Fortran. do while with function checking the prime number [duplicate] - fortran

Question

This question already has answers here:
Does Fortran preserve the value of internal variables through function and subroutine calls?
(3 answers)
Closed 2 years ago.
I'm recently studying Fortran, and trying to make a program to check the prime number. The function works fine without any loop. It can give 1 when the given number is a prime number and 0 otherwise. However, it doesn't work properly when it is used in do while loop. In the range of 2 ~ 10, it is supposed to give 1 (for 2), 1(for 2), 0(for 4), 1(for 5), 0(for 6), etc. But, it keeps showing only 0. I'm pretty new to programming, so I'm not sure what I'm missing. I know there are many answers related to prime numbers, but I don't see any issue like this.
** Function checking prime numbers **
module prime_function
contains
integer function isPrime(inp_num)
implicit none
integer :: inp_num
integer :: i = 1
integer :: temp1 = 0
do while (i < inp_num)
i = i + 1
if(mod(inp_num, i) == 0) then
exit
end if
end do
if(inp_num == i) then
temp1 = 1
else
temp1 = 0
end if
isPrime = temp1
end function
end module
program fortran_q
use prime_function
implicit none
integer :: ii, a
a = isPrime(10)
print *, "10 is prime number, so the return : ", a
a = isPrime(11)
print *, "11 is prime number, so the return : ", a
ii = 1
do while (ii < 10)
ii = ii + 1
print *, isPrime(ii)
end do
end program
** Results **
10 is prime number, so the return : 0
11 is prime number, so the return : 1
0
0
0
0
0
0
0
0
0

Answer 1

You have a classic issue for people new to Fortran. The initialization of i and temp0 implies the SAVE attribute. When you call isPrime for the first time the values are set to 1 and 0. On the next invocation, the values of i and temp0 are set to whatever their previous values were when isPrime was last executed. The belong program fixes the issue.
module prime_function
implicit none
private
public isprime
contains
function isPrime(inp_num) result(res)
integer res
integer, intent(in) :: inp_num
integer i, temp1
i = 1
temp1 = 0
do while (i < inp_num)
i = i + 1
if (mod(inp_num, i) == 0) exit
end do
res = 0
if (inp_num == i) res = 1
end function
end module
program fortran_q
use prime_function
implicit none
integer :: ii, a
a = isPrime(10)
print *, "10 is prime number, so the return : ", a
a = isPrime(11)
print *, "11 is prime number, so the return : ", a
ii = 1
do while (ii < 10)
ii = ii + 1
print *, isPrime(ii)
end do
end program