Fortran Syntax Basics? - gcc

Question

So I am trying to get my Fortran 95 code to work just for basic function and program definitions. I get practically an error for every line saying "Unexpected" or "Unclassified". I wonder if it is my compiler (gfortran used in cygwin terminal) or if I am supposed to put something at the beginning of the file? Here it is if anyone can tell me anything.
double precision :: pi = 3.14159265359
PROGRAM Diffraction
write (*,*) sinc(0)
write (*,*) sinc(pi)
write (*,*) 1_Slit(0, 1, 550E-9)
end PROGRAM Diffraction
function SINC(angle) result(sinc)
double precision :: sinc
double precision :: angle
if angle == 0.0 then
sinc == 1
else
sinc = (sin(angle)/angle)
endif
end function SINC
function I(angle, d, wl) result(I)
double precision :: I_0 = 0.01
double precision :: angle, d, wl, I
A = (d * pi)/wl
B = SIN(angle)
I = I_0 * (SINC(A*B)**f2)
return
end function I
The way I compile is:
gfortran Diffraction.f95

Answer 1

Generally speaking, it is a good idea to put all definitions in the main program or a module. So your "floating" definitions are a bit odd.
Your program should start with PROGRAM [name] followed by your used modules. In your case, there are no such modules. After this, it is good practice to write IMPLICIT NONE. This means, that no variables have a predefined type. Otherwise, every variable starting with I to N would be of type INTEGER and every other variable would be of type REAL.
The next part is the variable definition part, where your variables are defined. (The first line in your example.)
After this, the main part is following, where you execute your code.
The final part is the CONTAINS part, where your functions and subroutines are placed, which can use every variable, which is defined in the program (but this would be bad praxis...).
So your example (with some corrections) would look like:
PROGRAM Diffraction
IMPLICIT NONE
double precision :: pi = 3.14159265359d0
write (*,*) sinc(0.d0)
write (*,*) sinc(pi)
write (*,*) one_slit(0.d0, 1.d0, 550.d-9)
CONTAINS
function SINC(angle) result(snc)
double precision :: snc
double precision :: angle
if (angle == 0.d0) then
snc = 1.d0
else
snc = (sin(angle)/angle)
endif
end function SINC
function one_slit(angle, d, wl) result(I)
double precision :: I0 = 0.01d0, A, B
double precision :: angle, d, wl, I, f2=2.d0
A = (d * pi)/wl
B = SIN(angle)
I = I0 * (SINC(A*B)**f2)
end function one_slit
end PROGRAM Diffraction