ALGOL 68

# Sieve of Eratosthenes - Algol 68 version #
# The original Sieve of Eratosthenes algorithm #

PROC sieve = (INT limit, REF[]BOOL primes)VOID:
BEGIN
  # Initialise the array to all TRUE #
  FOR i FROM LWB primes TO UPB primes DO primes[i] := TRUE OD;

  # Set 0 and 1 to not be prime #
  primes[0] := primes[1] := FALSE;

  # For all numbers from 2 up to the square root of the limit #
  FOR i FROM 2 TO ENTIER sqrt(limit) DO
    # If the number is prime #
    IF primes[i] THEN
      # Mark all of its multiples as not prime #
      FOR j FROM i*i BY i TO limit DO
        primes[j] := FALSE
      OD
    FI
  OD
END; # sieve #

# Main program #
BEGIN
  INT limit = 100;
  [0:limit]BOOL primes;

  sieve(limit, primes);

  print(("Primes up to ", limit, ":", newline));
  FOR i FROM LWB primes TO UPB primes DO
    IF primes[i] THEN
      print((whole(i,0), " "))
    FI
  OD;
  print(newline)
END