topic Re: sequential difference in a single column with field calculator in Spatial Data Science Questions

sequential difference in a single column with field calculator

youngpark — Sun, 27 May 2018 15:45:42 GMT

Hello,

I have a column for elevation for a point data set. I would like to create a new column to show a difference between Row(n) and Row(n+1) values. well, I assume that the easiest way is calculating in EXCEL (=abs(A1-A2)) then make a join at ArcGIS. However, I would like to the process with "Field calculator" Does anyone know how to do it?

Thanks in advance

Re: sequential difference in a single column with field calculator

XanderBakker — Fri, 10 Dec 2021 22:04:57 GMT

To clarify something... you cannot read the next row and store the result in the current row. You can however, remember the previous row and store the difference in the current row. See the example below in ArcGIS Pro (same thing will apply in ArcMap):

You will have to use the Python parser and use the following code in the code block:

prev_val = None
def SequentialDif(curr_val):
    global prev_val
    if (prev_val == None): 
        prev_val = curr_val 
    dif_val = curr_val - prev_val
    prev_val = curr_val
    return dif_val‍‍‍‍‍‍‍‍

And use this as expression:

SequentialDif(!YourFieldName!)‍

Each record will have the difference between the current and the previous record of the given field.

Re: sequential difference in a single column with field calculator

DanPatterson_Retired — Sun, 27 May 2018 20:23:26 GMT

Is there any reason you abandoned your duplicate thread?

https://community.esri.com/message/772505-re-field-calculator-to-calculate-diff-row-values-in-a-single-column?commentID=772505#comment-772505

Re: sequential difference in a single column with field calculator

DanPatterson_Retired — Fri, 10 Dec 2021 22:05:00 GMT

and if you need the absolute difference, you can get that as well during the calculation process

Python parser of course

Code block below

Expression .... diff_between( !YourFieldName!, absol=True)

""" -----------------------------------------
diff_between(fld)
input:      a field
returns:    difference between successive records
expression: diff_between(!Shape!)
"""
val = 0.0
is_first = True
def diff_between(fld, absol):
    global val
    global is_first
    if is_first:
        is_first = False
        val = fld
        return 0.0
    diff = fld - val
    val = fld
    #val = diff
    if absol:
        return abs(diff)
    else:
        return diff
#__esri_field_calculator_splitter__
#diff_between(!YourFieldHere!, True)‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: sequential difference in a single column with field calculator

youngpark — Sun, 27 May 2018 23:09:36 GMT

Thanks Dan,

I should have changed just the field of question. I really appreciate your helps.