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Defining a subset of an array

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
//function DAQmxReadAnalogF64(taskHandle:Longint; numSampsPerChan:Longint; timeout:Double; fillMode:DAQmxFillMode;var readArray: Double; arraySizeInSamps:Longint; var sampsPerChanRead:Longint; reserved:Pointer):Longint; stdcall;
//"var readArray: Double;" is the variable in question

//Pass the address of element 0 of the array the contain all 40 data points
ReadData(Data^[0]); //Actual declaration: ReadData(var Data : array of double);
//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
ReadData(Data^[0]);
//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

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Pierre Cornelius

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;

Let me know if you need more info.

Regards
Pierre

alkisg

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! :-)

cfm

ASKER

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.

2266180

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