Member Avatar for hisan

Hi,
I want to know how to read the binary file from 10th byte to 90th byte in a file size of 100 bytes and need to write the read data into new binary file.
please help me in doing this.

  • 3 Contributors
  • 3 Replies
  • 100 Views
  • 1 Day Discussion Span
  • Latest Post Latest Post by hisan

Dani AI

Generated

A few clarifications and a couple of safer alternatives to the quick answers already posted by and .

The usual off-by-one confusion comes from human (1-based) versus programmatic (0-based) numbering. For a 1-based inclusive range start..end, compute zero_based_start = start - 1 and length = end - start + 1. Python slicing is half-open ([start:stop]) so the equivalent slice is [zero_based_start:end]. That formula avoids guessing whether you should seek(9) vs seek(10) or read 80 vs 81 bytes.

For a robust, memory-efficient extraction use a memory map so the OS handles paging instead of loading the whole file into Python objects:

import os
import mmap

infile = "binfile.dat"
start = 10    # 1-based inclusive
end = 90      # 1-based inclusive

size = os.path.getsize(infile)
if end > size:
    raise ValueError("input too small")

with open(infile, "rb") as f:
    with mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ) as m:
        chunk = m[start - 1:end]   # zero-based slice gives inclusive start..end

with open("out.dat", "wb") as out:
    out.write(chunk)

Quick troubleshooting/checks: confirm the output length equals (end - start + 1) using os.path.getsize; inspect a few leading bytes with binascii.hexlify if the result looks wrong. On very large files or unusual platforms, prefer a small seek/read loop or use readinto with a preallocated bytearray. See the Python docs for memory-mapped files and file size checks: mmap module docs and os.path.getsize.

Recommended Answers

All 3 Replies

Member Avatar for TrustyTony 
with open('binfile.dat', 'rb') as bf:
      bf.seek(10)
      open('out.dat', 'wb').write(bf.read(80))

Something like this. This is untested as I am not with computer now. Maybe you must seek(9) and/or read(81).

Edited by TrustyTony because: n/a
Member Avatar for woooee

You should be able to read the 100 bytes, split and write. The following code uses 25 bytes and splits from 10 through 20 (11 bytes) for simplicity.

alpha = "abcdefghijklmnopqrstuvwxy"
print alpha
print alpha[9:20]
Edited by woooee because: n/a
Member Avatar for hisan

The first solution really worked for me.. thanks

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.