nesting for statements?

[tpcolson]

I'm not what you'd call a "programmer" of any sort, so perhaps this
question may seem arcane and result in a plethora of "you idiot"
threads, but here goes:

ArcGIS 9.1 has a neat interface with python (2.1-2.4), allowing me to
do all sorts of spatial operations within python, namely, repetitive
functions.


In the below code, I'm trying to iterate thru multiple values of the
variable "Discretisation error factor" using a for statement that
temporarily populates "Disc", which is used as input in "Discretisation
error factor" in the gp.TopoToRaster._sa function.


For each iteration of "Discretisation error factor", I'm trying to name
the output file "outa", "outb", "out...."

Not quite sure how to implement that. There are lots of examples on
nested for loops out there, but nothing on combing that with a output
file naming sheme.

The gp.TopoToRaster_sa function by itself without all the for
statements works fine.

Appreciate any help on this, other wise I have to manually interpolate
hundreds of LiDAR point clouds, each, 10 times for diff. values of DEF.



# TopoToRaster_sample.py
# Description: Interpolate a series of point features onto a
rectangular raster using TopoToRaster
# Requirements: None
# Author: ESRI
# Date: 12\\01\\03

# Import system modules
import sys, string, os, win32com.client

# Create the Geoprocessor object
from win32com.client import Dispatch
gp = Dispatch("esriGeoprocessing.GpDispatch.1")

# Check out any necessary licenses
gp.CheckOutExtension("spatial")

# Iterate 2 thru 4 in increments of 2 for DEF
# Name the "2" dem "outa" and the "4" dem "outb"
for x in range(2,4):
Disc = int(x)
names = ["a","b"]
for y in names:
Out_Dem = "out"+y


try:

# Process: Topo to Raster...
gp.TopoToRaster_sa("C:\\temp\\falls_lidar.shp Z PointElevation",
Out_Dem, # Variable for name of output raster.
This should increment name of output based on the for statement
"5", # Output raster cell size: each pixel is 5
feet by 5 feet
"2103763.27 813746.12 2111850.32 822518.65",
#extent of raster borders, SPF, NC, NAD83
"20", # #Grid Margin
"", #Smallest z value to be used in
interpolation (optional)
"", #Largest z value to be used in interpolation
(optional)
"NO_ENFORCE", #Drainage option
"SPOT", #Spot data option
"40", #Maximum number of iterations (optional)
"", #Roughness penalty (optional)
Disc, #Discretisation error factor: This should
increment DEF based on the for statement
"0", #Vertical standard error (optional)
"", #Tolerance 1 (optional)
"" #Tolerance 2 (optional)
)
except:
print "ERROR OCCURED"
print gp.GetMessages()

 

[Paul Watson]

I'm not what you'd call a "programmer" of any sort, so perhaps this
question may seem arcane and result in a plethora of "you idiot"
threads, but here goes:

ArcGIS 9.1 has a neat interface with python (2.1-2.4), allowing me to
do all sorts of spatial operations within python, namely, repetitive
functions.


In the below code, I'm trying to iterate thru multiple values of the
variable "Discretisation error factor" using a for statement that
temporarily populates "Disc", which is used as input in "Discretisation
error factor" in the gp.TopoToRaster._sa function.


For each iteration of "Discretisation error factor", I'm trying to name
the output file "outa", "outb", "out...."

Not quite sure how to implement that. There are lots of examples on
nested for loops out there, but nothing on combing that with a output
file naming sheme.

The gp.TopoToRaster_sa function by itself without all the for
statements works fine.

Appreciate any help on this, other wise I have to manually interpolate
hundreds of LiDAR point clouds, each, 10 times for diff. values of DEF.



# TopoToRaster_sample.py
# Description: Interpolate a series of point features onto a
rectangular raster using TopoToRaster
# Requirements: None
# Author: ESRI
# Date: 12\\01\\03

# Import system modules
import sys, string, os, win32com.client

# Create the Geoprocessor object
from win32com.client import Dispatch
gp = Dispatch("esriGeoprocessing.GpDispatch.1")

# Check out any necessary licenses
gp.CheckOutExtension("spatial")

# Iterate 2 thru 4 in increments of 2 for DEF
# Name the "2" dem "outa" and the "4" dem "outb"
for x in range(2,4):
Disc = int(x)
names = ["a","b"]
for y in names:
Out_Dem = "out"+y


try:

# Process: Topo to Raster...
gp.TopoToRaster_sa("C:\\temp\\falls_lidar.shp Z PointElevation",
Out_Dem, # Variable for name of output raster.
This should increment name of output based on the for statement
"5", # Output raster cell size: each pixel is 5
feet by 5 feet
"2103763.27 813746.12 2111850.32 822518.65",
#extent of raster borders, SPF, NC, NAD83
"20", # #Grid Margin
"", #Smallest z value to be used in
interpolation (optional)
"", #Largest z value to be used in interpolation
(optional)
"NO_ENFORCE", #Drainage option
"SPOT", #Spot data option
"40", #Maximum number of iterations (optional)
"", #Roughness penalty (optional)
Disc, #Discretisation error factor: This should
increment DEF based on the for statement
"0", #Vertical standard error (optional)
"", #Tolerance 1 (optional)
"" #Tolerance 2 (optional)
)
except:
print "ERROR OCCURED"
print gp.GetMessages()

I think you want a filename generated for each pass through the loop. I
would suggest generating the filenames before you ever start the loop in
a list. Then, reference the filename list while you are in the loop.
You could also construct the filename while you are in the loop.

sor = 2 # start of range
eor = 4 # end of range

filenames = ['file' + chr((x-sor) + ord('a')) for x in range(sor, eor)]

for x in range(sor, eor):
print filenames[x - sor]

If you do not want to create all of the filenames in memory at one time,
you could generated them during the loop. However, this will not work
well when you code is run on a system not using ASCII character encoding.

for x in range(sor, eor):
print 'file' + chr((x - sor) + ord('a'))

Something using string.ascii_lowercase would work better. My guess is
that on the mainframe using EBCDIC character encoding that
string.ascii_lowercase still contains the LATIN SMALL LETTER characters,
just as it does on ASCII-based systems.

 

[tpcolson]

Hi. Thanks for the tip. However, implementing that example, the script
will only generate the second output "file", (or it's overwriting the
first one), so all I get when run is "fileb".

# Import system modules
import sys, string, os, win32com.client

# Create the Geoprocessor object
from win32com.client import Dispatch
gp = Dispatch("esriGeoprocessing.GpDispatch.1")

# Check out any necessary licenses
gp.CheckOutExtension("spatial")

# Iterate 2 thru 4 in increments of 2 for DEF
# Name the "2" dem "outa" and the "4" dem "outb"
sor = 2 # start of range
eor = 4 # end of range

filenames = ['file' + chr((x-sor) + ord('a')) for x in range(sor, eor)]

for x in range(sor, eor):
Out_Dem = filenames[x - sor]


try:

# Process: Topo to Raster...
gp.TopoToRaster_sa("C:\\temp\\test.shp Z PointElevation",
"C:\\temp\\"+Out_Dem, # Variable for name of
output raster. This should increment name of output based on the for
statement
"5", # Output raster cell size: each pixel is 5
feet by 5 feet
"2103763.27 813746.12 2111850.32 822518.65",
#extent of raster borders, SPF, NC, NAD83
"20", # #Grid Margin
"", #Smallest z value to be used in
interpolation (optional)
"", #Largest z value to be used in interpolation
(optional)
"NO_ENFORCE", #Drainage option
"SPOT", #Spot data option
"40", #Maximum number of iterations (optional)
"", #Roughness penalty (optional)
"0" #Discretisation error factor: This should
increment DEF based on the for statement
"0", #Vertical standard error (optional)
"", #Tolerance 1 (optional)
"" #Tolerance 2 (optional)
)
except:
print "ERROR OCCURED"
print gp.GetMessages()

 

[Peter Otten]

Hi. Thanks for the tip. However, implementing that example, the script
will only generate the second output "file", (or it's overwriting the
first one), so all I get when run is "fileb".

I think your looping code has the structure

for x in ["a", "b", "c"]:
pass
print x

This will print x once after the loop has completed. By then the value of x
is "c". To fix it, move the code from /after/ the loop /into/ the loop:

for x in ["a", "b", "c"]:
print x

In the code you give that would mean that you have to indent the

try:
# ...
except:
# ...

statement one more level.

Inside that loop you want a filename and an integer value in lockstep. The
simplest way to get that is zip():
.... Out_Dem = "out" + suffix
.... print Disc, Out_Dem
....
2 outa
4 outb

Again, you have to replace the print statement by your try...except. If you
fear that you will eventually run out of suffices, here is a function that
"never" does:
.... for c in chars:
.... yield c
.... for c in gen_suffix(chars):
.... for d in chars:
.... yield c + d
........ print i, s
....
0 a
1 b
2 c
3 aa
4 ab
5 ac
6 ba
7 bb
8 bc
9 ca
10 cb
11 cc
12 aaa
13 aab
14 aac
15 aba
16 abb
17 abc
18 aca
19 acb

That said, reading the tutorial or an introductory book on Python never
hurts...

Peter