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What is the proper way to do for and if statements in python?

Question asked by apmartincude on Aug 2, 2017
Latest reply on Aug 2, 2017 by xander_bakker

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I am having issues with loops and if statements in python. I keep getting an error on the last line. What am I doing wrong?

import arcpy
from arcpy.sa import *
from arcpy import env
 
sorder=r"C:\Temp\ARCGIS_PROJECT\Tutorial\Final\layers\streamorder"

 
desc= arcpy.Describe(sorder)
 
xmin= desc.extent.XMin
ymin=desc.extent.YMin
xmax= desc.extent.XMax
ymax= desc.extent.YMax
count= 0
 
for columns in range (xmin, xmax):
  pointval= arcpy.GetCellValue_management(sorder,"%s %s" % (columns++,ymin), "1")
  if (pointval== 1, pointval==2, pointval==3): 
      count++
print "count= %s" % (count)

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iamurray Aug 2, 2017 9:09 AM
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You need to have consistent spacing when indenting for and if statements. Your for statement is indented 2 spaces, while your if statement is indented 4 spaces from the for statement when it should be 2. Just make sure they are consistent, and that you aren't mixing spaces and tabs.

https://www.python.org/dev/peps/pep-0008/
```
for columns in range (xmin, xmax):
  pointval= arcpy.GetCellValue_management(sorder,"%s %s" % (columns++,ymin), "1") #indented 2 spaces
  if (pointval== 1, pointval==2, pointval==3): 
    count++ #indented 2 more spaces
```
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- xander_bakker @ iamurray on Aug 2, 2017 9:21 AM
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  You cannot use ++ to increment a counter (this is C/C++/JS notation). In Python you need to use:
  count += 1 (same for columns)
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rastrauch Aug 2, 2017 9:31 AM
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Since you are new to Python (I assume), you may want to look at some of the links Dan Patterson has The ...py... links He has some very advanced items, but there ar also formatting tips etc. Also, there are a few threads on geonet on "how to learn python" with many free options to get started. Getting a few of the formatting techniques down will help a lot. Also, there are threads on different "python IDEs" (development environments that make it easy to see syntax error) and many of these are free too. Just some suggestions. Learn a bit of that and it will make the Python experience a lot of fun. (in my opinion anyway )
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Dan_Patterson Aug 2, 2017 10:09 AM
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You shouldn't do comparisons like you are... use 'in' ... it is much cleaner, assuming that these are the only values you have.
```
pointval = 2
if pointval in (1, 2, 3):
    print("yes")
else:
    print("no")

yes
```
and you can mix object types
```
pointval = 'two'

if pointval in (1, 2, 'two', 3):
    print("yes")
else:
    print("no")
    
yes
```
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- xander_bakker @ Dan_Patterson on Aug 2, 2017 11:45 AM
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  Hi Andrew Martin ,
  
  I guess that this is (any many more questions you have posted during the past days are) still related to this challenge:
  How do you create point shapefiles on a raster to create watersheds?
  
  Maybe it would be a lot easier to extract the last line of the raster (Extract by Rectangle—Help | ArcGIS Desktop ) and use some raster algebra (What is Map Algebra?—ArcGIS Help | ArcGIS Desktop ) to set the values that you are not interested in to NoData like:
  new_raster = Con(ori_raster <= 3, ori_raster)
  
  ... and than simply convert the raster to points: Raster to Point—Help | ArcGIS Desktop
  
  This would result in a point featureclass with the points you are looking for.
  
  Another method is to use convert the raster (or subset of the raster) to numpy array and read out the last line, test for the raster value and populate a point featureclass with the result.
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  - xander_bakker @ xander_bakker on Aug 2, 2017 12:48 PM
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    Below an example of using a numy array to get the values you are interested in:
    The blue pixel have the value we are interested in (in my case 0). The red boxes are the points that are being created by the code below. In your case line 21 would be something as proposed by Dan Patterson: if val in (1, 2, 3):
    
    import arcpy def main(): arcpy.env.overwriteOutput = True ras_path = r'C:\GeoNet\WUE2\r007_WUE.TIF' # ourput featureclass fc_out = r'C:\GeoNet\WUE2\points.shp' # convert to numpy amd determine the sizes ras_np = arcpy.RasterToNumPyArray(ras_path) rows = ras_np.shape[0] cols = ras_np.shape[1] raster = arcpy.Raster(ras_path) points = [] row = rows-1 for col in range(cols): val = ras_np[row, col] if int(val) == 0: pntg = CreatePointGeometry(raster, row, col) points.append(pntg) arcpy.CopyFeatures_management(points, fc_out) def CreatePointGeometry(raster, row, col): xcell = raster.meanCellWidth ycell = raster.meanCellHeight xmin = raster.extent.XMin ymax = raster.extent.YMax sr = raster.spatialReference pnt = arcpy.Point((col + 0.5) * xcell + xmin, ymax - (row + 0.5) * ycell) return arcpy.PointGeometry(pnt, sr) if __name__ == '__main__': main()
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What is the proper way to do for and if statements in python?

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