confusion about mutable strings

confusion about mutable strings

[Ralph Douglass]

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I'm a little confused by some behavior I'm seeing.  Here is an example from
the toplevel:

# let foo () =
  let str = "ffff" in
  Printf.printf "%s\n%!" str;
  str.[0] <- 'r';
  str.[1] <- 'r';
  str.[2] <- 'r';
  str.[3] <- 'r';
  str
  ;;
val foo : unit -> string = <fun>
# foo ();;
ffff
- : string = "rrrr"
# foo ();;
rrrr
- : string = "rrrr"
#

Why isn't the output for the second printf also "ffff"?  It seems like it's
completely ignoring "let str =" the second time.

Any thoughts?  Is this a bug, or something I just don't know about?

3.10.1 on intel osx 10.5.1, installed from godi.

Thanks,
Ralph

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Re: [Caml-list] confusion about mutable strings

[Bünzli Daniel]

Each invocation of foo does not allocate a new string for str, "ffff"  
is a constant  string allocated once and you are updating this constant.

let str = String.copy "ffff"

will solve your problem.

Best,

Daniel

Re: [Caml-list] confusion about mutable strings

[Ralph Douglass]

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Sorry, I should have made clear that this is not a problem I wanted solved
for me, but rather a question about OCaml.  I've just never come across this
before because I don't usually mutate strings.

Observe the following:

# let foo () =
  let bar = [|'a';'b';'c'|] in
  Array.iter (Printf.printf "%c") bar;
  bar.(0) <- 'd';
  bar;;
val foo : unit -> char array = <fun>
# foo ();;
abc- : char array = [|'d'; 'b'; 'c'|]
# foo ();;
abc- : char array = [|'d'; 'b'; 'c'|]

Why does OCaml treat these two examples in such a different manner?  Is
there a reason why strings are magically special in this way?

On 2/10/08, Bünzli Daniel <daniel.buenzli@erratique.ch> wrote:
>
> Each invocation of foo does not allocate a new string for str, "ffff"
> is a constant  string allocated once and you are updating this constant.
>
> let str = String.copy "ffff"
>
> will solve your problem.
>
> Best,
>
> Daniel
>
>


-- 
Ralph

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Re: [Caml-list] confusion about mutable strings

[Loup Vaillant]

2008/2/11, Ralph Douglass <ralph@grayskies.net>:
>
> Observe the following:
>
> # let foo () =
>   let bar = [|'a';'b';'c'|] in
>   Array.iter (Printf.printf "%c") bar;
>   bar.(0) <- 'd';
>   bar;;
> val foo : unit -> char array = <fun>
>  # foo ();;
> abc- : char array = [|'d'; 'b'; 'c'|]
> # foo ();;
> abc- : char array = [|'d'; 'b'; 'c'|]
>
> Why does OCaml treat these two examples in such a different manner?  Is
> there a reason why strings are magically special in this way?

C strings are usually "magically special" in exactly the same way. It
has been talked about in another thread (dunno how to look for it).
Basically, this is a design choice meant for better performance.

Loup

Re: [Caml-list] confusion about mutable strings

[Ralph Douglass]

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Sorry, I was indeed informed today by someone at work that it was talked
about a week or two ago.  There is so much going on on the list that I just
missed it.  And now I added noise myself.  Sigh.

Thanks,

Ralph

On 2/11/08, Loup Vaillant <loup.vaillant@gmail.com> wrote:
>
> 2008/2/11, Ralph Douglass <ralph@grayskies.net>:
> >
> > Observe the following:
> >
> > # let foo () =
> >   let bar = [|'a';'b';'c'|] in
> >   Array.iter (Printf.printf "%c") bar;
> >   bar.(0) <- 'd';
> >   bar;;
> > val foo : unit -> char array = <fun>
> >  # foo ();;
> > abc- : char array = [|'d'; 'b'; 'c'|]
> > # foo ();;
> > abc- : char array = [|'d'; 'b'; 'c'|]
> >
> > Why does OCaml treat these two examples in such a different manner?  Is
> > there a reason why strings are magically special in this way?
>
> C strings are usually "magically special" in exactly the same way. It
> has been talked about in another thread (dunno how to look for it).
> Basically, this is a design choice meant for better performance.
>
> Loup
>



-- 
Ralph

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Re: [Caml-list] confusion about mutable strings

[Ashish Agarwal]

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Here's a link to the previous thread:
http://caml.inria.fr/pub/ml-archives/caml-list/2008/01/4ca0f4bdf642d5fdcda87a82c3f70a85.en.html


2008/2/11 Ralph Douglass <ralph@grayskies.net>:
> Sorry, I was indeed informed today by someone at work that it was talked
about a week or two ago. There is so much going on on the list that I just
missed it. And now I added noise myself. Sigh.
>
> Thanks,
>
> Ralph
>

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