uni/inf2100/pascal2100/examples/primes.pas

/* This program prints all primes up to Limit
{which is 1000 in this example}
using a technique called ’The sieve of Eratosthenes’.
*/

program Primes;

const Limit = 1000;

var prime : array [2..Limit] of Boolean;
    i      : integer;


procedure FindPrimes;
var i1 : integer;    I2 : Integer;
begin
    i1 := 2;
    while i1 <= Limit div 2 do begin
        i2 := 2 * i1;
        while i2 <= Limit do begin
            prime[i2] := false;
            i2 := i2+i1
        end;
        i1 := i1 + 1
        end
    end; {FindPrimes}

function NDigits (v : integer): integer;
/* How many digits are there in v (which is >= 0)? */
begin
    if v <= 9 then NDigits := 1
else NDigits := 1 + Ndigits(v div 10)
end; {NDigits}

procedure P4 (x : integer);
/* *Note* Equivalent to "printf("%4d",x);" in C. */
var NSpaces : integer;
begin
    NSpaces := 4 - NDigits(x);
    while NSpaces > 0 do begin
        write(’ ’);  NSpaces := NSpaces-1
    end;
    write(x);
end; {P4}

procedure PrintPrimes;
var i       : integer;
   NPrinted : integer;
begin
    i := 2;  NPrinted := 0;
    while i <= Limit do begin
        if prime[i] then begin
            if (NPrinted > 0) and (NPrinted mod 10 = 0) then write(eol);
            P4(i);  NPrinted := NPrinted + 1;
        end;
        i := i + 1;
    end;
    write(eol)
end; {PrintPrimes}


begin {main program}
    i := 2;
    while i <= Limit do begin prime[i] := true;  i := i+1 end;

    /* Find and print the primes:*/
    FindPrimes;  PrintPrimes;
end. {main program}