Task

Write a boolean function that tells whether a given integer is prime.



Remember that   1   and all non-positive numbers are not prime.

Use trial division.

Even numbers greater than   2   may be eliminated right away.

A loop from   3   to   √ n    will suffice,   but other loops are allowed.



Related tasks

   count in factors
   prime decomposition
   AKS test for primes
   factors of an integer
   Sieve of Eratosthenes
   factors of a Mersenne number
   trial factoring of a Mersenne number
   partition an integer X into N primes
   sequence of primes by Trial Division


#APPTYPE CONSOLE
 
FUNCTION ISPRIME(n AS INTEGER) AS INTEGER
    IF n = 2 THEN
        RETURN TRUE
    ELSEIF n <= 1 ORELSE n MOD 2 = 0 THEN
        RETURN FALSE
    ELSE
        FOR DIM i = 3 TO SQR(n) STEP 2
            IF n MOD i = 0 THEN
                RETURN FALSE
            END IF
        NEXT
        RETURN TRUE
    END IF
END FUNCTION
 
FUNCTION ISPRIME2(N AS INTEGER) AS INTEGER
    IF N <= 1 THEN RETURN FALSE
    DIM I AS INTEGER = 2
    WHILE I * I <= N
        IF N MOD I = 0 THEN
            RETURN FALSE
        END IF
        I = I + 1
    WEND
    RETURN TRUE
END FUNCTION
 
' Test and display primes 1 .. 50
DIM n AS INTEGER
 
FOR n = 1 TO 50
    IF ISPRIME(n) THEN
        PRINT n, " ";
    END IF
NEXT
 
PAUSE