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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 Im 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 Im 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