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digitalmars.D.learn - remaping arrays

reply BCS <BCS_member pathlink.com> writes:
What is the easiest way to paint something like "real[5][]" over something like
"real[]".
What I’m looking for would result in this:

real[] a;
real [5][] b;

b.length = a.length/b[0].length;
b.ptr = a.ptr;		// or whatever

now:

b[0] == a[0..5];
b[1] == a[5..10];
..
b[last] == a[(b.last-1)*5 .. b.last*5];


this should not requier any copying.
Aug 02 2005
next sibling parent reply "Regan Heath" <regan netwin.co.nz> writes:
On Tue, 2 Aug 2005 20:38:21 +0000 (UTC), BCS <BCS_member pathlink.com>  
wrote:

 What is the easiest way to paint something like "real[5][]" over  
 something like
 "real[]".
 What I’m looking for would result in this:

 real[] a;
 real [5][] b;

 b.length = a.length/b[0].length;
 b.ptr = a.ptr;		// or whatever

 now:

 b[0] == a[0..5];
 b[1] == a[5..10];
 ..
 b[last] == a[(b.last-1)*5 .. b.last*5];


 this should not requier any copying.


import std.stdio;

void main()
{
	static int[] a =  
[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24];
	int[][5] b;
	
	//paint
	foreach(int i, inout int[] row; b)
		row = a[(i*5)..(i*5)+5];
	
	//display
	foreach(int line, int[] row; b)
	{
		writef(line,": ");
		foreach(int i; row)
		{
			writef("%02d ",i);
		}
		writefln("");
	}
	
	writefln("");
	
	//proof
	writefln("No copying, see:");
	writefln("a: %x",a.ptr);
	foreach(int line, int[] row; b)
		writefln("%d: ",line,"%x",row.ptr);
}

Regan
Aug 02 2005
parent reply BCS <BCS_member pathlink.com> writes:
In article <opsuwik1iy23k2f5 nrage.netwin.co.nz>, Regan Heath says...

Not quite.

I need a dynamic array of arrays-of-const size



 real [5][] b;




you used an array-of-const size of dynamic arrays


	int[][5] b;
	


BTW, I havn't run you code yet, but I'll try it later.



 Aug 02 2005



  parent  "Regan Heath" <regan netwin.co.nz>  writes:


On Tue, 2 Aug 2005 23:51:48 +0000 (UTC), BCS <BCS_member pathlink.com>  
wrote:

 In article <opsuwik1iy23k2f5 nrage.netwin.co.nz>, Regan Heath says...

 Not quite.

 I need a dynamic array of arrays-of-const size



 real [5][] b;




 you used an array-of-const size of dynamic arrays


 	int[][5] b;
 	




I realised this, I thought maybe you had it backwards.

The basic problem is that a const-size array has a fixed memory location,  
so you cannot make it refer to another existing array, you have to copy  
into it.

So your options are, fixed-array and copy or dynamic array and no copy.  
AFAIKS.


 BTW, I havn't run you code yet, but I'll try it later.


NP.

Regan



 Aug 02 2005



prev sibling  parent reply Derek Parnell <derek psych.ward>  writes:


On Tue, 2 Aug 2005 20:38:21 +0000 (UTC), BCS wrote:


 What is the easiest way to paint something like "real[5][]" over something like
 "real[]".
 What I’m looking for would result in this:
 
 real[] a;
 real [5][] b;
 
 b.length = a.length/b[0].length;
 b.ptr = a.ptr;		// or whatever
 
 now:
 
 b[0] == a[0..5];
 b[1] == a[5..10];
 ..
 b[last] == a[(b.last-1)*5 .. b.last*5];
 
 this should not requier any copying.


At first I thought you couldn't do this, but I did some tests and here is
my result...

<code>
import std.stdio;

void main()
{
    real[5][] a;
    real[] b;

    // Populate the unmapped array with some test data.
    real k = 6.996; // starting number
 
    for (int i = 0; i < 7; i++) // create 7 real[5] arrays.
    {
        a.length = a.length + 1;

        for( int j = 0; j < 5; j++, k+=1.569)
            a[$-1][j] = k;
    }

    // Remap the array.
    b = cast(real[])a;
    b.length = a.length * 5;

    // Display the remapped array.
    foreach(int i, real x; b)
        writefln("Elem[%2d] = %s", i, x);
}
</code>
-- 
Derek
Melbourne, Australia
3/08/2005 10:32:50 AM



 Aug 02 2005



  parent  "Regan Heath" <regan netwin.co.nz>  writes:


On Wed, 3 Aug 2005 10:34:23 +1000, Derek Parnell <derek psych.ward> wrote:

 On Tue, 2 Aug 2005 20:38:21 +0000 (UTC), BCS wrote:


 What is the easiest way to paint something like "real[5][]" over  
 something like
 "real[]".
 What I’m looking for would result in this:

 real[] a;
 real [5][] b;

 b.length = a.length/b[0].length;
 b.ptr = a.ptr;		// or whatever

 now:

 b[0] == a[0..5];
 b[1] == a[5..10];
 ..
 b[last] == a[(b.last-1)*5 .. b.last*5];

 this should not requier any copying.


 At first I thought you couldn't do this, but I did some tests and here is
 my result...

 <code>
 import std.stdio;

 void main()
 {
     real[5][] a;
     real[] b;

     // Populate the unmapped array with some test data.
     real k = 6.996; // starting number
    for (int i = 0; i < 7; i++) // create 7 real[5] arrays.
     {
         a.length = a.length + 1;

         for( int j = 0; j < 5; j++, k+=1.569)
             a[$-1][j] = k;
     }

     // Remap the array.
     b = cast(real[])a;
     b.length = a.length * 5;

     // Display the remapped array.
     foreach(int i, real x; b)
         writefln("Elem[%2d] = %s", i, x);
 }
 </code>


You've made a liar out of me. I should learn to read all the replied  
before posting.

The memory layout of "real[5][]" and the fact that a "real[5]" is not an  
'array struct' makes painting as real[] possible, very clever.

Regan



 Aug 02 2005