BBC BASIC Programmers' Reference

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=====Allocating arrays using the Windows or SDL 2.0 API=====

//by Richard Russell, December 2006//\\ \\  You can allocate arrays, using **DIM**, either on BASIC's //heap// (global and PRIVATE arrays) or on the //stack// (LOCAL arrays). Using a combination of these will satisfy the needs of most programs. For example a skeleton program might be structured as follows:\\ \\ 
<code bb4w>
        DIM globalarray1(...), globalarray2(...)
        REPEAT
          PROCdo_something
          PROCdo_something_else
        UNTIL finished
        END

        DEF PROCdo_something
        LOCAL localarray1(), localarray2()
        DIM localarray1(...), localarray2(...)
        REM .....
        ENDPROC

        DEF PROCdo_something_else
        LOCAL localarray3(), localarray4()
        DIM localarray3(...), localarray4(...)
        REM .....
        ENDPROC
</code>
\\  Here the memory occupied by arrays **localarray1()** and **localarray2()** is freed on exit from the first procedure and can be reused by **localarray3()** and **localarray4()** in the second procedure. This is a flexible arrangement and some combination of global and local arrays will usually suffice.\\ \\  However there may very rarely be situations which cannot satisfactorily be resolved using the capabilities of **DIM**. One is when the size of an array exceeds the amount of memory available to BBC BASIC for Windows (a total of 256 Megabytes). Another is when the memory occupied by the arrays cannot be allocated and freed in the required sequence, for example:\\ \\ 

  - Create first array
  - Create second array
  - Destroy first array
  - Destroy second array
\\  Because LOCAL arrays are allocated on the //stack// they can only be destroyed in the reverse order from which they were created.\\ \\  In these cases it is possible to utilise the capabilities of the **Windows or SDL 2.0 API** to allocate and free the memory used by arrays. By setting BASIC's internal pointers to the allocated memory such arrays can be used exactly as if they were declared using **DIM**, but the restrictions on size and sequence of creation and destruction are removed.\\ \\  The routines listed below illustrate how this can be achieved. For simplicity a separate routine is listed for 1-dimensional and 2-dimensional arrays. It should be relatively easy to see how this can be extended to arrays with more dimensions: 

**BBC BASIC for Windows:**
<code bb4w>
        DEF PROCdim1d(RETURN A(),S%,D1%)
        LOCAL A%
        SYS "GlobalAlloc", 64, 5+S%*(D1%+1) TO A%
        ?A%=1 : A%!1=D1%+1
        !^A() = A%
        ENDPROC

        DEF PROCdim2d(RETURN A(),S%,D1%,D2%)
        LOCAL A%
        SYS "GlobalAlloc", 64, 9+S%*(D1%+1)*(D2%+1) TO A%
        ?A%=2 : A%!1=D1%+1 : A%!5=D2%+1
        !^A() = A%
        ENDPROC

        DEF PROCundim(RETURN A())
        SYS "GlobalFree", !^A()
        !^A() = 0
        ENDPROC
</code>

**BBC BASIC for SDL 2.0:**
<code bb4w>
      DEF PROCdim1d(RETURN A(),S%,D1%)
      LOCAL a%%
      SYS "SDL_malloc", 5+S%*(D1%+1) TO a%%
      IF @platform% AND &40 ELSE a%% = !^a%%
      ?a%%=1 : a%%!1=D1%+1
      PTR(A()) = a%%
      ENDPROC

      DEF PROCdim2d(RETURN A(),S%,D1%,D2%)
      LOCAL a%%
      SYS "SDL_malloc", 9+S%*(D1%+1)*(D2%+1) TO a%%
      IF @platform% AND &40 ELSE a%% = !^a%%
      ?a%%=2 : a%%!1=D1%+1 : a%%!5=D2%+1
      PTR(A()) = a%%
      ENDPROC

      DEF PROCundim(RETURN A())
      SYS "SDL_free", PTR(A())
      PTR(A()) = 0
      ENDPROC
</code>

In each case the parameters supplied to the **PROCdim** procedure are the name of the array to be created, the size (in bytes) of each array element and the array's dimension(s). The element size should be specified as follows:\\ \\ 

| **byte** e.g. array&()\\ | size=**1**\\ |
| **32-bit integer** e.g. array%()\\ | size=**4**\\ |
| **40-bit float** e.g. array()\\ | size=**5**\\ |
| **string** e.g. array$()\\ | size=**6**\\ |
| **64-bit float** e.g. array#()\\ | size=**8**\\ |
| **string** e.g. array$()\\ | size=**8**+\\ |
| **64-bit integer** e.g. array%%()\\ | size=**8**+\\ |
| **80-bit float** e.g. array()\\ | size=**10**+\\ |
 + BB4W version 6 only\\ \\  Note that if you free a **string** array using **PROCundim** you must first empty all the elements (set them to zero-length strings), for example:\\ \\ 
<code bb4w>
        array$() = ""
        PROCundim(array$())
</code>
You could use the above procedures to allocate and free arrays in a sequence which cannot otherwise be achieved:\\ \\ 
<code bb4w>
        PROCdim1d(arr1%(),4,99)
        arr1%(50) = 12345678
        PROCdim1d(arr2%(),4,99)
        arr2%(99) = 87654321
        PRINT arr1%(50)
        PROCundim(arr1%())
        PRINT arr2%(99)
        PROCundim(arr2%())
</code>