How to check for a prime number in Delphi

Question

I want to read all prime numbers between 1 and 10000 into a dynamic array and all non-prime numbers into another dynamic array and then read the prime array into riche

Answer 1

Perhaps the main problem you have is that you mix all the different aspects (looping over candidates, prime checking, display etc.) into a single function.

Life becomes much easier if you decompose the task into small pieces that just perform a single task. Start with a function to test if a number is prime.

function IsPrime(N: Integer): Boolean;
var
  M: Integer;
begin
  Assert(N > 0);
  if N = 1 then // annoying special case
  begin
    Result := False;
    exit;
  end;
  for M := 2 to (N div 2) do
  begin
    if N mod M = 0 then
    begin
      Result := False;
      exit;
    end;
  end;
  Result := True;
end;

Now you might wish to make a list containing primes:

var 
  Primes: TList<Integer>;
  N: Integer;
....
// create Primes
for N := 1 to 10000 do
  if IsPrime(N) then
    Primes.Add(N);

This is not the most efficient way to enumerate primes. But it's probably where you should start and I've mainly written this answer to encourage you to separate code into small logical methods that do specific focused tasks.

Answer 2

procedure TForm1.btnprimeClick(Sender: TObject); var K, I, iCount : Integer; begin for K := iStart to iEnd do begin iCount := 0; for I := 2 to iEnd do begin if K mod I = 0 then begin Inc(iCount); end; end; if iCount = 1 then begin memData.Lines.Add(IntToStr(K)); end; end; end;