CODEWITHSUNDEEP
algorithmfortranfortran90
CrackSmoker9000
CrackSmoker9000

Reputation: 319

Generate and sum prime numbers

I am attempting to sum up all the prime numbers below 2 million. I've gone over my code for hours but I cannot find what is causing it to print out the faulty number.

logical function isprime(n) result(response)

    implicit none

    integer :: i
    integer, intent(in) :: n
    integer :: upto

    if (n <= 1) then
        response = .false.
        return
    end if

    upto = int(n**.5)

    do i = 3, upto, 2
        if (mod(n, i) == 0) then
            response = .false.
            return
        end if
    end do

    response = .true.

end function isprime

program problem10

    implicit none

    integer :: n
    logical :: isprime
    integer, parameter :: int64 = selected_int_kind(16)
    integer(kind = int64) :: total

    do n = 1, 2000000
        if (isprime(n)) then
            total = total + n
        end if
    end do

    print *, total

end program problem10

It's printing 1179908152 when it should be 142913828922

Upvotes: 1

Views: 796

Answers (1)

M Oehm
M Oehm

Reputation: 29126

The sum overflows 32-bit-integers. Make total a 64-bit integer and also make sure to initialise it:

integer, parameter :: int64 = selected_int_kind(16)  ! dec. digits
integer (kind = int64) :: total

total = 0

Your program also thinks 2 isn't prime, because for n == 2, upto is 2, which is divisible by 2. It is safe to delete the + 1 term from upto, at least as long your integers don't exceed 16,777,216 when you calculate the square root with 32-bit float arithmetics.

Finally, you can speed up your program by not considering even numbers when summing (and start with a sum of 2, of course). Your loop in isprime could also handle even numbers as special case and iterate over odd numbers only.

Upvotes: 4

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