Common Problems

Forget the type of a FUNCTION.
```
FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER, INTENT(IN) :: a, b

   DoSomthing = SQRT(a*a + b*b)
END FUNCTION  DoSomthing
```
If there is no type, you will not be able to determine if the returned value is an INTEGER, a REAL or something else.
Forget INTENT(IN).
```
REAL FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER :: a, b

   DoSomthing = SQRT(a*a + b*b)
END FUNCTION  DoSomthing
```
Actually, this is not an error. But, without INTENT(IN), our Fortran compiler will not be able to check many potential errors for you.

Change the value of a formal argument declared with INTENT(IN).

REAL FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER, INTENT(IN) :: a, b

   IF (a > b) THEN
      a = a - b
   ELSE
      a = a + b
   END IF
   DoSomthing = SQRT(a*a + b*b)
END FUNCTION  DoSomthing

Since formal argument a is declared with INTENT(IN), its value cannot be changed.

Forget to store a value to the function name.
```
REAL FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER, INTENT(IN) :: a, b
   INTEGER             :: c

   c = SQRT(a*a + b*b)
END FUNCTION  DoSomthing
```
When the execution of this function reaches END FUNCTION, it returns the value stored in DoSomething. However, in the above, since there is no value ever stored in DoSmething, the returned value could be anything (i.e., a garbage value).
Function name is used in the right-hand side of an expression.
```
REAL FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER, INTENT(IN) :: a, b

   DoSomething = a*a + b*b
   DoSomething = SQRT(DoSomething)
END FUNCTION  DoSomthing
```
In the above, function name DoSomething appears in the right-hand side of the second expression. This is a mistake. Only a special type of functions, RECURSIVE functions, could have their names in the right-hand side of expressions.
Only the most recent value stored in the function name will be returned..
```
REAL FUNCTION  DoSomething(a, b)
   IMPLICIT  NONE

   INTEGER, INTENT(IN) :: a, b

   DoSomething = a*a + b*b
   DoSomething = SQRT(a*a - b*b)
END FUNCTION  DoSomthing
```
In the above, the value of SQRT(a*a-b*b) rather than the value of a*a + b*b is returned. In fact, this is obvious. Since the name of a function can be considered as a special variable name, the second assignment overwrites the previous value.