Factorial primes: Difference between revisions

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=={{header|ALGOL 68}}==

Basic task. Assumes LONG INT is at least 64 bits.

{{libheader|ALGOL 68-primes}} (NB: the library source is on rosettacode).

<lang algol68>BEGIN # find some factorial primes - primes that are f - 1 or f + 1 #

# foe some factorial f #

PR read "primes.incl.a68" PR # include prime utilities including #

# is ~~probably~~ prime #

# is prime PROC based on the one in the primality by trial division task #

PROC is prime = ( LONG INT p )BOOL:

IF p <= 1 OR NOT ODD p THEN

p = 2

ELSE

BOOL prime := TRUE;

FOR i FROM 3 BY 2 TO SHORTEN ENTIER long sqrt(p) WHILE prime := p MOD i /= 0 DO SKIP OD;

prime

FI;

# end of code from the primality by trial divisio task #

PROC show factorial prime = ( INT fp number, INT n, CHAR fp op, LONG INT fp )VOID:

print( ( whole( fp number, -2 ), ":", whole( n, -4 )

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f *:= n;

IF LONG INT fp = f - 1;

is ~~probably~~ prime( fp )

is prime( fp )

THEN

fp count +:= 1;

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FI;

IF LONG INT fp = f + 1;

is ~~probably~~ prime( fp )

is prime( f + 1 )

THEN

fp count +:= 1;