Expression

[Terry Reedy]

Lydia wrote:
> I am working on calculating one of the fields in a feature class based 
> on other 2 fields. The logic is,
> A (the resulting field) is calculated from B, but C and D have part of 
> the value that could fill the blank of B, which meaning that combine 
> three fields of values can make A.
>  
> Field A is what I need.
>  
> The data looks like: .
>  
> A   B   C   D
>  2  2
>  5  5
>  4        4
>  6               6
>  
>  
>  cur = gp.UpdateCursor(data)
>     row = cur.Next()
>     gp.CalculateField_management(data, "A", "[B]", "VB", "")

Those indents are wrong, and would cause a syntax error, so this must 
not be the code you actually ran.  When posting, please copy and paste 
instead of retyping.

Does the last line set A from B? Should it be inside the loop?

>  while row:
>  
>         cur.UpdateRow(row)

Or does this magically set A from B?

>  
>         if  not(row.GetValue("C") == 'NULL'):

This should be the same as
     if row.GetValue("C") != 'NULL':
which I find easier to read.

 From what you said before, you only want to set A from C or D if B is 
blank.  If so, this section should be indented under
     if row.GetValue("B") == 'NULL':

>               row.SetValue("A",row.GetValue("C"));
>  
>         elif not(row.GetValue("D") == 'NULL'):
>               row.SetValue("A",row.GetValue("D"));
>             
>         row = cur.Next()
>        
>     del cur
>     del row

Again, indent is erroneous.


> But the out looks like only B was calculated to A successfully. C&D are 
> not in A.

I have no idea why not.  A main way to debug is to insert print 
statements into the code in appropriate places and see if what is 
printed matched what you expect.

> I guess there must be something wrong with the code, but I am very new 
> to Python, and not familiar with the expression. Could anybody help ?  
> PS. I am coding Python with ARCGIS.

I am not familiar with ARCGIS.

tjr