topic Re: calculation within an IF statement in arcGIS Pro in ArcGIS Pro Questions

calculation within an IF statement in arcGIS Pro

MáireDolan — Fri, 10 Aug 2018 15:24:25 GMT

I'll begin by stating I am very new to the python world!

My problem is this....

I am trying to use field calculator in ArcGIS Pro to return a calculation of values in other fields based on an if statement, if that makes sense! Not sure if it can be done?

The Calculation is

Qrp=Sum(Qs*Ls)/SumLs

Basically thus;

weightAADT(!rp!,!Qs!,!Ls!)

Code Block

def weightAADT(rp, Qs, Ls):
if rp=="Urban Two Lane":
return sum(Qs * Ls) / sum(Ls)

my error is "TypeError: 'float' object is not iterable"

Any help appreciated

Re: calculation within an IF statement in arcGIS Pro

FreddieGibson — Fri, 10 Aug 2018 16:24:08 GMT

I would assume that the problem is you're giving the sum built-in function a single number when it expects a collection of numbers.

2. Built-in Functions — Python 3.7.0 documentation

sum(iterable[, start])https://docs.python.org/3/library/functions.html#sum

Sums start and the items of an iterable from left to right and returns the total. start defaults to 0. The iterable’s items are normally numbers, and the start value is not allowed to be a string.

Are your values Qs and Ls single float values or are they a collection of numbers?

Re: calculation within an IF statement in arcGIS Pro

DanPatterson_Retired — Fri, 10 Aug 2018 16:34:43 GMT

The field calculator does one record/row at a time, hence your error as Freddie says.

If you want to vectorize your calculations, you need to use numpy with arcpy's TableToNumPyArray to get the field values into an array format, then arcpy.da.ExtendTable to append the result column onto the original table. Otherwise dump the sum as the error indicates

Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 09:18:13 GMT

Thanks Freddie, that makes sense.

Qs & Ls values are indeed single float values. What I was trying to achieve was to sum the calculation/values for each row containing the value of the if statement.

Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 09:19:33 GMT

Thanks Dan,

I've never used numpy, could you give me an example of its use?

Re: calculation within an IF statement in arcGIS Pro

DanPatterson_Retired — Mon, 13 Aug 2018 10:24:43 GMT

If each row contains a Qs and an Ls value, then you can calculate

Qs * Ls as a field calculation …. nothing more.

If the final result is supposed to be the sum of all the (Qs*Ls) values divided by the sum of the Ls values as in

sum(Qs * Ls) / sum(Ls)

then you do that final calculation after the fact by doing a field statistics Sum on that product field and the Ls field.

That equation is a single result which isn't suitable for field calculations

Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 10:30:45 GMT

Thanks Dan,

I have done the above, but was trying to see if I could do the same in one step without the statistics table.

Trying to figure out the most efficient way of doing same.

If the statistics table is it, then I'll go with that.

Re: calculation within an IF statement in arcGIS Pro

DanPatterson_Retired — Sat, 11 Dec 2021 12:07:02 GMT

How do you want to report it?

Are you using a toolbox and just want one number reported?

Do you actually need the Qs*Ls column data at all?

If not, then if would be possible to just produce a small script to get the values in a searchcursor, do the math and spit out the answer.

or use numpy

input = "c:/data/usa.gdb/some_table"
arr = arcpy.da.TableToNumPyArray(input, ('Qs', 'Ls'))

qs = arr['Qs']

ls = arr['Ls']

# ---- fake data for now
qs
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

ls
array([4., 3., 1., 4., 1., 4., 3., 3., 3., 4.])

#---- now the math in numpy

qs_ls_sum = np.sum(qs*ls)

ls_sum = np.sum(ls)

result = qs_ls_sum/ls_sum

result
4.666666666666667
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Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 11:01:58 GMT

I need the results as field attributes in the original database. The results are needed for further calculations within that database.

Re: calculation within an IF statement in arcGIS Pro

DanPatterson_Retired — Mon, 13 Aug 2018 12:29:06 GMT

I am not sure what the calculation is then.. maybe show a sample couple of rows because there is no way you are going to get that formula in each row.

I suspect that your calculation is (Qs*Ls)/Ls … without the 'sum' thing.

You need to clarify why a sum is needed in any event

Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 13:19:43 GMT

Fair enough Dan, I'll try and explain...

I have a database with 525 entries, which I will call 'sites'. Each site has a 'RP' field with 1 of 4 possible attributes, i.e. Urban Two Lane, Rural Two Lane etc.

I need to carry out a calculation, based on the RP attribute, that returns the sum of [Length of site (Ls) * Traffic Flow @ site (Qs)] divided by sum Ls.

So a calculation needs to be applied to each site belonging to a defined RP, before summing up for that RP, if that makes sense.

Below is a very simplified version of fields and attributes contained in said database

Site_ID	Ls	Qs	RP
R125	1.25	4445	Urban Two Lane
R148	0.98	8956	Rural Two Lane
R524	1.35	2350	Urban Two Lane
R385	0.88	10253	Urban Dual Carriageway
R449	1.01	15236	Rural Dual Carriageway
R621	1.1	1052	Urban Two Lane

To be fair Dan I have used the summarise feature followed by a join to get the required result, my query was if there was a better way of doing it in Pro with field calculator alone.

Re: calculation within an IF statement in arcGIS Pro

DanPatterson_Retired — Mon, 13 Aug 2018 13:24:32 GMT

better way... no... only way... yes …. well, except for doing it in numpy from the start and producing the summary result table directly, but that would be a parallel solution

Re: calculation within an IF statement in arcGIS Pro

MáireDolan — Mon, 13 Aug 2018 13:32:24 GMT

Thanks Dan