- Fortran 90 has
pointer variables,
but their
behavior
is slightly different than the
pointer variables of C++,
due to the
lack of the dereference operator
- Also, a pointer variable
can
only point to
designated targets
- Syntax to define a pointer variable:
type, POINTER :: varName
(POINTER is an attribute)
- Example:
REAL, POINTER :: ptr
- Syntax to define a target variable:
type, TARGET :: varName
(TARGET is another type of attribute)
- Example:
REAL, TARGET :: r
- When a
pointer variable ptr
points to a
target variable r,
then the expression
ptr
is an alias for the expression:
r
- Example:
REAL, POINTER :: ptr REAL, TARGET :: r r = 1111 ptr => r ! Make ptr points to r ptr = 4444 ! Same as "r = 4444"
The variable r is updated !
- Example Program:
(Demo above code)
- Prog file: click here
- An important usage of
pointer variables
is to construct
sub-matrices
- A
sub-matrix
of a given matrix can be
constructed as follows:
- Define an
array (2-dimensional array for a matrix)
of
pointer variables
The array of pointer variables will become the ub-matrix
- To complete the
sub-matrix
construction,
make
each pointer variable
point to
the desired element in
the actual matrix
This way, a selected portion (i.e., a sub-matrices) is formed.
- Define an
array (2-dimensional array for a matrix)
of
pointer variables
- More on this when we cover arrays...
- Interestingly, a
function
can return a
pointer variable
Example:
FUNCTION f(a, b) REAL, TARGET :: a, b REAL, POINTER :: f IF ( a > b ) THEN f => a ELSE f => b END IF END FUNCTION
- How to use a function that returns a pointer variable:
(Assign the return value to a pointer variable, of course....)
Example:
REAL, POINTER :: ptr ptr => f(x, y) ! function "f" returns a pointer
The pointer variable ptr will point to either x or y
- Example Program:
(Demo above code)
- Prog file: click here