as a feature request like "make the FC handle better some cases"
#9
Updated by Nathan Woodrow about 11 years ago
- Resolution set to duplicate
- Status changed from Feedback to Closed
In that case it's the same as #8369