Question

I have a text file which does not confirm to standards. So I know the (end,start) positions of each column value.

Sample text file :

#     #   #   #
Techy Inn Val NJ

Found the position of # using this code :

  1 f = open('sample.txt', 'r')
  2 i = 0
  3 positions = []
  4 for line in f:
  5     if line.find('#') > 0:
  6         print line
  7         for each in line:
  8             i += 1
  9             if each == '#':
 10                 positions.append(i)

1 7 11 15 => Positions

So far, so good! Now, how do I fetch the values from each row based on the positions I fetched? I am trying to construct an efficient loop but any pointers are greatly appreciated guys! Thanks (:

Answer 1

Off the top of my head:

f = open(.......)
header = f.next() # get first line
posns = [i for i, c in enumerate(header + "#") if c = '#']
for line in f:
    fields = [line[posns[k]:posns[k+1]] for k in xrange(len(posns) - 1)]

Update with tested, fixed code:

import sys
f = open(sys.argv[1])
header = f.next() # get first line
print repr(header)
posns = [i for i, c in enumerate(header) if c == '#'] + [-1]
print posns
for line in f:
    posns[-1] = len(line)
    fields = [line[posns[k]:posns[k+1]].rstrip() for k in xrange(len(posns) - 1)]
    print fields

Input file:

#      #  #
Foo    BarBaz
123456789abcd

Debug output:

'#      #  #\n'
[0, 7, 10, -1]
['Foo', 'Bar', 'Baz']
['1234567', '89a', 'bcd']

Robustification notes:

1. This solution caters for any old rubbish (or nothing) after the last # in the header line; it doesn’t need the header line to be padded out with spaces or anything else.
2. The OP needs to consider whether it’s an error if the first character of the header is not #.
3. Each field has trailing whitespace stripped; this automatically removes a trailing newline from the rihtmost field (and doesn’t run amok if the last line is not terminated by a newline).

Final(?) update: Leapfrooging @gnibbler’s suggestion to use slice(): set up the slices once before looping.

import sys
f = open(sys.argv[1])
header = f.next() # get first line
print repr(header)
posns = [i for i, c in enumerate(header) if c == '#']
print posns
slices = [slice(lo, hi) for lo, hi in zip(posns, posns[1:] + [None])]
print slices
for line in f:
    fields = [line[sl].rstrip() for sl in slices]
    print fields