Unexpected behaviour of strings initialized with quotes

Unexpected behaviour of strings initialized with quotes

[Arthur Chargueraud]

I am surprized by a difference of behaviour between the strings
"bbbb" and (String.make 4 'b').
I have been programming in Caml for a while, and I've always
assumed that the two expression would be rather equivalent.

The idea is that when writing:  let f() = "bbbb"
Any call to function f() will return the same string (ie same pointer).
This is of course not the case when writing:  let f() = String.make 4 'b'

Below is a program where this behaviour is causing a problem:

   code:
         for i = 0 to 3 do
            let s = "bbbb" in
            s.[i] <- 'a';
            Printf.printf "string s is now %s\n" s;
         done;

   outputs:
         string s is now abbb
         string s is now aabb
         string s is now aaab
         string s is now aaaa

What it means is that this code is just equivalent to:

         let s = "bbbb" in
         for i = 0 to 3 do
            s.[i] <- 'a';
            Printf.printf "string s is now %s\n" s;
         done;

which I find really unexpected. I was waiting for:

         string s is now abbb
         string s is now babb
         string s is now bbab
         string s is now bbba

which is what happens when using (String.make 4 'b') inside the loop.


This is not a real problem, since it is not usual to modify
strings initialized with quotes, but I am just wandering
about the reason of such a behaviour...


Arthur Chargueraud

Re: [Caml-list] Unexpected behaviour of strings initialized with quotes

[Martin Jambon]

On Wed, 23 Nov 2005, Arthur Chargueraud wrote:

> I am surprized by a difference of behaviour between the strings
> "bbbb" and (String.make 4 'b').

...

> This is not a real problem, since it is not usual to modify
> strings initialized with quotes, but I am just wandering
> about the reason of such a behaviour...

It's actually easier to write String.copy "abc" instead of "abc"
than:
let f =
   let constant = "abc"
   fun x ->
      ...
      do_something_with constant
      ...

instead of:
let f x =
   ...
   do_something_with "abc"
   ...

which happens all the time.

--
Martin Jambon, PhD
http://martin.jambon.free.fr

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