topic Re: Newbie for loop question in Python Questions

Newbie for loop question

AndrewMosley — Thu, 17 Nov 2011 18:52:14 GMT

Hi there,

I am trying to move an ID number from the attribute table of a census block file to the attribute table of a roads file. The blocks are all separate feature classes in a gdb and the roads file is one single layer. So far I've decided that the simplest way to do this may be to select all roads with their center in each block, get the STFID from the block attribute table, then assign that STFID to the roads selected in the roads attribute table. The code I've got so far will select all roads at the same time, instead of running through the process one at a time. Current syntax as follows:

>>> import arcpy
>>> #Set geoprocessing environment
>>> arcpy.env.workspace = ("H:\Dr. Hines\New File Geodatabase.gdb")
>>> arcpy.env.overwriteOutput = True
>>> #Create list of all blocks
>>> fcList = arcpy.ListFeatureClasses("*blk00")
>>> #Create loop
>>> for fc in fcList:
...      blk = fcList
...      arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN")
...      for fc in fcList:
...         rows = arcpy.InsertCursor(blk)
...         row = rows.newRow()
...         row.STFID = ()
...         fieldVal = row.getValue("STFID")
...         arcpy.CalculateField_management("Analysis Roads","STFID","fieldVal")
...
Runtime error <type 'exceptions.RuntimeError'>: ERROR 999999: Error executing function.

any help is greatly appreciated!

Re: Newbie for loop question

MathewCoyle — Thu, 17 Nov 2011 19:24:21 GMT

Looks like improper indentation, but hard to tell. Try writing the whole thing out in IDLE and then copy it in to ArcGIS, if you must run it using the Python window in Arc.

Edit: Also with your path strings, use r in front ex. r"H:\Dr. Hines\New File Geodatabase.gdb"

Re: Newbie for loop question

markdenil — Sat, 11 Dec 2021 19:32:50 GMT

Why are there parenthisis around the string in
arcpy.env.workspace = ("H:\Dr. Hines\New File Geodatabase.gdb")
??

You can wrap a code block in code tags when you place it in the message window
    It makes it easier to read

Use the # button above the text entry window...

Re: Newbie for loop question

AndrewMosley — Sat, 11 Dec 2021 19:32:53 GMT

import arcpy
#Set geoprocessing environment
arcpy.env.workspace = "H:\Dr. Hines\New File Geodatabase.gdb"
arcpy.env.overwriteOutput = True
#Create list of all blocks
fcList = arcpy.ListFeatureClasses("*blk00")
#Create loop
for fc in fcList:
      blk = fcLis
      arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN")
      for fc in fcList:
           rows = arcpy.InsertCursor(blk)
           row = rows.newRow()
           row.STFID = ()
           fieldVal = row.getValue("STFID")
           arcpy.CalculateField_management("Analysis Roads","STFID","fieldVal")

There we go, hope that's easier to read. Not sure why I had parentheses around the path name...still not functional, however- the indenting is correct I just did not put it in code block here.

Re: Newbie for loop question

StephanieWendel — Sat, 11 Dec 2021 19:32:55 GMT

So you to give the block number ID to each of the road feature classes you have? Does Analysis Roads already have a field called STFID? If you already have this field you would be better off to use an update cursor to update the values of the STFID field. Do each of your blk00 contain only one feature or multiple?

Here is a rough example using the Update Cursor instead. I don't think this will work exactly with the select by location, but it should help show you want you may need to modify to use the Update Cursor instead. Have a look through the update cursor info http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/UpdateCursor/000v0000003m000000/

import arcpy
#Set geoprocessing environment
arcpy.env.workspace = "H:\Dr. Hines\New File Geodatabase.gdb"
arcpy.env.overwriteOutput = True
#Create list of all blocks
fcList = arcpy.ListFeatureClasses("*blk00")
#Create loop
for fc in fcList:
      selected = arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN", fc)
      highway = "Analysis Roads"
      rows = arcpy.UpdateCursor(higway)
          for row in rows:
              if row is in selected:
                  row.STFID = fc.STFID #this is assuming your feature classes have a STFID field too, you can change this to the right field name. 
                  rows.updateRow(row)

del row
del rows

Re: Newbie for loop question

RichardFairhurst — Sat, 11 Dec 2021 19:32:58 GMT

import arcpy
#Set geoprocessing environment
arcpy.env.workspace = r"H:\Dr. Hines\New File Geodatabase.gdb" # \ is an escape character in python strings and needs the raw flag or to be doubled to act as a character
arcpy.env.overwriteOutput = True
#Create list of all blocks
fcList = arcpy.ListFeatureClasses("*blk00")
#Create loop
for fc in fcList:
      blk = fcLis # Looks like a misspelling.  Shouldn't this be fcList
      arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN")
      for fc in fcList: # this is bad.  Never loop the same object within a loop of itself.  Create a new list object to loop if you need an embedded loop.
           rows = arcpy.InsertCursor(blk)
           row = rows.newRow()
           row.STFID = ()
           fieldVal = row.getValue("STFID")
           arcpy.CalculateField_management("Analysis Roads","STFID","fieldVal")

Not sure if the things noted above fix everything, but they are suspect as possible errors.

Re: Newbie for loop question

GeorgeNewbury — Mon, 21 Nov 2011 12:26:37 GMT

The 'Select Layer By Location' tool takes in a layer, not a feature class. So remember if you run it as a tool outside of ArcMap you'll need to make a feature layer of the Roads.

You could also add a selection for rows with no 'STFID' value before the select layer by location. So something like:

1. Select Rows from 'Analysis Roads' where STFID is null/empty
2. Subset Selection 1 using Select Layer by Location.

That way you're not having to do as much geometry calculations toward the end of the script.

I often troubleshoot things like this by running the tools from the toolbox, manually. Then copy them as python snippets in and hook them together.

Re: Newbie for loop question

AndrewMosley — Sat, 11 Dec 2021 19:33:01 GMT

Ok I've made some changes and it's come close to working as it should but gives me a syntax exception saying line 6 is invalid. For whatever reason it doesn't want to name the variable and perform the operation in the same line of code. Is there a way to define a variable as the output of a previous operation? Something like:

arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN",fc)
selected = outputOfPreviousLineOperation <--- ?

selected = arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN",fc)
returns "Parsing error <type 'exceptions.SyntaxError'>: invalid syntax (line 6)" but without selected = ..it functions as it should.

Here is the current code:

>>> import arcpy
>>> #Set geoprocessing environment
>>> arcpy.env.workspace = r"I:\Dr. Hines\New File Geodatabase.gdb"
>>> arcpy.env.overwriteOutput = True
>>> #Create list of all blocks
>>> fcList = arcpy.ListFeatureClasses("*blk00")
>>> #Create loop
>>> for fc in fcList:
...     X = arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN", fc)
...     highway = "Analysis Roads"
...     rows = arcpy.UpdateCursor(highway)
...     for row in rows:
...         if row is in X:
...             row.STFID = fc.STFID
...             rows.updateRow(row)
...             del row
...             del rows

Re: Newbie for loop question

JamesHood — Sun, 12 Dec 2021 16:28:59 GMT

A syntax error means that you have written it out, tabbed, spelled something incorrectly.

for fc in fcList:
    X = arcpy.SelectLayerByLocation_management("Analysis Roads","HAVE_THEIR_CENTER_IN", fc)
    highway = X  #I think you want highway to = X which is the selection of analysis roads from the previous #operation? 
    rows = arcpy.UpdateCursor(highway)
    for row in rows:
        if row is in X:#I dont think this is correct if statement... should be something like if X==ROW
            row.STFID = fc.STFID
            rows.updateRow(row)
     del row #these del row, del rows should be tabbed outside of the loop, not inside
     del rows

Re: Newbie for loop question

MathewCoyle — Sat, 11 Dec 2021 19:33:03 GMT

Holding the selection as a variable is unnecessary, Update Cursor will "see" selections and process on those only (when referencing a selectable layer of course).

This should work, I am not sure what you are trying to do with the row.STFID = fc.STFID line though.

import arcpy
#Set geoprocessing environment
arcpy.env.workspace = r"I:\Dr. Hines\New File Geodatabase.gdb"
arcpy.env.overwriteOutput = True
#Create list of all blocks
fcList = arcpy.ListFeatureClasses("*blk00")
highway = "Analysis Roads"
#Create loop
for fc in fcList:
    arcpy.SelectLayerByLocation_management(highway,"HAVE_THEIR_CENTER_IN", fc)
    rows = arcpy.UpdateCursor(highway)
    for row in rows:
        row.STFID = fc.STFID
        rows.updateRow(row)
    del row
    del rows

Re: Newbie for loop question

AndrewMosley — Wed, 30 Nov 2011 17:32:21 GMT

Using the above code returns this error:
Runtime error <type 'exceptions.AttributeError'>: 'unicode' object has no attribute 'STFID'

I am trying to move the STFID value from the blocks layers to the roads layer.
STFID of block1------\
STFID of block2------ \ STFID in roads layer (all null/empty values)
STFID of block3------ /
STFID of block4------/

I'm not sure why it says the unicode value has no attribute STFID, as all layers in this do have an STFID layer which are all string of the same length.

In the end though, I want the selected roads to be assigned the STFID value of the block they were selected by.

Re: Newbie for loop question

MathewCoyle — Wed, 30 Nov 2011 17:54:04 GMT

I suspected that might happen, your fc variable is a feature class and has no attribute STFID. It is simply a unicode reference to the feature class in the list.

Does each block feature class have a single unique STFID value? You will have to retrieve that value somehow, either assigned manually, read from a field the same way you do with the roads, or some other way.

If you don't understand this concept you should try some more simplistic operations to get a grasp of basic arcpy functionality. Esri has some good free online training courses to get you started.
Try working through this, it gives a great intro to arcpy and python in general.
http://training.esri.com/gateway/index.cfm?fa=catalog.webCourseDetail&courseid=1868

Re: Newbie for loop question

AndrewMosley — Wed, 30 Nov 2011 17:59:48 GMT

Yes, each block has a STFID value. However they are all the same with each block's attribute table, as this is the value I want to assign to any roads within this block's geometry.

Re: Newbie for loop question

MathewCoyle — Wed, 30 Nov 2011 18:30:03 GMT

Have you tried looking at the intersect tool?

Re: Newbie for loop question

AndrewMosley — Sat, 11 Dec 2021 19:33:06 GMT

The roads pass through multiple blocks so the intersect tool will return too many results (more than 1).
I'm wondering if something like this would work:

for fc in fclist:
    arcpy.selectLayerByLocation_management(highway,"Have_their_center_in", fc)
    arcpy.GetValue(STFID from the current fc table)
    arcpy.UpdateCursor(STFID for selected roads in roads table)

though im not sure on how the coding would look exactly.