C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Printing 2D Arrays with Pointers          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Super KHB Guest Posted: Thu Oct 28, 2004 6:42 am    Post subject: Printing 2D Arrays with Pointers I know that I am close, but it wont work. HELP!!!! void PrintA( int * , int); void main() { int num[3][5] = { {1, 2,3,4,5}, {6,7,8,9,10}, {11,12,13,14,15}, }; int *num_ptr; num_ptr = &num[0][0]; PrintA( num_ptr, 5 ); } void PrintA( int *a, int cols ) { for (int r = 0; r < 3 ; r++) {for ( int c = 0; c < 5; c++ ) cout << *(*(a + r) + c) << " "; cout << endl; } } Back to top Siemel Naran Guest Posted: Thu Oct 28, 2004 7:12 am    Post subject: Re: Printing 2D Arrays with Pointers "Super KHB" <look (AT) message (DOT) end> wrote Quote: void PrintA( int *a, int cols ) { for (int r = 0; r < 3 ; r++) {for ( int c = 0; c < 5; c++ ) cout << *(*(a + r) + c) << " "; cout << endl; } } Should the line Quote: cout << *(*(a + r) + c) << " "; be cout << *(*(a + 5*r) + c) << " "; Also, instead of the hardcoded constant "5", do you mean to use the variable "cols"? Back to top Karl Heinz Buchegger Guest Posted: Thu Oct 28, 2004 9:44 am    Post subject: Re: Printing 2D Arrays with Pointers Super KHB wrote: Quote: I know that I am close, but it wont work. HELP!!!! void PrintA( int * , int); void main() { int num[3][5] = { {1, 2,3,4,5}, {6,7,8,9,10}, {11,12,13,14,15}, }; Here you have a memory layout looking like this: num +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+ Quote: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+ and the compiler keeps track of calulating the correct offset from the beginning when given 2 indices Quote: int *num_ptr; num_ptr = &num[0][0]; PrintA( num_ptr, 5 ); } void PrintA( int *a, int cols ) { for (int r = 0; r < 3 ; r++) {for ( int c = 0; c < 5; c++ ) cout << *(*(a + r) + c) << " "; Here you assume a memory layout like this: num +------+ +----+----+----+----+----+ Quote: o--------->| 1 | 2 | 3 | 4 | 5 | | +----+----+----+----+----+ +------+ +----+----+----+----+----+ o------------------>| 6 | 7 | 8 | 9 | 10 | | +----+----+----+----+----+ +------+ +----+----+----+----+----+ o------------>| 11 | 12 | 13 | 14 | 15 | | +----+----+----+----+----+ +------+ ( an array of pointers, where each pointer points to an array of int ) Totally different. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Venki Guest Posted: Thu Oct 28, 2004 11:35 am    Post subject: Re: Printing 2D Arrays with Pointers Quote: cout << *(*(a + r) + c) << " "; try this cout << *(a+((cols * r) + c))<< " "; Back to top Old Wolf Guest Posted: Thu Oct 28, 2004 7:46 pm    Post subject: Re: Printing 2D Arrays with Pointers "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote: Quote: "Super KHB" <look (AT) message (DOT) end> wrote: void PrintA( int *a, int cols ) { for (int r = 0; r < 3 ; r++) {for ( int c = 0; c < 5; c++ ) cout << *(*(a + r) + c) << " "; cout << endl; } } Should the line cout << *(*(a + r) + c) << " "; be cout << *(*(a + 5*r) + c) << " "; We are dealing with a pointer to int so there should be exactly one indirection. I would write it as: a[r * cols + c] (NB. Actually I wouldn't write this because I think it is UB. However we have just had a big discussion with no conclusion on that topic in another thread so I don't want to start it up again here). Back to top Siemel Naran Guest Posted: Fri Oct 29, 2004 7:32 am    Post subject: Re: Printing 2D Arrays with Pointers "Old Wolf" <oldwolf (AT) inspire (DOT) net.nz> wrote in message Quote: "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote: Should the line cout << *(*(a + r) + c) << " "; be cout << *(*(a + 5*r) + c) << " "; Oops, that should be cout << *(a + 5*r + c) << " "; Quote: We are dealing with a pointer to int so there should be exactly one indirection. I would write it as: a[r * cols + c] Thanks for pointing out. Quote: (NB. Actually I wouldn't write this because I think it is UB. However we have just had a big discussion with no conclusion on that topic in another thread so I don't want to start it up again here). What is UB? 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