topic Re: arcpy: maximum of a field of a feature class in Python Questions

arcpy: maximum of a field of a feature class

RamB — Fri, 26 Apr 2019 12:34:25 GMT

I would like to get the maximum of a field in a feature class. If no features are selected, I get the right value. But if some features are selected, I am getting the maximum of a field from within these selected features. However, I would like to get maximum of a field from the whole table i.e., from both selected and un-selected features.

how may I do it?

Thank you

Re: arcpy: maximum of a field of a feature class

DanPatterson_Retired — Fri, 10 Dec 2021 22:21:54 GMT

in_fc = 'C:/Your_spaceless_path/CoordGeom.gdb/Polygons'

arr = arcpy.da.TableToNumPyArray(in_fc, 'Shape_Area')  # pick a field

# nan functions account for nulls, cast to the dtype of the field using astype 
np.nanmax(arr.astype('float'))  

155.0‍‍‍‍‍‍‍‍

Re: arcpy: maximum of a field of a feature class

RamB — Fri, 26 Apr 2019 13:12:33 GMT

Thank you, may be I was not clear. Your code gives same answer as mine. Let me try again.

Imagine you have 3 features [0,1,2] in a fc. They have a income field which is an integer [10,12,14] respectively .

case1: I have selected no feature selected, I ask for max of fc , I get 14 >> correct

case2: I have selected features [0,1], i ask for max of fc , I get 12 >> incorrect, I still want 14, because I am asking max of fc, not max of selection of fc.

case3: I have selected features [1,2], i ask for max of fc, I get 14, >> correct

I hope it clarifies a bit?

Re: arcpy: maximum of a field of a feature class

RamB — Fri, 26 Apr 2019 13:15:59 GMT

I mean, I am asking max of income field in fc.

Re: arcpy: maximum of a field of a feature class

JoeBorgione — Fri, 26 Apr 2019 13:45:53 GMT

Just need to ask: have you actually looked at the values of your selected versus non-selected and done the summation manually? Sounds like one of your fields is 0 or None value. What is your objective of comparing your selected sum versus the non-selected? If you know the sum of the income field without any selections, why not write that value to a variable if you need to make comparisons to it?

Re: arcpy: maximum of a field of a feature class

RamB — Fri, 26 Apr 2019 14:02:21 GMT

User selects few features, sets their income to max(income in whole of fc) +1 , so now the max of income of whole fc has changed. User goes to another area in the extent, selects a few polygons, sets their income to max(income in whole of fc) + 1, so the max of income in the whole fc has changed again. This continues till all polygons are finished.

Re: arcpy: maximum of a field of a feature class

DanPatterson_Retired — Fri, 26 Apr 2019 14:45:14 GMT

My answer gives you the maximum whether you have a selection or not.

If you need that maximum, that is the way to get it without worrying about selections or not.

If you need maximum-ish (ie there is a selection) then work with arcpy

If you want the best of both worlds, they you can work with both approaches

Re: arcpy: maximum of a field of a feature class

RamB — Mon, 29 Apr 2019 11:41:06 GMT

would your solution of accessing a disk file be affected by:

1. lock imposed by arcmap

2. hard-coded path on the file system?

Re: arcpy: maximum of a field of a feature class

DanPatterson_Retired — Mon, 29 Apr 2019 12:21:29 GMT

If you see the requirements for the function, it just requires the path to the featureclass's table. Locks aren't for reading, only modifying a featureclass.

You could try it and report back

Re: arcpy: maximum of a field of a feature class

LukeWebb — Fri, 10 Dec 2021 22:21:57 GMT

Dans Answer is correct.

in_fc = 'C:/Your_spaceless_path/CoordGeom.gdb/Polygons'

arr = arcpy.da.TableToNumPyArray(in_fc, 'Shape_Area')  # pick a field

# nan functions account for nulls, cast to the dtype of the field using astype 
np.nanmax(arr.astype('float'))  

155.0

If this is returning different numbers based on selection, then this is because you are not entering "in_fc" as shwon above.

E.g.

in_fc = 'C:/Your_spaceless_path/CoordGeom.gdb/Polygons'

Is different than:

in_fc = 'Polygons'

One points to a layer in a map, and will return results based on selections and things, the other points to data on a disk, and will return counts of whatever is inside that dataset.

The only possible thing that could go wrong using this, is if a user is inside an Edit session, then there features are not saved to the "dataset on disk" until they save Edits.

Re: arcpy: maximum of a field of a feature class

JoshuaBixby — Fri, 10 Dec 2021 22:22:00 GMT

Just use a Describe object to retrieve the underlying feature class of the feature layer.

featlyr_name = "rangerdistrict" # name of existing feature layer
fld_max = "GIS_ACRES" # field for determining maximum value

max(fld for fld, in arcpy.da.SearchCursor(featlyr_name,fld_max))

fc = arcpy.Describe(featlyr_name).featureclass.catalogPath
max(fld for fld, in arcpy.da.SearchCursor(fc,fld_max))
‍‍‍‍‍‍‍

Re: arcpy: maximum of a field of a feature class

RamB — Wed, 01 May 2019 11:28:05 GMT

Thank you Dan Patterson‌ Joshua Bixby‌ Luke Webb

I did two different approaches and both worked.

Approach 1. I first got a max for the selection, then switched the selection and got a second max, then took max of the both. I felt it was slow, but it works and this switching also updated my labels in display, which kind of confirmed the display of new max assigned to the features.

Approach 2. I did not follow the advice of giving a hard coded path, because user can have the file anywhere on the system. So instead, I used layer.dataSource to get the path. Then I used np.amax to get the max of the array (attribute table).

So broadly I followed the advice of path and the advice of np.array and also liked the list comprehension technique by Joshua. that also works.