Nally Kaunda-Bukenya wrote: > I am very sorry to bother you, but I need help understanding the code that > you assisted me with. It does exactly what I needed done, I can't thank > you enough for that. I am just learning Python, and would appreciate all > the help. please see my comments below and help me understand the code.
Please don't email me directly, send questions to the list instead to give more people a chance to answer. > Code below: > > #WHAT IS A DICT? SAME AS LIST? This is a very basic question which is dealt with in every tutorial. Look here for a starting point: http://wiki.python.org/moin/BeginnersGuide/NonProgrammers > #iterate over the Outfalls_ND table twice, the first time to calculate the > #sums per OUTFALL_ID and put them into a dict. With the second pass the > #Tot_Outf_Area column is updated > import arcgisscripting > def rows(cur): > while True: > row = cur.Next() > if row is None: > break #?? > yield row #?? break terminates a loop, yield gives back a result without terminating the function which is then called "generator". For now I recommend that you treat this generator as a black box. > gp = arcgisscripting.create() > gp.Workspace = "C:\\NPDES\\NPDES_PYTHON.mdb" > TABLE = "Outfalls_ND" > GROUP = "OUTFALL_ID" #does upper case mean anything?? (group below is lower > case?) Python is case-sensitive, so GROUP and group (and Group or grouP) are different names. I am following a convention here that uppercase names are used for constants, i. e. the value of GROUP never changes after the first assignment. > SUM = "AREA_ACRES" > TOTAL = "Tot_Outf_Area" > > aggregate = {} > cur = gp.UpdateCursor(TABLE) > for row in rows(cur): #is this diff from for rows in cur? > group = row.GetValue(GROUP) #loop thru rows and search for outfall_ID > amount = row.GetValue(SUM) #loop thru rows and search for area_acres > aggregate[group] = aggregate.get(group, 0.0) + amount #need help > w/explanation on aggregate fn > #aggregate[group] is what is it? is this a list of unique outfall_ID's? > #aggregate.get (group, 0.0) how do you get 0.0? > #I tried this code on a diff field and got this error: TypeError: > unsupported operand #type(s) for +: 'float' and 'NoneType' > > cur = gp.UpdateCursor(TABLE) > for row in rows(cur): > group = row.GetValue(GROUP) > row.SetValue(TOTAL, aggregate[group]) #?? > cur.UpdateRow(row) Sorry, I don't think answering any of the above questions worthwhile until you've worked through an introductory text. Once you /have/ a basic understanding you can often find out the answers by asking python itself. Just fire up the interactive interpreter and type >>> help(dict.get) If the explanation given there is not sufficient you can use the online documentation as a fallback: http://docs.python.org/library/stdtypes.html#mapping-types-dict Good look with your efforts! Peter -- http://mail.python.org/mailman/listinfo/python-list
