Dear FPC,

I'm considering movingn from GPC to FPC. One thing I like very much about GPC is its schema types. I can declare a matrix like this:

matrix_type(num_rows,num_columns:integer)=
   array [1..num_rows,1..num_columns] of real;

and then create a new matrix with:

var
   M:^matrix_ptr;
   num_rows,num_columns:integer;
begin
   readln(num_rows,num_columns);
   M:=new(M,num_rows,num_columns);

So far as I can tell, FPC supports one-dimentional dynamic arrays, but not more complicated schema types. Is that true?

Yours, Kevan

FPC supports multi dimensional dyn. arrays with

var
  a : array of array of real;
begin
  setlength(a,num_rows,num_columns);

...

Dear Florian,

Interesting. What about schema types that are no simple arrays? Like this one:

type
fancy_type(a,b,c:integer)=record
   first:array [0..a,0..b] of integer;
   second:array [0..a,0..b] of real;
   third: array [0..c] of char;
end;
var
   fancy:^fancy_type;
begin
   fancy:=new(fancy,10,20,300);

How do I apply setlength to "first" and "second" after declaring the fancy_type?

Yours, Kevan

fancy:=new(fancy_type); // or something similar, I usually use GetMem not New
SetLength(fancy.first, 10, 20);
SetLength(fancy.second, 10, 20);
SetLength(fancy.third, 300);

Dear FPC,

Nice. I see how it works. Thank you very much (Felipe and Florian) for your answers. I'll try porting some code to FPC.

Yours, Kevan

While I was doing a build, I got the error:

"The procedure entry point Nt could not be located in the dynamic link library ntdll.dll"

I exited Lazarus, restarted, but now this error appears instead of Lazarus. Can this be solved without re-installing or rebooting? (It's a remotely operated number-crunching machine).

One important different to keep in mind between dynamic arrays and regular arrays (including those defined using schema types): dynamic arrays are reference counted and copies are always shallow copies. This means that in this code:

var
   a, b: array of real;
begin
   setlength(a,10);
   b:=a;
   a[5]:=1.0;
end.

at the end, b[5] will also be 1.0. To make a deep copy, use the following code after assignment:

setlength(b,length(b));

Note that this will only make the "first" level unique. I.e., if you have:

var
   a, b: array of array of real;
begin
   setlength(a,10,10);
   b:=a;
   setlength(b,length(b));
   a[5][5]:=1.0;
   writeln(b[5][5]);
end.

then this will still print 1.0. I don't think there is an easier way to make a full deep copy than using a loop to iteratively call setlength on all sub-arrays as well:

var
   a, b: array of array of real;
   i: longint;
begin
   setlength(a,10,10);
   b:=a;
   setlength(b,length(b));
   for i:=0 to high(b) do
    setlength(b[i],length(b[i]));
   a[5][5]:=1.0;
   writeln(b[5][5]);
end.

Jonas

The proper way to do a copy is
b:=copy(a);
:)

It's only a one level deep copy though.

I would also add that it's wise to employ overloaded commands for each data structure that contains dynamic arrays. There may be nested dynamic arrays, and Delphi at least would not respect copying over nested data structures that contained Dynamic Arrays, so I write Copy, Empty, SetSize routines for all my data types that contain dynamic arrays.

I would like to point out that it would be nice if we could have lamda expressions. Does FPC already have this or is it planned?