Parsing ascii file

[diablo]

Hello ,

I have a file that contains the following data (example) and does NOT have
any line feeds:

11 22 33 44 55 66 77 88 99 00 aa bb cc
dd ....to 128th byte 11 22 33 44 55 66 77 88 99
00 aa bb cc dd .... and so on

record 1 starts at 0 and finishes at 128, record 2 starts at 129 and
finishes at 256 and so on. there can be as many as 5000 record per file. I
would like to parse the file and retreive the value at field at byte 64-65
and conduct an arithmetical operation on the field (sum them all up).

Can I do this with python?

if I was to use awk it would look something like this :

cat <filename> | fold -w 128 | awk ' { SUM=SUM + substr($0,64,2) } END
{print SUM}'
Regards
Dean

[Peter Otten]

diablo wrote:

Hello ,

I have a file that contains the following data (example) and does NOT have
any line feeds:

11 22 33 44 55 66 77 88 99 00 aa bb cc
dd ....to 128th byte 11 22 33 44 55 66 77 88
99
00 aa bb cc dd .... and so on

record 1 starts at 0 and finishes at 128, record 2 starts at 129 and
finishes at 256 and so on. there can be as many as 5000 record per file. I
would like to parse the file and retreive the value at field at byte 64-65
and conduct an arithmetical operation on the field (sum them all up).

Can I do this with python?

if I was to use awk it would look something like this :

cat <filename> | fold -w 128 | awk ' { SUM=SUM + substr($0,64,2) } END
{print SUM}'

Is it an ascii or a binary file? I'm not entire sure from your description.
In the following I assume binary data, but it should be easy to modify the
value() function if those two bytes are ascii digits.

import struct, sys
from itertools import imap

def fold(instream, width=80):
while 1:
line = instream.read(width)
if not line: break
yield line

def value(line, start=64): # may be an "off by one" bug
# return int(line[start:start+2]))
return struct.unpack("h", line[start:start+2])[0]

if __name__ == "__main__":
try:
filename = sys.argv[1]
except IndexError:
instream = sys.stdin
else:
instream = file(filename)

print sum(imap(value, fold(instream, 128)))

Peter

[Eddie Corns]

"diablo" <dl******@btinternet.com> writes:

Hello , I have a file that contains the following data (example) and does NOT have
any line feeds: 11 22 33 44 55 66 77 88 99 00 aa bb cc
dd ....to 128th byte 11 22 33 44 55 66 77 88 99
00 aa bb cc dd .... and so on record 1 starts at 0 and finishes at 128, record 2 starts at 129 and
finishes at 256 and so on. there can be as many as 5000 record per file. I
would like to parse the file and retreive the value at field at byte 64-65
and conduct an arithmetical operation on the field (sum them all up). Can I do this with python? if I was to use awk it would look something like this : cat <filename> | fold -w 128 | awk ' { SUM=SUM + substr($0,64,2) } END
{print SUM}'

You can use stdin.read(128) to get consecutive records and slicing to extract
the fields. Something like:

from sys import stdin
sum = 0
while True:
record = stdin.read(128)
if not record: break
sum += int(record[64:65])
print sum

Frankly, I'd stick with the Awk version unless it's a pedagogical exercise.
Actually I'd go further and have a script that simplys sums up all the numbers
in the input and add 'cut' into the pipeline to extract the columns first.

Eddie