=====Arrays of structures (LBB)=====

//by Richard Russell, December 2014//\\ \\  One of the language extensions provided by [[http://www.lbbooster.com/|LB Booster]] is native **arrays of structures**. These can be useful for structured data storage within a program, or when calling a Windows API function (for example [[http://msdn.microsoft.com/en-us/library/windows/desktop/dd162814.aspx|Polygon]]) which requires an array of structures as one of its parameters.\\ \\  The syntax for declaring an array of structures is very similar to that for declaring a single structure; the only difference is the addition of the dimension(s) in parentheses:\\ 
<code lb>
  STRUCT sname(dims), member1 AS type, member2 AS type... 
</code>
Arrays of structures may have any number of dimensions, limited only by available memory. As with ordinary numeric and string arrays the index value runs from 0 to the specified maximum dimension.\\ \\  Accessing an individual member, in order to change or read its value, again uses a syntax very similar to that used for a single (scalar) structure:\\ 
<code lb>
  sname(index).member.struct = newvalue
  PRINT sname(index).member.struct
</code>
\\  To pass the entire array of structures as a parameter to an API function use the following syntax:\\ 
<code lb>
  sname() AS struct
</code>
Note the use of a pair of parentheses with nothing in between.\\ \\  Here are a couple of examples of the use of arrays of structures. Firstly an adaptation of the program by Dennis McKinney at the [[http://lbpe.wikispaces.com/ArraysAndStructs|Lliberty BASIC Programmer's Encyclopedia]]:\\ 
<code lb>
  'define array of 100 structures
       struct test(99),_
         a as char[20],_
         b as long,_
         c as char[20]

  'for example purposes, fill the whole array of structures
       for i = 0 to 99
         'put some data into the struct
         test(i).a.struct = "Carol - " + str$( i)
         test(i).b.struct = i
         test(i).c.struct = "Andy - " + str$( i)
       next i

  'for example, read all of the structures
       for i = 0 to 99
         A$ = test(i).a.struct
         B = test(i).b.struct
         C$ = test(i).c.struct
         print A$ + " " + str$(B) + " " + C$
       next i

  'To retrieve the third element from the 50th structure:
       C$ = test(49).c.struct
       print C$

  'To change the value of the third element of the 50th structure:
       test(49).c.struct = "Changed" 'change one or more values

  'just for example
       C$ = test(49).c.struct
       print C$

  [quit]
       end
</code>
\\  Secondly, an example of passing an array of structures to an API function:\\ 
<code lb>
      nomainwin
      open "Structure array" for graphics as #w
      #w "trapclose [quit]"
      hw = hwnd(#w)
      calldll #user32, "GetDC", hw as ulong, hdc as ulong

      npoints = 3
      struct points(npoints-1), x as long, y as long
      for i = 0 to npoints-1
          points(i).x.struct = 120 + 120 * cos(2*i)
          points(i).y.struct = 150 + 120 * sin(2*i)
      next
      calldll #gdi32, "Polygon", hdc as ulong, _
        points() as struct, npoints as long, r as long
      wait

  [quit]
      close #w
      end
</code>