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