>I need a function which randomly takes a subarray of n elements from a
given array
> (where each element of the input array can only be once in the subarray).
>Of course I can just make a while loop, where a random element is each
>time picked from the input array, but when this element is allready in
>the output array from a previous turn of the loop, this needs to be
>repeated. Maybe this can be quite time consuming
One good idea which someone on this list suggested some time ago was to
swap the element that you have just found (by random (x), where x
indicates the number of elements in a zero-based array) with the last
element in the arraray and reduce x by one for the next time. This
changes the order of input data of course. Example code below.
hth Malcolm
type
TIntArray = array of integer;
procedure GetSubset(arr, result: TIntArray);
var
HighArr,SwapInt,i,j:integer;
begin
HighArr:=high(arr);
assert(HighArr>high(result),'Something wrong!');
for i:=low(result) to high(result) do begin
j:=random(HighArr+1);
result[i]:=arr[j];
SwapInt:=arr[HighArr];
arr[HighArr]:=arr[j];
arr[j]:=SwapInt;
dec(HighArr);
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
a,b : TIntArray;
i : integer;
s : string;
begin
SetLength(a,10);
for i:=low(a) to high(a) do a[i]:=i;
SetLength(b,3);
GetSubSet(a,b);
s:='';
for i:=low(b) to high(b)-1 do begin
s:=s+IntToStr(b[i])+',';
end;
s:=s+IntToStr(b[high(b)]);
ShowMessage(s);
end;
initialization
randomize;
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