I'm trying to write a Fortran code which checks if a user input number is a prime or not. The code is given below,
program cp
implicit none
integer(kind=8) :: nmb, ind
real*16 :: sqnmb, real_nmb
print*, "Largest value for a is ", huge(nmb)
DO
print *,"Enter the number larger than 0:"
read (*,*) nmb
IF ( nmb <= 0) THEN
print *, "Please enter a number larger than 0."
END IF
IF ( nmb > 0) THEN
EXIT
END IF
END DO
sqnmb = sqrt(real(nmb))
do ind= 2, int(sqnmb), 1
IF (mod(nmb,ind) == 0) THEN
print *, "The number", nmb ,"is not a prime, it is divisible by",ind
STOP
END IF
end do
print *, "The number", nmb ,"is a prime"
end program cp
The program works fine for all numbers below 9223372036854775807(19 digits) which is the largest number that can be defined with the integer(kind=8) type. However, I want to calculate for numbers with 25 digits. How can we define larger integers in Fortran?
