Talk:ALGOL 68-primes: Difference between revisions
→Source code: Added operators, etc. for extracting a list of primes from a sieve of primes
(→Source code: Include 0 in the sieve) |
(→Source code: Added operators, etc. for extracting a list of primes from a sieve of primes) |
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FOR i FROM 4 BY 2 TO UPB prime DO prime[ i ] := FALSE OD;
FOR i FROM 3 BY 2 TO ENTIER sqrt( UPB prime ) DO
IF prime[ i ] THEN
FOR s FROM i * i BY i + i TO UPB prime DO prime[ s ] := FALSE OD
FI
OD;
prime
END; # PRIMESIEVE #
# modes and operators for extracting primes from a sieve #
INT first n primes extract op = 0;
INT primes up to extract op = 1;
MODE PRIMEEXTRACT = STRUCT( INT extract op, INT n );
# returns a PRIMEEXTRACT with the first n primces extract op #
OP EXTRACTFIRST = ( INT n )PRIMEEXTRACT: PRIMEEXTRACT( first n primes extract op, n );
# returns a PRIMEEXTRACT with primes up to extract op #
OP EXTRACTPRIMESUPTO = ( INT n )PRIMEEXTRACT: PRIMEEXTRACT( primes up to extract op, n );
# returns an array of primes extracted from prime #
# as specified by by the extract op and number of primes in ex #
PRIO PRIMESFROMSIEVE = 9, FROMPRIMESIEVE = 9;
OP PRIMESFROMSIEVE = ( PRIMEEXTRACT ex, []BOOL prime )[]INT:
IF extract op OF ex = first n primes extract op
THEN
# extract the first n OF ex primes #
# count the primes up n OF ex #
INT p count := 0;
FOR i TO UPB prime WHILE p count < n OF ex DO
IF prime[ i ] THEN p count +:= 1 FI
OD;
# construct a list of the first n OF ex primes or p count, #
# whichever is smaller #
[ 1 : p count ]INT low prime;
INT low pos := 0;
FOR i WHILE low pos < UPB low prime DO
IF prime[ i ] THEN low prime[ low pos +:= 1 ] := i FI
OD;
low prime
ELSE
# extract primes up to n OF ex primes #
INT max prime = n OF ex;
# count the primes up to max prime #
INT p count := 0;
FOR i TO max prime DO IF prime[ i ] THEN p count +:= 1 FI OD;
# construct a list of the primes up to max prime #
[ 1 : p count ]INT low prime;
INT low pos := 0;
FOR i WHILE low pos < UPB low prime DO
IF prime[ i ] THEN low prime[ low pos +:= 1 ] := i FI
OD;
low prime
FI # PRIMESFROMSIEVE # ;
# alternative spelling of the operator name #
OP FROMPRIMESIEVE = ( PRIMEEXTRACT ex, []BOOL prime )[]INT: ex PRIMESFROMSIEVE prime;
# END primes.incl.a68 #</lang>
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