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digitalmars.D.learn - Why does multidimensional arrays not allocate properly?

reply Jot <Jot datacentralave.com> writes:
auto x = new int[][](n,m);

But one cannot freely assign anywhere in x:

x[3,6] = 4 crashes.

I, can, of course, convert everything to a linear matrix and 
index by i+w*j, but what's the point of having multidimensional 
matrices in D if they don't allocate them fully?
Jan 22 2017
parent reply rikki cattermole <rikki cattermole.co.nz> writes:
On 22/01/2017 9:05 PM, Jot wrote:

 auto x = new int[][](n,m);

 But one cannot freely assign anywhere in x:

 x[3,6] = 4 crashes.

 I, can, of course, convert everything to a linear matrix and index by
 i+w*j, but what's the point of having multidimensional matrices in D if
 they don't allocate them fully?


It does allocate them fully, you're indexing them wrong.

void main() {
	auto x = new int[][](1, 2);
	x[0][1] = 3;	
}
Jan 22 2017
parent reply Jot <Jot datacentralave.com> writes:
On Sunday, 22 January 2017 at 08:07:26 UTC, rikki cattermole 
wrote:

 On 22/01/2017 9:05 PM, Jot wrote:

 auto x = new int[][](n,m);

 But one cannot freely assign anywhere in x:

 x[3,6] = 4 crashes.

 I, can, of course, convert everything to a linear matrix and 
 index by
 i+w*j, but what's the point of having multidimensional 
 matrices in D if
 they don't allocate them fully?


 It does allocate them fully, you're indexing them wrong.

 void main() {
 	auto x = new int[][](1, 2);
 	x[0][1] = 3;	
 }


No, that isn't the reason, it was cause I was going past the end 
when I added some new code(the [3,6] was suppose to be [3][6]).

I tried it before and it was crashing before I added the new code 
and visualD seems to not be updating variable values properly 
anymore so I can't really debug ;/


In anycase, what is the correct notation for indexing?

x = new int[][](width, height)

and x[height][width] or x[width][height]?
Jan 22 2017
next sibling parent albert-j <djftgls ifdflv.com> writes:
 In anycase, what is the correct notation for indexing?

 x = new int[][](width, height)

 and x[height][width] or x[width][height]?


It's x[width][height], but because indexing is 0-based, largest 
valid indexes are
x[width-1][height-1].
Jan 22 2017
prev sibling parent Nicholas Wilson <iamthewilsonator hotmail.com> writes:
On Sunday, 22 January 2017 at 08:18:35 UTC, Jot wrote:

 On Sunday, 22 January 2017 at 08:07:26 UTC, rikki cattermole 
 wrote:

 On 22/01/2017 9:05 PM, Jot wrote:

 auto x = new int[][](n,m);

 But one cannot freely assign anywhere in x:

 x[3,6] = 4 crashes.

 I, can, of course, convert everything to a linear matrix and 
 index by
 i+w*j, but what's the point of having multidimensional 
 matrices in D if
 they don't allocate them fully?






If you want multidimensional array (matrices, tensors) use either
std.experimental.ndslice or mir.ndslice (they are (effectively) 
the
same package, one is a dev version of the other).
see https://github.com/libmir/


 It does allocate them fully, you're indexing them wrong.

 void main() {
 	auto x = new int[][](1, 2);
 	x[0][1] = 3;	
 }


 No, that isn't the reason, it was cause I was going past the 
 end when I added some new code(the [3,6] was suppose to be 
 [3][6]).

 I tried it before and it was crashing before I added the new 
 code and visualD seems to not be updating variable values 
 properly anymore so I can't really debug ;/


 In anycase, what is the correct notation for indexing?

 x = new int[][](width, height)

 and x[height][width] or x[width][height]?


The trick is to remember that in D int[][] is effectively 
(int[])[].
As such indexing the outer dimension gives you the inner 
dimension.

so x[height-1][width-1] will give you the "last" element.

visually if - is an int
[ - - - - - - - - -]
[ - - - - 9 - - - -]
[ - - - - - - - - -]
the first index (i.e index it once and it) gives you the row, 
index it again to get the value.

so that 9 is
x[1][4]
Jan 22 2017