asldh@acad2.anc.alaska.edu (01/14/91)
Welp, I hope that nobody out there is too offended, but I need to ask a
Really Stupid Question. Ready? I am writing in MS-DOS Turbo C or Microsoft C,
and I need to be able to pass the data contained in a two-dimensional char
array from one function to another. I looked in all of my programming
references and found NOTHING on passing multi-dimensional arrays, and the
one-dimensional array method did not seem to work. I have constructed a
workaround at the moment, which consists of declaring a global array large
enough to handle any of the arrays I need to pass, and then filling it as
needed. However, this "solution" offends me -- it smells of old-time BASIC
methodology... :-} Can anyone offer me a more elegant method? I do not need
to alter the contents of the arrays (at present), but I do need to read them.
Thanks in advance for listening to the ramblings of another lost soul...
(If this is the WRONG newsgroup, please direct your flames to alt.nul...
Thanks!)
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These opinions are not necessarily even mine!
rose@glacier.unl.edu (David Rosecrans) (01/22/91)
In <1991Jan13.130957.1@acad2.anc.alaska.edu> asldh@acad2.anc.alaska.edu writes: > Welp, I hope that nobody out there is too offended, but I need to ask a >Really Stupid Question. Ready? I am writing in MS-DOS Turbo C or Microsoft C, >and I need to be able to pass the data contained in a two-dimensional char >array from one function to another. I looked in all of my programming >references and found NOTHING on passing multi-dimensional arrays, and the >one-dimensional array method did not seem to work. I have constructed a >workaround at the moment, which consists of declaring a global array large >enough to handle any of the arrays I need to pass, and then filling it as >needed. However, this "solution" offends me -- it smells of old-time BASIC >methodology... :-} Can anyone offer me a more elegant method? I do not need >to alter the contents of the arrays (at present), but I do need to read them. >Thanks in advance for listening to the ramblings of another lost soul... >(If this is the WRONG newsgroup, please direct your flames to alt.nul... >Thanks!) Someone asked the question a few weeks ago and I sent I reply directly because I thought that alot of people would respond with the same answer I would give as it turnde out they all either said it couldn't be done (not true) or used some variation on allocating space and doing the multi-dimensional addressing themselves. Though the last one is fine I think what I'll show here is cleaner and easier to use once its set up. After the initial extra work of dynamically allocating the space you can use it just like any other array. Of course you also should free the space once you are done. Anyway here we go, and by the way this does require that you be comfortable with how arrays and pointers (which are almost interchangable) work in C. Lets start with the following decleration. int **t; This says that t is a pointer to a pointer to an int. To graphically represent this: ____ ____ ____ | t*-----> *----->int | |____| |____| |____| Since we can think of a pointer to an element as an array of the same type, so t could also be an array of pointers to ints, and to carry it a bit farther a pointer to an int is functionally similar to an array of ints, we are all familiar with pointers to characters and arrays of characters being the same. ____ ____ ____ ____ ____ ____ ____ | t*----> *----> int| int| int| | ..... |____| |____| |____|____|____|____|____ | *-----> int| int| | | .... |____| |____|____|____|____|____ | | . . |____| ......... | | . . |____| ......... | | . . |____| . . | . | . . | . | . . The nice thing is that we can use it in the same way too. If we say t[3] we get the fourth element of what t points to. That in itself is a pointer to an int so we can use the array index again, which will give us t[3][3] which actually gets an int. So after we allocate the structure the only thing strange is the decleration in passing it. but that is easy to generalize. One '*' is a one-dimensional array, two '**' is a two dimensional array and so on. So enough pointer basics, here's malloc_2d as an example of how to allocate multidimensional arrays it should be easy to modify for any N and it may even generalize, but I've never needed more then a 3d array so I've never tried it. void **malloc_2d(int rows, int cols, size_t size) { void **t; int i; if ((t = (void **) malloc(rows * sizeof(void *))) == NULL) return NULL; for (i = 0; i < rows; i++) if ((t[i] = (void *) malloc(cols * size)) == NULL) return NULL; return t; } You'll may want to free up what ever space is allocated when it fails to allocate the rows, but you will get the idea. To use it: . . . . int **a = malloc_2d(4,5,sizeof(int)); int **b = malloc_2d(4,5,sizeof(int)); int **c; /* should check that malloc 2d worked and need to load something usefull in. */ c = add(a,b,4,5); . . . . int **add(int **a, int **b, int rows, int cols) { int **c = malloc_2d(rows,cols,sizeof(int)); int i; int j; if (c != NULL) { for (i = 0; i < rows; i++) for (j = 0; j < cols; j++) c[i][j] = a[i][j] + b[i][j]; } return c; } So, thats it. I hope it helps. It is also pretty efficient at least on machines were additions are faster than multiplies. David Rosecrans University of Nebraska/Lincoln rose@fergvax.unl.edu ----------------------------- Nothing cute shall appear here.