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digitalmars.D.learn - Is [ 0, 1, 2 ] an immutable array?

reply Ali Cehreli <acehreli yahoo.com> writes:
Does the expression [ 0, 1, 2 ] form an immutable array? If so, is the
assignment to a[0] undefined below? Is it trying to modify an immutable element?

    int[] a = [ 0, 1, 2 ];
    a[0] = 42;

The reason for my thinking that [ 0, 1, 2] is an array is because it has the
.dup property and this works too:

    int[] a = [ 0, 1, 2 ].dup;

Thank you,
Ali
Aug 11 2009
next sibling parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
Ali Cehreli wrote:

 Does the expression [ 0, 1, 2 ] form an immutable array? If so, is the
assignment to a[0] undefined below? Is it trying to modify an immutable element?
 
     int[] a = [ 0, 1, 2 ];
     a[0] = 42;
 
 The reason for my thinking that [ 0, 1, 2] is an array is because it has the
.dup property and this works too:
 
     int[] a = [ 0, 1, 2 ].dup;
 
 Thank you,
 Ali
 



Nope, it's an ordinary, mutable array. :) To create an immutable array 
you can do like this:

   // This is an immutable array of ints:
   immutable int[] a = [ 0, 1, 2 ];

   // This is an array of immutable ints:
   immutable(int)[] a = [ 0, 1, 2 ];

The .dup property simply creates a copy of the array, which can be 
useful whether the array is immutable or not.

(Note that there will be some changes in array syntax/semantics from the 
next version of DMD2. In particular, arrays of type T[] will be 
unresizable. Resizable arrays will have a new type, denoted T[new].)

-Lars
Aug 12 2009
prev sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 11 Aug 2009 19:05:56 -0400, Ali Cehreli <acehreli yahoo.com> wrote:


 Does the expression [ 0, 1, 2 ] form an immutable array? If so, is the  
 assignment to a[0] undefined below? Is it trying to modify an immutable  
 element?

     int[] a = [ 0, 1, 2 ];
     a[0] = 42;

 The reason for my thinking that [ 0, 1, 2] is an array is because it has  
 the .dup property and this works too:

     int[] a = [ 0, 1, 2 ].dup;


No, it's a mutable array.  It's one of the quirks of D2 that bugs me.  A  
string literal is an immutable array but a normal array literal actually  
allocates new space on the heap for the array every time you use it.  So  
if you assign the same literal to 2 different variables, they are 2  
separate copies of the array.

I think the behavior should be identical to strings.

I think there's even a bugzilla for that...

-Steve
Aug 12 2009
parent reply Ali Cehreli <acehreli yahoo.com> writes:
Steven Schveighoffer Wrote:


     int[] a = [ 0, 1, 2 ];
     a[0] = 42;





 No, it's a mutable array.  It's one of the quirks of D2 that bugs me.  A  
 string literal is an immutable array but a normal array literal actually  
 allocates new space on the heap for the array every time you use it.  So  
 if you assign the same literal to 2 different variables, they are 2  
 separate copies of the array.
 
 I think the behavior should be identical to strings.


I agree. I thought that D was a good first language to teach, so I've started
to write a tutorial; but I am having big difficultly extracting the semantics
of arrays and slices.

I still can't understand how to explain dynamic arrays and slices even to
myself yet. :D

Could anyone point me to documentation that would clarify these issues for me,
for a person who thinks he knows C and C++ arrays and vectors pretty well? :p

Andrei, can we review your chapter on arrays please? ;)

Thank you,
Ali
Aug 12 2009
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Wed, 12 Aug 2009 12:41:20 -0400, Ali Cehreli <acehreli yahoo.com> wrote:


 Steven Schveighoffer Wrote:


     int[] a = [ 0, 1, 2 ];
     a[0] = 42;





 No, it's a mutable array.  It's one of the quirks of D2 that bugs me.  A
 string literal is an immutable array but a normal array literal actually
 allocates new space on the heap for the array every time you use it.  So
 if you assign the same literal to 2 different variables, they are 2
 separate copies of the array.

 I think the behavior should be identical to strings.


 I agree. I thought that D was a good first language to teach, so I've  
 started to write a tutorial; but I am having big difficultly extracting  
 the semantics of arrays and slices.

 I still can't understand how to explain dynamic arrays and slices even  
 to myself yet. :D


Hold off.  Arrays and slices are about to change drastically (see thread  
on T[new] in digitalmars.D)

-Steve
Aug 12 2009