Primes whose first and last number is 3: Difference between revisions
Primes whose first and last number is 3 (view source)
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There were 2251 primes of the form 3x3 below one million.</pre>
=={{header|Delphi}}==
{{works with|Delphi|6.0}}
{{libheader|SysUtils,StdCtrls}}
<syntaxhighlight lang="Delphi">
function IsPrime(N: int64): boolean;
{Fast, optimised prime test}
var I,Stop: int64;
begin
if (N = 2) or (N=3) then Result:=true
else if (n <= 1) or ((n mod 2) = 0) or ((n mod 3) = 0) then Result:= false
else
begin
I:=5;
Stop:=Trunc(sqrt(N+0.0));
Result:=False;
while I<=Stop do
begin
if ((N mod I) = 0) or ((N mod (I + 2)) = 0) then exit;
Inc(I,6);
end;
Result:=True;
end;
end;
procedure GetDigits(N: integer; var IA: TIntegerDynArray);
{Get an array of the integers in a number}
var T: integer;
begin
SetLength(IA,0);
repeat
begin
T:=N mod 10;
N:=N div 10;
SetLength(IA,Length(IA)+1);
IA[High(IA)]:=T;
end
until N<1;
end;
procedure ShowFirstLast3Prime(Memo: TMemo);
var I,Cnt1,Cnt2: integer;
var IA: TIntegerDynArray;
var S: string;
function FirstLast3(N: integer): boolean;
var I: integer;
begin
GetDigits(N,IA);
Result:=(IA[0]=3) and (IA[High(IA)]=3);
end;
begin
Cnt1:=0; Cnt2:=0;
S:='';
for I:=0 to 1000000-1 do
if IsPrime(I) then
if FirstLast3(I) then
begin
Inc(Cnt1);
if I<4000 then
begin
Inc(Cnt2);
S:=S+Format('%5D',[I]);
If (Cnt2 mod 5)=0 then S:=S+CRLF;
end;
end;
Memo.Lines.Add(S);
Memo.Lines.Add('Count < 1,000 = '+IntToStr(Cnt2));
Memo.Lines.Add('Count < 1,000,000 = '+IntToStr(Cnt1));
end;
</syntaxhighlight>
{{out}}
<pre>
3 313 353 373 383
3023 3083 3163 3203 3253
3313 3323 3343 3373 3413
3433 3463 3533 3583 3593
3613 3623 3643 3673 3733
3793 3803 3823 3833 3853
3863 3923 3943
Count < 1,000 = 33
Count < 1,000,000 = 2251
Elapsed Time: 181.797 ms.
</pre>
=={{header|F_Sharp|F#}}==
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