Solved

Defining a subset of an array

Posted on 2006-11-24
788 Views
Hi, need to pass an array to a C compiled ddl, read the returned array and break it up into 4 smaller arrays.

The array I want to read back, is one long array, containing 4 sets of smaller arrays. I want to assign the address of the first element of each subset of data to a variable (of type array) so that I have 4 smaller arrays.

Consider the sample code:

const
Samples = 10;
type
TArray = array of Double;
var
Data, Data0, Data1, Data2, Data3: ^TArray;
begin
SetLength(Data^, 4*Samples);

//The actual Delphi call to the C DLL
//"var readArray: Double;" is the variable in question

//Pass the address of element 0 of the array the contain all 40 data points
//Set the address of Data0, the fisrt 10 data points
Data0 := Data;
SetLength(Data0^, Samples);

//Set the address of Data1, the second 10 data points
Data1 := Data^[10];
SetLength(Data1^, Samples);

//Set the address of Data2, the third 10 data points
Data2 := Data^[20];
SetLength(Data2^, Samples);

//Set the address of Data3, the forth 10 data points
Data3 := Data^[30];
SetLength(Data3^, Samples);
end;//The code above does not work in practise, but I'd it to be implemented in a simular manner

//This is how I can get it to work
const
Samples = 10;
type
TArray = array of Double;
var
Data, Data0, Data1, Data2, Data3: TArray;
n: integer;
begin
SetLength(Data^, 4*Samples);

//Pass the address of element 0 of the array the contain all 40 data points
//Set the address of Data0, the fisrt 10 data points
Data0 := Data;
SetLength(Data0^, Samples);

SetLength(Data1^, Samples);
SetLength(Data2^, Samples);
SetLength(Data3^, Samples);

for n := 0 to 9 do
begin
Data1^[n] := Data^[n+10];
Data2^[n] := Data^[n+20];
Data3^[n] := Data^[n+30];
end;
end;

This is a problem as I need to process up to 5E6 for each data set

0
Question by:cfm
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

• Help others & share knowledge
• Earn cash & points

LVL 28

Accepted Solution

2266180 earned 250 total points
ID: 18006893
you won't be able to do that with just dynamic arrays since they are not just a series of elements in the memory. there is otehr information saved as well.

shortly:
- the data starts at @a[0]
- the data about teh array (length, element size?, etc) is stored at @a
- if you get the address of element X from the array, you still need a valid address for the dynamic array itself in order to cast and successfully use.

the workaround is to use static arrays. small example:

procedure TForm1.FormCreate(Sender: TObject);
type TElement=integer;
PArray=^Tarray;
TArray=array [0..30000] of TElement;
var a:TArray;
i:integer;
s:array[1..4] of PArray;
ss:string;
l:integer;
begin
l:=16;
for i:=1 to l do// initialize some test data
a[i-1]:=i;
for i:=1 to 4 do// initialize the subsets
s[i]:=pointer(integer(@a[0])+(i-1)*(l div 4){the number of elements in the subset}*sizeof(TElement){the length of the type of the elements});
ss:='';
for i:=1 to 4 do// show the first element of each subset
ss:=ss+inttostr(s[i]^[0])+' ';
showmessage(ss);
end;
0

LVL 14

Expert Comment

ID: 18008610
It seems your returned array is of fixed length i.e. 40 bytes and you want to reference it as 4 separate arrays each of 10 bytes. If my understanding is correct, then you simply need to use a multidimensional array to reference the data accordingly. Something like this:

var
Data: array[1..4, 1..10] of double;
begin
after your call to DAQmxReadAnalogF64 the Data variable can be referenced as follows:
Data[1,1] = data set 1 point 1
Data[1,2] = data set 1 point 2
Data[2,1] = data set 2 point 1
Data[3,1] = data set 3 point 1
etc.

another option is to structure the data something like this:

type
TDataPoints = array[1..10] of double;
TDataPointsSet = array[1..4] of TDatapoints;

Regards
Pierre
0

LVL 9

Expert Comment

ID: 18009330
IF Samples is constant, you may also use a record:

const
Samples = 10;
type
MyArray = packed record
case byte of
0: (a: array[1..4*Samples] of integer);
1: (b, c, d, e: array [1..Samples of integer);
end;

In this case, record.a overlaps with record.b, record.c, record.d and record.e so it does what you want with an easier syntax:
with record do
begin
a[1] := ...
b[1] := ...
end;

I like the other (Ciuly's and Pierre's) ideas better, though! :-)
0

Author Comment

ID: 18025793
PierreC and alkisq

Did try playing with multidimensional arrays and records earlier.  Ran into trouble getting the variable accepted by the function in the dll.
0

LVL 28

Expert Comment

ID: 18025814
I'm just curious: what wasn't good enough?
0

Featured Post

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

creating threads in delphi 1 202
How to convert memory stream to PDF file 6 230
Firemonkey Listview item popup menu ? 1 73
Automatic field translation delphi 10.2 6 45
Hello everybody This Article will show you how to validate number with TEdit control, What's the TEdit control? TEdit is a standard Windows edit control on a form, it allows to user to write, read and copy/paste single line of text. Usua…
In my programming career I have only very rarely run into situations where operator overloading would be of any use in my work.  Normally those situations involved math with either overly large numbers (hundreds of thousands of digits or accuracy re…
Six Sigma Control Plans
Why Skyport?
Suggested Courses
Course of the Month5 days, 9 hours left to enroll