On Tue, Aug 16, 2011 at 11:26 AM, Thomas <th.vanderv...@gmail.com> wrote:
> Hi everyone,
>
> I have been struggling with this, hopefully, simple problem now for
> quite sometime, What I want to do is:
>
> *) read a file line by line
> *) modify each line
> *) write it back to a different file
>
> This is a bit of sample code that reproduces the problem:
>
> ==========================================================
> (def old-data (line-seq (reader "input.txt")))
>
> (defn change-line
> [i]
> (str i " added stuff"))
>
> (spit "output.txt" (map change-line old-data))
> ==========================================================
> #cat "output.txt"
> clojure.lang.LazySeq@58d844f8
>
> Because I get the lazy sequence I think I have to force the execution?
> but where
> exactly? And how?
The spit function expects a string; you need to pr-str the object.
However, that will output it like the REPL would: (line1 line2 line3
...)
You probably want no parentheses, and separate lines. So you'll want
something more like
(with-open [w (writer-on "output.txt")]
(binding [*out* w]
(doseq [l (map change-line old-data)]
(println l))))
The output part is lazy now, so you might want to consider making the
input part lazy as well:
(with-open [r (reader-on the-input-file)
w (writer-on "output.txt")]
(binding [*out* w]
(doseq [l (line-seq r)]
(println (change-line l)))))
(note: untested, and assumes suitable reader-on and writer-on
functions such as from contrib)
Then it will be able to process files bigger than can be held in main
memory all at once.
--
Protege: What is this seething mass of parentheses?!
Master: Your father's Lisp REPL. This is the language of a true
hacker. Not as clumsy or random as C++; a language for a more
civilized age.
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