Here is the source (lets call it test4.F):
do 10 i=1
write(*,*) i
10 continue
end
With gfortran:
$gfortran test4.F
$./a.out
-259911288
with ifort:
$ifort test4.F
$./a.out
0
I know the syntax is incorrect. So both should throw compile-time errors.
Your test program is indeed not valid Fortran, but there isn't a syntax problem.
When the file has a .F suffix both gfortran and ifort assume that the program uses fixed form source. In fixed form source, blanks in statements aren't significant (outside character contexts). Your program is equivalent to:
do10i=1
write(*,*) i
10 continue
end
With comments:
do10i=1 ! Set the real variable do10i to value 1
write(*,*) i ! Write the undefined integer variable i
10 continue ! Do nothing
end ! End execution
Because of implicit typing you have a defaul real variable do10i and so there isn't a syntax error for a do control, and there isn't a do control that sets i to 1.
Instead your program is invalid because you are referencing the value of i although it hasn't first been defined (because it isn't a loop variable). But this isn't an error the compiler has to complain about at compile time (or even run time). A compiler is allowed to print any value it likes. gfortran chose one, ifort another.
Liberal use of implicit none and avoiding fixed-form source are good ways to avoid many programming errors. In this case, use of an end do instead of continue would also alert the compiler to the fact that you intended there to be some looping.
Which version of gfortran are you using? With gfortran-6, the following
PROGRAM MAIN
USE ISO_FORTRAN_ENV, ONLY:
1 COMPILER_VERSION,
2 COMPILER_OPTIONS
C EXPLICIT TYPING ONLY
IMPLICIT NONE
C VARIABLE DECLARATIONS
INTEGER I
DO 10 I = 1
WRITE (*,*) I
10 CONTINUE
PRINT '(/4A/)',
1 'THIS FILE WAS COMPILED USING ', COMPILER_VERSION(),
2 ' USING THE OPTIONS ', COMPILER_OPTIONS()
END PROGRAM MAIN
indeed throws the error
main.F:13:13:
DO 10 I = 1
1
Error: Symbol ‘do10i’ at (1) has no IMPLICIT type
I suspect that test4.F does not include the implicit none statement. Consequently, DO 10 I = 1 is interpreted as an implicitly defined real variable DO10I = 1. Here is a sample of a fixed form statement showing what are now (post Fortran 90) considered significant blanks followed by an equivalent statement without the blanks
DO I=1 , M AX ITER S
DO I = 1 , MAXITERS
Upshot, always use explicit variable declarations in all your programs. Better yet, only write Fortran in free source form main.f90. Free source form is more compatible with modern interactive input devices than fixed form. The maximum line length is 132 characters, compared to the older limit of 72 characters. This reduces the possibility of text exceeding the limit, which could lead the compiler to misinterpret names. Here's the same code in free source form
program main
use ISO_Fortran_env, only: &
stdout => OUTPUT_UNIT, &
compiler_version, &
compiler_options
! Explicit typing only
implicit none
! Variable declarations
integer :: i
do i = 1, 3
write (stdout, *) i
end do
print '(/4a/)', &
' This file was compiled using ', compiler_version(), &
' using the options ', compiler_options()
end program main
and
gfortran-6 -std=f2008ts -o main.exe main.f90
./main.exe
yields 1 2 3
This file was compiled using GCC version 6.1.1 20160802 using the options -mtune=generic -march=x86-64 -std=f2008ts
DO 31 = 3.1. This was a typo for DO 31 = 3,1, but the compiler “correctly” parsed it as declaring an implicit variable named DO31. The rocket blew up, and the postmortem determined that the design of Fortran was partly to blame.
implicit none statement.