How do i find the 40th byte from a binary file using Scheme language?

Question

Hi I am trying to find the the 40th byte from a binary file using scheme commands.

(define FileName "D:/work_dsi/Stories/WriterOptions/Tests/block_english.model")

(define fPtr (open-input-file FileName))

(define sRec "1")

(((sRec (read-byte fPtr)))

    do
     ((eof-object? sRec ))
    (print sRec)
    (define sRec (read-byte fPtr))
)
(close-input-port fPtr)

i tried with this expression for a better understanding of the concept , but couldn't achieve anything.

Answer 1

You need these functions:

• with-input-from-file: allows you to read from given file (file is set as current-input-port)
• port->bytes: you get all bytes from given port or current input port, result is byte string
• bytes-ref: get nth byte from byte string

(require racket/port)

(with-input-from-file
    "D:/work_dsi/Stories/WriterOptions/Tests/block_english.model"
  (lambda ()
    (bytes-ref (port->bytes) 39)))

This function will return number. For reading text files, you can convert that number with integer->char to get character.

Answer 2

A version using only standard R7RS functions (In particular, open-binary-input-file and read-u8 from the file library:

(import (scheme file))
(define (read-nth-byte filename n)
  (call-with-port
   (open-binary-input-file filename)
   (lambda (inp)
     (do ((i 1 (+ i 1))
          (byte (read-u8 inp) (read-u8 inp)))
         ((= i n) byte)
       (if (eof-object? byte) (error "Premature end of file"))))))

To avoid reading a large chunk of the file into memory for large values of n, this one reads and discards bytes until it hits the nth byte. (It would be nice if there was a seek function in the standard...)