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digitalmars.D.learn - Trying to understand multidimensional arrays in D

reply Profile Anaysis <PA gotacha.com> writes:
I'm a bit confused by how D does arrays.

I would like to create a array of matrices but I do not seem to 
get the correct behavior:

     int[][4][4] matrix_history;

What I would like is to have a 4x4 matrix and have a history of 
it. Just n 4x4 matrices but each matrix is a fixed size but there 
can be an arbitrary(dynamic) number.

I would like, for example,

matrix_history[0] to be the first 4x4 matrix,
matrix_history[1] to be the second 4x4 matrix,
...

and I would, in fact, like to be able to append a matrix like

matrix_history ~= some_4x4matrix.

I try to assign like

matrix_history[total-1] = new int[][](8,8);

or append

matrix_history ~= new int[][](4,4);

but the append fails with

Error: cannot append type int[][] to type int[][4][4]

which is confusing because the type per entry in the matrix 
history is of type int[][].

e.g., I could wrap the int[][] in a struct and then just have a 
singular array of these matrices and, to me, the logic should be 
the same. e.g.,

struct matrix
{
     int[4][4] data;
}

then

matrix[] matrix_history.

and

matrix_history ~= new matrix;

so, the logic should be the same between two. This method works 
but the more direct method doesn't seem to.

If I do

auto x = matrix_history[0];

x is not a int[4][4] but of type int[4](as reported by the 
debugger), which is very confusing.

it seems that the way D indexes multidimensional arrays is not 
logical nor consistent from my perspective.

auto x = matrix_history[0] returns an array of size 4.
auto x = matrix_history[0][0] returns an 2d array of size 4x4.
auto x = matrix_history[0][0][0] returns an int(as expected).


does this mean that have

     int[][4][4] matrix_history;

backwards?

     int[4][4][] matrix_history;

this creates even a more set of problems.

I guess I will have to revert to wrapping the matrix in a struct 
to get the natural extension of single arrays unless someone can 
clue me in on what is going on.
Jan 25
next sibling parent reply Ivan Kazmenko <gassa mail.ru> writes:
On Thursday, 26 January 2017 at 01:47:53 UTC, Profile Anaysis 
wrote:
 does this mean that have

     int[][4][4] matrix_history;

 backwards?

     int[4][4][] matrix_history;

 this creates even a more set of problems.
In short, you are right, `int[4][4][]` is a dynamic array of `int[4][4]`. In turn, `int[4][4]` is a static length-4 array of `int[4]`, and that is a static length-4 array of `int`. It's quite logical once you learn how to read it: if T is a type, then T[] is a dynamic array of that type, and T[4] is a static length-4 array of that type. So, if I have `int[2][5][7] a;` somewhere, the very last element is `a[6][4][1]`. If you are inclined to think in terms of this difference, the simple rule of thumb would be that the order of dimensions in the declaration is reversed. Also, note that if you want to have, for example, a dynamic array of 5 dynamic arrays of the same length 7 (modeling a C rectangular array, or a D static array, but with possibility to change the length of each row, as well as the number of rows), you would go with `auto a = new int [] [] (5, 7);` (initialization) The static array of 5 static arrays of length 7 is still `int [7] [5] a;` (type declaration) So the reverse only happens in type declarations. (On the contrary, declarations in C or C++ looks rather unintuitive from this perspective: `T a[4][5][6]` is means that `a` is an array of 4 arrays of 5 arrays of 6 arrays of `T`. Note how we have to read left-to-right but then wrap around the string to get the meaning.) Additionally, reading about various kinds of arrays in D might help: https://dlang.org/spec/arrays.html And more in-depth material about array slicing: http://dlang.org/d-array-article.html Ivan Kazmenko.
Jan 25
parent reply Profile Anaysis <PA gotacha.com> writes:
On Thursday, 26 January 2017 at 02:29:07 UTC, Ivan Kazmenko wrote:
 On Thursday, 26 January 2017 at 01:47:53 UTC, Profile Anaysis 
 wrote:
 does this mean that have

     int[][4][4] matrix_history;

 backwards?

     int[4][4][] matrix_history;

 this creates even a more set of problems.
In short, you are right, `int[4][4][]` is a dynamic array of `int[4][4]`. In turn, `int[4][4]` is a static length-4 array of `int[4]`, and that is a static length-4 array of `int`. It's quite logical once you learn how to read it: if T is a type, then T[] is a dynamic array of that type, and T[4] is a static length-4 array of that type. So, if I have `int[2][5][7] a;` somewhere, the very last element is `a[6][4][1]`. If you are inclined to think in terms of this difference, the simple rule of thumb would be that the order of dimensions in the declaration is reversed.
Thanks, knowing the last element is important ; Basically I just need to know the proper index. For me, having the array declared in symbolic form that matches the indexing, like in C/C++, is quicker, easier to remember, and harder to forget. I don't really care too much beyond that. They could be declared any way... but I find myself getting confused in D because of little things like this that don't carry over while almost everything else is.
 Also, note that if you want to have, for example, a dynamic 
 array of 5 dynamic arrays of the same length 7 (modeling a C 
 rectangular array, or a D static array, but with possibility to 
 change the length of each row, as well as the number of rows), 
 you would go with
 `auto a = new int [] [] (5, 7);` (initialization)
 The static array of 5 static arrays of length 7 is still
 `int [7] [5] a;` (type declaration)
 So the reverse only happens in type declarations.
 (On the contrary, declarations in C or C++ looks rather 
 unintuitive from this perspective: `T a[4][5][6]` is means that 
 `a` is an array of 4 arrays of 5 arrays of 6 arrays of `T`.  
 Note how we have to read left-to-right but then wrap around the 
 string to get the meaning.)
lol, I don' tknow what the last sentence means. wrap around the string? Do you mean look at the variable? For me the interpretation above is the most logical because it is a sequential operation in my mind, if you will. x of y of z and the chain can be cut off anywhere and the interpretation still be the same. Since I am a native speaker of English, which is a left to right language, it just makes sense. I, am, of coursed biased because I started with C/C++ rather than D.
 Additionally, reading about various kinds of arrays in D might 
 help:
 https://dlang.org/spec/arrays.html

 And more in-depth material about array slicing:
 http://dlang.org/d-array-article.html

 Ivan Kazmenko.
Jan 25
parent Ivan Kazmenko <gassa mail.ru> writes:
On Thursday, 26 January 2017 at 05:20:07 UTC, Profile Anaysis 
wrote:
 (On the contrary, declarations in C or C++ looks rather 
 unintuitive from this perspective: `T a[4][5][6]` is means 
 that `a` is an array of 4 arrays of 5 arrays of 6 arrays of 
 `T`.  Note how we have to read left-to-right but then wrap 
 around the string to get the meaning.)
lol, I don' tknow what the last sentence means. wrap around the string? Do you mean look at the variable? For me the interpretation above is the most logical because it is a sequential operation in my mind, if you will. x of y of z and the chain can be cut off anywhere and the interpretation still be the same.
This means that in `T a[4][5][6]`, the type `T[4][5][6]` is spread on both sides of the variable name `a`. In the interpretation, "`a` is an array of 4 arrays of 5 arrays of 6 arrays of `T`", note that 4, 5 and 6 are from the right side but T is from the left side. That's what I meant by wrap around.
Jan 26
prev sibling next sibling parent reply Jonathan M Davis via Digitalmars-d-learn writes:
On Thursday, January 26, 2017 01:47:53 Profile Anaysis via Digitalmars-d-
learn wrote:
 I'm a bit confused by how D does arrays.

 I would like to create a array of matrices but I do not seem to
 get the correct behavior:

      int[][4][4] matrix_history;
Like in C/C++, types are mostly read outward from the variable name in D. In both C/C++ and D, int* foo; is a pointer to an int. It's read outward from the variable name, so you get the pointer and then what it points to. Similarly, int** foo; is a pointer to a pointer to an int. In C/C++, a static array would be written like int arr[4][3]; and again, it's read outward from the type. It's a static array of 4 static arrays of 3 ints. This gets increasingly confusing as the types get more complicated, but it's critical for understanding how function pointers are written in C/C++. For D, it's a lot less critical, because we have a cleaner function pointer declaration syntax, but the same basic rules mostly apply (const, immutable, and shared is where they start breaking the rules a bit, but they're pretty straightforward and consistent with just pointers and arrays). That int arr[4][3]; in C/C++ can then be accessed like so arr[3][2] = 5; and that would get the 3rd element in the first array and the second element in the second array without exceeding the bounds of the array. D follows the same declaration rules except that it has the array bounds all on the left-hand side of the variable. So, in C/C++, you have int arr[4][3]; whereas in D, the same array would be int[3][4] arr; and you would still access it like so arr[3][2] = 5; without exceeding the bounds of the array, whereas arr[2][3] = 5; _would_ exceed the bounds of the array, because the second array has 3 elements in it, and that asks for the 4th. This tends to be very confusing at first, because most folks usually expect the indices to always in the same order, when they're not. They are so long as the sizes is always on the right-hand side, which occurs with dynamic arrays, but in D, static arrays go on the left. C/C++ would have the exact same ordering problem as D if it put the sizes on the left, because it uses basically the same rules for how types are written. But they put it on the right, separating from the type, which makes the indices clearer but splits the type in two. So, both approaches have their pros and cons. In any case, the idea that the type is read outward from the variable name extends to types in general. In particular, if you have int[][4][4] arr; as in your example, you have a static array of 4 static arrays of 4 dynamic arrays of int. You can append to the innermost static array arr[3][3] ~= [1, 2, 3]; but you can't append to arr. If you want a dynamic array of static arrays, then you need to do int[4][4][] arr; Then you can append a 4x4 static array to arr. However, your attempts at creating a static array were not actually creating static arrays. auto arr = new int[4]; and auto arr = new int[](4); both allocate a dynamic array of length 4. The code semantics are identical. However, once we go beyond one dimension, it starts mattering - and getting confusing. Take this auto arr = new int[][](4, 5); static assert(is(typeof(arr) == int[][])); assert(arr.length == 4); arr is a dynamic array of length 4 that contains dynamic arrays of length 5 of int. This on the other hand auto arr = new int[4][](5); static assert(is(typeof(arr) == int[4][])); assert(arr.length == 5); makes it so that arr is dynamic array of length 5 that contains static arrays of length 4 of int. auto arr = new int[4][5]; static assert(is(typeof(arr) == int[4][])); assert(arr.length == 5); has the exact same semantics. So, the right-most number is always the length of the outer, dynamic array, and whether the interior is more dynamic arrays or static arrays depends on whether the numbers are between the brackets or the parens. Another thing to note is that when you have int[][], it is a dynamic array, whereas int[4][4] is a static array. So, whenever you see the compiler give you the type int[][], it's talking about a dynamic array, not a static array. The numbers have to be there for it to be a static array. When looking at the type of an array (as opposed to a expression using new), numbers between the subscripts mean a static array, whereas a lack of numbers means a dynamic array, and the type of a dynamic array does not change depending on its length. Also, even if you had declared matrix_history correctly int[4][4][] matrix_history; this code would be wrong
 matrix_history ~= new int[][](4,4);
because int[][](4, 4) is allocating a dynamic array of dynamic arrays of ints, not a static array of 4 static arrays of 4 ints. In addition, AFAIK, you can't just new up a static array of 4 static arrays of int. You can new up dynamic arrays but not static arrays. The static arrays need to be in something to be on the heap. But that's not really what you wanted anyway. Take a simpler example. int[] arr; arr ~= 5; arr ~= 42; Note that you're not newing up the 5 or the 42. If you were newing up the ints, you'd actually have int*[] arr; So, with your matrix_history, when you append a static array to it, you're just appending a value - either a variable or an array literal. e.g. int[4][4] sa; matrix_history ~= sa; or matrix_history ~= [[1, 2, 3, 4], [5, 6, 7, 8], [8, 7, 6, 5], [4, 3, 2, 1]]; Well, hopefully that's not too much information at once, and hopefully it helps. But I'd suggest reading https://dlang.org/spec/arrays.html http://ddili.org/ders/d.en/arrays.html http://dlang.org/d-array-article.html to try and better understand arrays in general. The last one applies more to dynamic arrays, but depending on your current understanding, it could really help you figure out what's going on (be warned though that it does not use the official terminology; per the language spec, int[] is a dynamic array no matter what memory it points to, and there is no special term for the GC-allocated buffer that the dynamic array points to when you use new int[], whereas that article refers to int[] as a slice and the GC-allocated buffer that you get with new int[] as the dynamic array; however, aside from that problem with the terminology, it's a fantastic article and should be quite enlightening). - Jonathan M Davis
Jan 25
next sibling parent reply Profile Anaysis <PA gotacha.com> writes:
On Thursday, 26 January 2017 at 03:02:32 UTC, Jonathan M Davis 
wrote:
 On Thursday, January 26, 2017 01:47:53 Profile Anaysis via 
 Digitalmars-d- learn wrote:
      [...]
Like in C/C++, types are mostly read outward from the variable name in D. In both C/C++ and D, [...]
Thanks. I'll just have to play around with them a bit until it sinks in. I think my problem was declaring them wrong which would always lead to weird errors. I am using static arrays because the size of the matrix is fixed. I need to allocate them though because that is what my matrix_history contains. I guess I can do that with new int[n][n] type of thing? (I think I tried that before. Anyways, probably would work fine now but I already move don to wrapping it in a struct. It provides more flexibility in my case.
Jan 25
parent Jonathan M Davis via Digitalmars-d-learn writes:
On Thursday, January 26, 2017 05:44:04 Profile Anaysis via Digitalmars-d-
learn wrote:
 I am using static arrays because the size of the matrix is fixed.
 I need to allocate them though because that is what my
 matrix_history contains.
If I understood correctly, you want a dynamic array of static arrays where the static array is therefore an entry in your "matrix history." That does not require that you allocate a static array. It just requires that you allocate the dynamic array of static arrays. For instance, if you had an integer instead of a matrix, you would have something like int[] arr; and when you appended to it, you would just append the value. e.g. arr ~= 42; arr ~= 77; arr ~= 9; No allocation of the values is required. The dynamic array may very well do some reallocating to fit the new values (depending on its current capacity), but the values themselves are not allocated. So, if you have a dynamic array of static arrays int[4][4][] arr; then you just append each static array. e.g. int[4][4] value; // fill in value... arr ~= value; or arr ~= [[9, 9, 9, 9], [7, 7, 7, 7], [2, 2, 2, 2], [3, 3, 3, 3]]; Again. You're not allocating the values that you're appending at all. At most, the dynamic array ends up being reallocated to make room for more elements. - Jonathan M Davis
Jan 26
prev sibling parent reply Profile Anaysis <PA gotacha.com> writes:
On Thursday, 26 January 2017 at 03:02:32 UTC, Jonathan M Davis 
wrote:
 On Thursday, January 26, 2017 01:47:53 Profile Anaysis via 
 Digitalmars-d- learn wrote:
      [...]
Like in C/C++, types are mostly read outward from the variable name in D. In both C/C++ and D, [...]
Actually, I think the notation is simply wrong. // Matrix order testing auto x = new int[][][][](1,2,3,4); auto y = new int[1][2][][](3,4); for(int i = 0; i < 1; i++) for(int j = 0; j < 2; j++) for(int k = 0; k < 3; k++) for(int l = 0; l < 4; l++) { x[i][j][k][l] = i*j*k*l; //x[l][k][j][i] = i*j*k*l; //y[i][j][k][l] = i*j*k*l; //y[l][k][j][i] = i*j*k*l; y[k][l][j][i] = i*j*k*l; } It is inconsistent with dynamic arrays and mixing them creates a mess in the order of indices. I best someone was asleep at the wheel when programming the code for static arrays. (probably someone different than who programmed the dynamic arrays) This is a bug IMO.(unfortunately one that can't be fixed ;/)
Jan 25
parent reply Mike Parker <aldacron gmail.com> writes:
On Thursday, 26 January 2017 at 05:50:03 UTC, Profile Anaysis 
wrote:

 It is inconsistent with dynamic arrays and mixing them creates 
 a mess in the order of indices.

 I best someone was asleep at the wheel when programming the 
 code for static arrays. (probably someone different than who 
 programmed the dynamic arrays)

 This is a bug IMO.(unfortunately one that can't be fixed ;/)
No, there's no inconsistence and there's no bug. Shorten things down a bit: ``` auto x = new int[][](3,4); auto y = new int[3][](4); writefln("%s, %s", x[0].length, y[0].length); ``` This will print 4, 3. Why? auto a = int[](4); In this case, a is an array of int. The allocation makes space for 4 values of type int. auto b = int[][](4); Here, b is an array of int[]. The allocation makes space for values of type int[]. The following are the equivalent declarations for static arrays: auto c = int[4]; auto d = int[][4]; In other words int[][4] is the static equivalent of new int[][](4). Following from that: auto e = new int[][](4, 3); This creates an array of 4 values of type int[], each of which is an array that holds 3 values of type int. The static equivalent: auto f = int[3][4]; So based on that, you should be able to see that the problem is not in the implementation of static arrays in D, but a mismatch in your declarations. auto x = new int[][][][](1,2,3,4); auto y = new int[1][2][][](3,4); // Does not match declaration of x To get what you want, change the declaration of x: auto x = new int[][][][](4, 3, 2, 1);
Jan 26
parent Mike Parker <aldacron gmail.com> writes:
Sorry. I mistyped some of my examples. Obviously dropped some 
news:

auto a = new int[](4);
auto b = new int[][](4);

And the static arrays should be:
int[4] c;
int[][4] d;

And I would also like managed to overlook there is no static 
array in sight here:

auto y = new int[1][2][][](3,4);

The 1 and 2 are not creating static arrays. They are dynamic 
arrays. The following are exactly the same:

auto a1 = new int[](2);
auto a2 = new int[2];

The syntax in a2 is the original syntax for allocating space for 
dynamic arrays. The () was added later.

The point I made about changing the declaration of x remains.
Jan 26
prev sibling parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 01/25/2017 05:47 PM, Profile Anaysis wrote:

 a 4x4 matrix and have a history of it. Just
 n 4x4 matrices but each matrix is a fixed size but there can be an
 arbitrary(dynamic) number.
I don't think using aliases is recommended yet. It can simplify things a lot: import std.stdio; alias Row = int[4]; alias Matrix = Row[4]; alias History = Matrix[]; Row makeRow(int value) { Row row; row = value; // Special array syntax; all elements are now 'value' return row; } Matrix makeMatrix() { Matrix matrix; foreach (int i, ref row; matrix) { row = makeRow(i + 1); } return matrix; } void main() { History history; foreach (i; 0..3) { history ~= makeMatrix(); } writeln(history); } As others have said, D's array definition is natural because unlike C's inside-out (or is that outside-in?) syntax, it follows from the alias syntax. Replacing History inside main with Matrix[], etc.: History history; // is the same as: Matrix[] history; // is the same as: Row[4][] history; // is the same as: int[4][4][] history; Ali
Jan 29