Gfortran won't compile real variables
I wrote a simple Fortran90 program to calculate the area of a triangle. The user enters the three sides of the triangle, and then the program draws out that area. Simple enough.
MODULE Triangle_Operations
IMPLICIT NONE
CONTAINS
FUNCTION Area(x,y,z)
REAL :: Area ! function type
REAL, INTENT( IN ) :: x, y, z
REAL :: theta, height
theta = ACOS((x**2+y**2-z**2)/(2.0*x*y))
height = x*SIN(theta); Area = 0.5*y*height
END FUNCTION Area
END MODULE Triangle_Operations
PROGRAM Triangle
USE Triangle_Operations
IMPLICIT NONE
REAL :: a, b, c, Area
PRINT *, 'Welcome, please enter the &
&lengths of the 3 sides.'
READ *, a, b, c
PRINT *, 'Triangle' area: ', Area(a,b,c)
END PROGRAM Triangle
When I compile this with gfortran
gfortran triangle1.f90
, I get the following message:
triangle1.f90:16.25:
REAL :: a, b, c, Area
1
triangle1.f90:14.8:
USE Triangle_Operations
2
Error: Symbol 'area' at (1) conflicts with symbol from module 'triangle_operations', use-associated at (2)
triangle1.f90:19.13:
READ *, a, b, c
1
Error: Symbol 'a' at (1) has no IMPLICIT type
triangle1.f90:19.16:
READ *, a, b, c
1
Error: Symbol 'b' at (1) has no IMPLICIT type
triangle1.f90:19.19:
READ *, a, b, c
1
Error: Symbol 'c' at (1) has no IMPLICIT type
Why exactly was there an error for variables a,b,c
? I have clearly identified them as real.
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The problem is that you have defined twice Area
- once in your main program and once in the module you are importing - and the name conflicts are in conflict. You probably feel like you need to define Area
in the main program as a delay from earlier (darker) times when calling a function without an explicit interface. In modern Fortran, modules provide interfaces automatically, and a statement is use Triangle_operations
sufficient.
To fix the problem, remove delcaration Area
from your main program eg. turn on
REAL :: a, b, c, Area
in
REAL :: a, b, c
Subsequent errors in your compilation output are the result of the first error associated with Area
. This whole line is invalidated, so the types of ads a
, b
and c
are not processed, and this leads to the fact that the compiler will complain about missing types in the next time they meet. These errors will go away once you do the fix suggested above.
If your intention was for your main program to have a named variable Area
to store the result of calling a module function, you can rename the module symbol, eg.
use triangle_operations, triangleArea => Area
and then do the following:
real a, b, c, Area Area = triangleArea(a,b,c)
in your main program.
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