Understanding fortran integers

integer_test.f90

! compile and run using either of the following commands
! and investigate the output.
!
! gfortran integer_test.f90; a.out
! gfortran -fdefault-integer-8 integer_test.f90; a.out

program main
    implicit none

    ! define some integer types
    integer, parameter :: &
            iintegersd = kind(1), &
            iintegers1 = selected_int_kind(1), &
            iintegers2 = selected_int_kind(2), &
            iintegers4 = selected_int_kind(4), &
            iintegers8 = selected_int_kind(8), &
            iintegers16 = selected_int_kind(16)

    ! variables to represent the above types
    integer :: i
    integer(iintegersd) :: id   ! kind = 'default'
    integer(iintegers1) :: i1   ! kind = 1 byte
    integer(iintegers2) :: i2   ! kind = 1 byte
    integer(iintegers4) :: i4   ! kind = 2 bytes
    integer(iintegers8) :: i8   ! kind = 4 bytes
    integer(iintegers16) :: i16 ! kind = 8 bytes

    print *, "---- What are the values of kind (bytes for each type) ----"
    print *, "i           = ", kind(i)
    print *, "iintegersd  = ", iintegersd
    print *, "iintegers1  = ", iintegers1
    print *, "iintegers2  = ", iintegers2
    print *, "iintegers4  = ", iintegers4
    print *, "iintegers8  = ", iintegers8
    print *, "iintegers16 = ", iintegers16


    print *,
    print *, "----- Assign 1 to all integer types -----"

    i = 1
    id = 1_iintegersd
    i1 = 1_iintegers1
    i2 = 1_iintegers2
    i4 = 1_iintegers4
    i8 = 1_iintegers8
    i16 = 1_iintegers16

    print *, "The variables are:"
    print *, "i", i
    print *, "id", id
    print *, "i1", i1
    print *, "i2", i2
    print *, "i4", i4
    print *, "i8", i8
    print *, "i16", i16

    print *,
    print *, "max values for each variable:"
    print *, "huge(i) ", huge(i)
    print *, "huge(id) ", huge(id)
    print *, "huge(i1) ", huge(i1)
    print *, "huge(i2) ", huge(i2)
    print *, "huge(i4) ", huge(i4)
    print *, "huge(i8) ", huge(i8)
    print *, "huge(i16)", huge(i16)

    print *,
    print *, "test equality:"
    if (i == 1) then
        print *, "i == 1"
    endif

    if (i == id) then
        print *, "i == id"
    endif

    if (i == i1) then
        print *, "i == i1"
    endif

    if (i == i2) then
        print *, "i == i2"
    endif

    if (i == i4) then
        print *, "i == i4"
    endif

    if (i == i8) then
        print *, "i == i8"
    endif

    if (i == i16) then
        print *, "i == i16"
    endif

    print *,
    print *, "----- Examine selected_int_kind() and kind() -----"
    do i=1, 20
        if (selected_int_kind(i) > kind(10**i)) then
            print *, i, &
                    selected_int_kind(i), &
                    kind(10**i), &
                    ' x'
        else
            print *, i, &
                    selected_int_kind(i), &
                    kind(10**i), &
                    ' _/'
        endif
    end do

end program main

! Lessons learned
!
! integer(kind=8) != integer(kind=kind(8))
!                 != integer(kind=selected_int_kind(8))
!
! kind=8       -> 8 is the bytes
! kind=kind(8) -> kind(8) returns the "default" bytes
!   that store the integer 8
! kind=selected_int_kind(8) -> selected_int_kind(8) returns
!   returns the bytes required to store an integer with
!   8 decimal digits i.e the maximum exponent
!
! selected_int_kind(i) >= kind(10^i) because 10^i takes on the
! default integer size even though that may lead to an overflow/
! underflow. kind(10^i) is the same for all values of i, so other
! than kind(1) use a variable i.e. kind(var) or avoid it.