Adding script with math functions to ModelBuilder?

Question

I am very new to ModelBuilder + python.

I am trying to make a model. Don't know if it matters to upload it. If you want i can.

I want to calculate variance from a points-shapefile with elevation data and want to add this script to my model:

### x = (column: GRID_CODE, from a shapefile from model builder)
average = sum(x) / len(x)
variance = sum((average - value) ** 2 for value in x) / (len(x)-1)
### output to a new shapefile

How could i make this script working? Searched a lot here and at arcgis help but couldn't find something.

Can't understand how to "grab" data from shapefile, process them through script and then print the output.

Answer 1

if i understand the question - one way would be to use a SearchCursor - the cursor can vary slightly depending on your version of arcgis, but basically use the search cursor to total the value and calculate the average and variance. something similar to:

numberOfRows=0
total=0
data=[]
with arcpy.da.SearchCursor('yourshapefile.shp','GRID_CODE') as sCursor:
 for row in sCursor:
 total=row[0]+total
 data.append(row[0])
 numberOfRows+=1
ave=total/numberOfRows

you could then calculate the variance (using your equation)

var=sum([(ave-value)**2 for value in data])/(numberOfRows-1)

or typically numpy is also installed with arcgis, so you could import numpy as np and use

var=np.var(data)

which you could simply print to standard output.