writing binary values (bytes) to a file
prhlava
Hello all,
I have started to play with the clojure a day ago and today I have
almost finished porting simple program (from plt-scheme).
The place I got stuck in is - how to write a binary value (multiple
bytes) in one write operation to a file... The code uses
java.io.FileWriter, but this wants string.
What I need to write is 3 RHS bytes from an integer, (the pix is
Integer in following code):
(. ofile write (str
(char (bit-shift-right pix 16))
(char (bit-shift-right pix 8))
(char pix)))
)
writes wrong stuff. Is this unicode thing, my noob status thing or
something else?
Kind regards,
Vlad
pmf
> (. ofile write (str
> (char (bit-shift-right pix 16))
> (char (bit-shift-right pix 8))
> (char pix)))
> )
>
takes an array, not the one that takes a string.
Constructing the array is a bit tricky:
; Define your int
(def pix 652187261)
; Define your array, typed as array of char
(def payload
(into-array Character/TYPE [(char (bit-shift-right pix 16))
(char (bit-shift-right pix 8))
(char pix)]))
; Create your FileWriter
(def ofile (java.io.FileWriter. "somefile.bin"))
; Write and close
(.write ofile payload)
(.close ofile)
wwmorgan
binary data. If possible, create a FileOutputStream like this:
(ns my.ns
(:import (java.io FileOutputStream)))
(def ofile (FileOutputStream. "/some/file/somewhere"))
and use one of the write methods of FileOutputStream.
Graham Fawcett
> Dealing with byte-arrays, you will want to use the write-method that
> takes an array, not the one that takes a string.
>
> Constructing the array is a bit tricky:
>
> ; Define your int
> (def pix 652187261)
>
> ; Define your array, typed as array of char
> (def payload
> (into-array Character/TYPE [(char (bit-shift-right pix 16))
> (char (bit-shift-right pix 8))
> (char pix)]))
Char's on the JVM are not bytes, and are wider than 8 bits -- so if
you're packing binary data, you should use Byte and byte instead of
Character and char. (Note too that the pix value is greater than 2^24,
so you need four bytes to store it if you're using a byte-array.)
As wwmorgan mentioned, FileOutputStreams can write out byte-buffers,
so the following seems to work:
; Define your int
(def pix 652187261)
; Define your array, typed as array of byte (not char!)
(def payload
(into-array Byte/TYPE [(byte (bit-shift-right pix 24))
(byte (bit-shift-right pix 16))
(byte (bit-shift-right pix 8))
(byte pix)]))
; Create your FileWriter
(def ofile (java.io.FileOutputStream. "/tmp/somefile.bin2"))
; Write and close
(.write ofile payload)
(.close ofile)
The created file contains four bytes: 26 df 96 7d.
Bonus question for the bored reader: write a function,
(byte-array-maker N), that takes a width N, and returns a function
that takes an Integer as input and returns an N-width byte array
containing the Integer in big-endian order. E.g.
((byte-array-maker 4) 652187261)
=> [4-byte array: 26, df, 96, 7d]
Cheers,
Graham
prhlava
Hello again,
Thank you all for the posts and explanations,
After getting the clojure SVN version and few tweaks in the code, the
working result looks like:
(with-open [ofile (new java.io.FileOutputStream
(str result-directory
"/"
(make-filename x y))
(boolean true))] ; I am appending only
(. ofile write
(into-array Byte/TYPE
[(byte (bit-shift-right pix 16))
(byte (bit-shift-right pix 8))
(byte pix)])) ; the pix is of the Integer type
Vlad
Mark Volkmann
It looks like with-open accepts any number of bindings. Does it just
call close on the first one when the body finishes or on all of them?
If I wanted to figure this out for myself, how would I find the source
code for with-open?
--
R. Mark Volkmann
Object Computing, Inc.
Stuart Halloway
The metadata points to the source:
user> (meta #'with-open)
{:doc "bindings => name init\n\n Evaluates body in a try expression
with name bound to the value of\n init, and a finally clause that
calls (.close name).", :ns #<Namespace clojure.core>, :arglists
([bindings & body]), :file "core.clj", :name with-open, :macro
true, :line 1752}
If you look through the archive I believe somebody posted code that
uses the metadata to locate the source and display it at the REPL.
Stu
Graham Fawcett
Hi,
If you're using Slime in Emacs, type in the word "with-open" (or put
your cursor on it in a Clojure source file) and press M-. (alt-period
or Escape, period).
Best,
Graham
Mark Volkmann
I looked for occurrences of "(meta " throughout the clojure and
clojure-contrib repositories, but I didn't find a function that prints
the code for a given function. That would be incredibly useful for
learning! Does anybody know what function does that and whether it is
part of clojure-contrib?
Jeff Bester
a similar problem last week and ended up writing a generic library
that converts various sizes of integers to big or little endian byte
arrays and then back again.
Code is located at: http://github.com/jbester/cljext/tree/master/cljext%2Fbinpack.clj
Jeffrey
prhlava
Hello Graham,
> Bonus question for the bored reader: write a function,
> (byte-array-maker N), that takes a width N, and returns a function
> that takes an Integer as input and returns an N-width byte array
> containing the Integer in big-endian order. E.g.
>
> ((byte-array-maker 4) 652187261)
> => [4-byte array: 26, df, 96, 7d]
Vlad
prhlava
Hello Jeffrey,
Thanks for the link, more food for study on how to write a generic
library...
Basically, I am scheme/lisp noob, learning it in my spare time, the
progress is slow but rewarding...
Few observations about my own experience with starting with lisp:
1. concentrate on meaning and what fuctions do, not on the form and
"(((())))"
2. name your functions well
3. use tools to have your code idented properly
4. learn ways of creating abstracions and recognising when one is
asking to be written
(this thread kickstarted 4.)
Kind regards,
Vlad
Graham Fawcett
Best,
Graham
>
> Vlad
> >
>