uucigj@gandalf.sbc.com (Gregg Jensen (5-3531)) (10/17/90)
Novice question: how do you use new to allocate space for a two
dimension array?
char **matrix = new ??????;
Gregg Jensen
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---------------------------------------------------------------------- pena@fuug.fi (Olli-Matti Penttinen) (10/24/90)
In article <1990Oct17.153438.12299@swbatl.sbc.com> uucigj@gandalf.sbc.com (Gregg Jensen (5-3531)) writes: >Novice question: how do you use new to allocate space for a two >dimension array? >char **matrix = new ??????; It's tempting to assume that char **matrix = new char[XMAX][YMAX]; would do the trick. However, the real problem lies elsewhere. When you use a real multidimensional array, you only need room for the actual elements, since the compiler knows how the calculate the offset of any element at run time. With dynamic arrays, it's a different thing: you either (a) explicitly calculate offsets char *matrix = new char[XMAX * YMAX]; ... matrix[i * YMAX + j] = ...; or (b) in addition to the actual data, allocate an extra vector of XMAX pointers (in this case char *) and initialize them to point at elements 0, YMAX, 2 * YMAX, ... , (XMAX -1) * YMAX, respectively: char *data = new char[XMAX * YMAX]; char *ptrs = new (char(*[XMAX])); for (int i = 0; i < XMAX; i++) matrix[i] = data + i * YMAX; ... matrix[i][j] = ...; This works because by definition a[i][j] is actually *(*(a + i)+j). Note, however that this seemless integration of dynamic multi-dimensional arrays doesn't necessarily work for class objects, whose [], *, &, etc. operators have different semantics from their mundane counterparts. Note also that this approach is readily extensible to 3, 4, ... dimensional cases, because every 3-D array can be considered a 1-D array whose elements happen to be 2-D arrays (or 1-D arrays of 1-D arrays, for that matter) ad infinitum. To make life easier, I wrote a couple of years ago a fairly portable K&R C function, that only uses malloc() once to allocate and initialize an arbitrary dimensioned array of any type. It is less than 100 lines of code, I think. If anyone is interested, let me know. I hope this helps, ==pena -- Olli-Matti Penttinen <pena@innopoli.fi> | "When in doubt, use brute force." | Brainware Oy | --Ken Thompson | Tekniikantie 17 +-----------------------------------+ 02150 ESPOO, Finland Tel. +358 0 4375 320 Fax. +358 0 4553 117 -- Olli-Matti Penttinen <pena@innopoli.fi> | "When in doubt, use brute force." | Brainware Oy | ==Ken Thompson | Tekniikantie 17 +-----------------------------------+ 02150 ESPOO, Finland Tel. +358 0 4375 320 Fax. +358 0 4553 117
gregs@hpmtlx.HP.COM ($Greg_Stander) (10/26/90)
> Novice question: how do you use new to allocate space for a two > dimension array? > > char **matrix = new ??????; Of course if dimensions are constants, this works and is usually easier. #include <stream.h> #include <stdio.h> const int XMAX = 3; const int YMAX = 20; typedef char array_type[XMAX][YMAX]; main() { array_type &myarray = *(array_type *)new char[XMAX * YMAX]; // Init to some values for(int row=0; row<XMAX; row++) for(int col=0; col<YMAX; col++) myarray[row][col] = 'z'; for(row=0; row<XMAX; row++) sprintf(myarray[row], "This is row %d", row); // Show as array for(row=0; row<XMAX; row++) cout << "myarray[" << row << "] = " << myarray[row] << "\n"; // Show as individiual characters char *show = (char *)myarray; for(int index = 0; index<XMAX*YMAX; index++) cout << show[index] << (((index+1)%YMAX)?"":"\n"); } Greg Stander