Using IF/ELSE Statement for field calculation in ArcPy?

Question

I would like to calculate the slope value in %, but I don’t want to have any negative values. If there are negative values in the slope field, I would like to multiple them by -1 and if not, multiple them by +1 (or do nothing).

My input is a line shapefile. First I added a field called "slope", then I calculate the slope based on other fields.

enter image description here

arcpy.AddField_management(ShapeFileLine, "Slope", "DOUBLE", 10, 3)
arcpy.CalculateField_management(ShapeFileLine, "Slope", '((((!END_Z! - !START_Z!)/!LENGTH_3D!))*100)!',"PYTHON")

What I should do if I want I only have positive values? In this case they are all negatives, but sometimes the positive and negative values are mixed

Answer 1

I agree with @smiller answer about how to do an If Else in a code block.

However, in this particular case, you could also just use the absolute value function which will always return the positive version of a number, regardless if the input is positive or negative https://docs.python.org/2/library/functions.html#abs

Which means it would look something like:

arcpy.CalculateField_management(ShapeFileLine, "Slope", 'abs((((!END_Z! - !START_Z!)/!LENGTH_3D!))*100)',"PYTHON")

(FYI: It looked like you had an extra ! at the end of your code block originally, so I removed that because it looked like a miss-type.)

• Anahita Kp: yes. you are right! I have tested both of the suggestions. they worked.@smiller @John

Answer 2

Include a codeblock to use if/else.

expression = "posSlope(!Slope!)"
codeblock = """def posSlope(slope):
 if slope < 0:
 return slope * -1
 else:
 return slope""" 
arcpy.CalculateField_management(ShapeFileLine, "Slope", expression, "PYTHON_9.3", codeblock)

Or if you wanted to do it all at once (I assume startz/endz are both float):

expression = "calcSlope(!END_Z!, !START_Z!, !LENGTH_3D!)" 
codeblock = """def calcSlope(endz, startz, length3d):
 slope = ((endz - startz)/length3d) * 100 
 if slope < 0:
 return slope * -1
 else:
 return slope""" 
arcpy.CalculateField_management(ShapeFileLine, "Slope", expression, "PYTHON_9.3", codeblock)

Another example: https://community.esri.com/thread/87299

• Anahita Kp: it works. but the quotations needs to be edited.

Answer 3

Just take the absolute value. For example, using an UpdateCursor in arcpy:

import arcpy
fc = r'C:/path/to/your.gdb/featureclass'
with arcpy.da.UpdateCursor(fc, "Slope") as cursor:
 for row in cursor
 row[0] = abs(row[0])
 cursor.updateRow(row)