Parsing info into a 2d list

[Matthew Walsh]

Whats wrong with the following code?
using pythonWin I get the following error:

Traceback (most recent call last):
  File "C:\Python23\Lib\site-packages\pythonwin\pywin\framework\scriptutils.py",
line 310, in RunScript
    exec codeObject in __main__.__dict__
  File "C:\Documents and Settings\Administrator\My
Documents\file_parser.py", line 16, in ?
    rownum = range(eval(nrows))
TypeError: object doesn't support item assignment

>>CODE
f = open('f:\input.dat')
feed= f.read()
lines = feed.split('\n')
ncols = lines[0].split()[1]
nrows = lines[1].split()[1]
xllcorner = lines[2].split()[1]
yllcorner = lines[3].split()[1]
cellsize = lines[4].split()[1]
NoData = lines[5].split()[1]

colnum = range(eval(ncols))
rownum = range(eval(nrows))

for r in rownum:
    for c in colnum:
        slope[r][c]= eval(lines[6+r].split()[c])
        
f.close()
>>END OF CODE