Re: Variable-length arrays (from a C programmer)

[Stephen Leake <stephen.leake@gsfc.nasa.gov>]

Paul T. McAlister wrote:
> 
> I have an application where I will be displaying a number of different
> screens full of text messages.
> I want these messages and screens to be data table driven.
> 
> In "C" (see example), I can define an array of messages for each page,
> then define an array of pages for the whole thing.
> I don't have to worry about string length or number of items defined in
> the arrays, the compiler handles all of that for me.
> 
> I have not been able to figure out how to do the equivalent thing in
> Ada.
> I don't know how to define strings without setting the length to the
> maximum length string I will have, which wastes a lot of space.
> I think I know how to define the variable length arrays for the messages
> for each screen, but I don't know how to define an array type of these
> arrays of string for each page (since they are different types because
> they are different lengths).
> I am using Ada83.
> 
> I know this is a lengthy question, but any help would be greatly
> appreciated.


> 
> - - - - - - - - - - - -
> 
> #include <stdio.h>
> 
> // structure for each message
> typedef struct
> {
>   char *text;   // pointer to message text
>   int   xPos;   // x position on screen
>   int   yPos;   // y position on screen
> }
> MSG_STRUCT;
> 
> // structure for each page
> typedef struct
> {
>   MSG_STRUCT  *msg;      // pointer to array of message structures
>   int         num_msgs;   // number of message on this page
> }
> PAGE_STRUCT;
> 
> // page 1 messages
> MSG_STRUCT page1[] =
> { {"Greetings Earthling", 0, 0}
> , {"Take me to your leader", 1, 0}
> };
> 
> // page 2 messages
> MSG_STRUCT page2[] =
> { {"Screen 2", 0, 0}
> , {"This text is some lines farther down", 5, 0}
> , {"and this is even FARTHER down", 10, 0}
> };
> 
> // page 3 messages
> MSG_STRUCT page3[] =
> { {"This is the third screen", 0, 0}
> , {"and by now you should be getting", 1, 10}
> , {"the point of what it is", 2, 12}
> , {"I am trying to do!!", 3, 14}
> };
> 
> // messages for all pages
> PAGE_STRUCT pages[] =
> { {page1, (sizeof(page1) / sizeof(page1[0]))}
> , {page2, (sizeof(page2) / sizeof(page2[0]))}
> , {page3, (sizeof(page3) / sizeof(page3[0]))}
> };
> 
> void main()
> {
>   int iPage;
>   int iMsg;
>   int nPage;
> 
>   // find number of pages
>   nPage = sizeof(pages) / sizeof(pages[0]);
> 
>   // for each page
>   for (iPage = 0; iPage < nPage; ++iPage)
>   {
>     printf("\n");   // put blank line between pages
> 
>     // for each message
>     for (iMsg = 0; iMsg < pages[iPage].num_msgs; ++iMsg)
>     {
>       printf(pages[iPage].msg[iMsg].text);  // print message
>       printf("\n");                         // terminate line
>     }
>   }
> }
> 
>   --PTM                                   8-)
> --------------------------------------
> Paul McAlister
> Paul.McAlister@AlliedSignal.com
> 913.768.2238
> --------------------------------------