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Dynamic Arrays

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• Dynamic arrays
  • F90 has support for dynamic arrays:

    You can write a program where the size of an array is undetermined at the time of programming The array is created dynamically when you run the program (Basically, it's the same as new double[] in C++, but fancier !!)

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  • Although C++ has dynamic arrays (using new - click here ) C++ cannot specify the size and shape of the array

    F90 provides much better support for dynamic arrays than C/C++

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  • Defining, allocating and deallocating a ONE-dimensional dynamic arrays:

    Defining a ONE-dimensional dynamic array: REAL, DIMENSION( : ), ALLOCATABLE :: A ^^^^^^^^^^^

    Allocating space for a ONE-dimensional dynamic array: ALLOCATE(A(10)) !! A(1..10) ALLOCATE(A(-5:20)) !! A(-5..20) ALLOCATE(A(N)) !! A(1..N)

    Deallocating space of a dynamic array: DEALLOCATE(A)

    • Example Program: (Dynamic arrays) --- click here        

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  • Defining, allocating and deallocating TWO-dimensional dynamic arrays: it's very similar to one-dim. array:

    Defining a TWO-dimensional dynamic array: REAL, DIMENSION( : , : ), ALLOCATABLE :: A

    Allocating space for a TWO-dimensional dynamic array: ALLOCATE(A(5, 10)) !! A(1..5, 1..10) ALLOCATE(A(1:3, -5:20)) !! A(1..3, -5..20) ALLOCATE(A(M, N)) !! A(1..M, 1..N)

    Deallocating space of a dynamic array: DEALLOCATE(A)

    • Example Program: (Dynamic arrays) --- click here        

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• Detecting error in dynamic array allocation
  • It is possible that due to memory shortage, the ALLOCATE statement fails

  • The safer way to execute an ALLOCATE statement is to add a status variable :

    INTEGER :: errCode ALLOCATE( A(size), STAT = errCode)

  • The integer variable errCode will contain a value 0 (ZERO) if the allocation was successful.

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• Checking if a dynamic array is valid
  • Another common error is not allocate space for a dynamic array

  • To check if an allocatable variable is currently assiciated with a valid variable;

    ALLOCATED( Allocatable-Variable )

    The ALLOCATED intrinsic function will return TRUE if and it returns FALSE otherwise.

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  • Example Usage:

    REAL, DIMENSION( : ), ALLOCATABLE :: A ... IF ( ALLOCATED (A) ) THEN print *, "A has been allocated" ELSE print *, "A has not been allocated" END IF

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• Pointer variable that points to an array variable
  • Recall the use of pointer variables:

    REAL, TARGET :: r REAL, POINTER :: ptr ptr => r ! Make ptr points to r ptr = 1234 ! Same as "r = 1234"

    Notice that a pointer variables has a type

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  • Defining pointer variable to array variables:

    Type, DIMENSION( ... ), POINTER :: varName;

    Example: defining a pointer variable to a 2-dim array:

    REAL, TARGET, DIMENSION(3,3) :: A REAL, DIMENSION(:,:), POINTER :: B B => A ! Make B points to A B(1,1) = 1234 ! Same as "A(1,1) = 1234"

    • Example Program: (Pointer to array variable) --- click here            

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• Restrictions on dynamic arrays and how to overcome them....
  1. Allocatable variables cannot be shared using a COMMON block (i.e., globally shared variables).

     (for an explanation of what a COMMON block is: click here )

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     Sharing dynamic arrays (making a dynamic array globally available) is achieve through a module

     (for an explanation on how to define global variable using modules: click here )

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     Example: defining a global dynamic matrix in a module

     MODULE myModule REAL, DIMENSION(:, :), ALLOCATABLE :: myMatrix END MODULE

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  2. Functions cannot return ALLOCATABLE variables.

     However...

     Functions can return a pointer to a dynamic array See this webpage on functions that return pointers: click here . Therefore, you can return an ALLOCATABLE arrays, by using a function that returns a POINTER variable.

     Example: returning a dynamically allocated array

     FUNCTION pivot_row( A ) real, dimension(:, :) :: A real, dimension( : ), pointer :: pivot_row -----------+ | integer, dimension ( 2 ) :: location | real, dimension(:), ALLOCATABLE, TARGET :: R <---------+ location = MAXLOC( A ) !! Find row + column !! index of MAX elem. R = ALLOCATE( SIZE(A(1, :) ) !! Allocate array R = A( location(1), : ) !! Copy pivot row pivot_row => R !! Return pointer to !! the dyn array END FUNCTION

     How to use a function that returns a pointer: Define an INTERFACE Use => pointer assignment !!! PROGRAM Main INTERFACE FUNCTION pivot_row( A ) real, dimension(:, :), target :: A real, dimension( : ), pointer :: pivot_row END FUNCTION END INTERFACE real, dimension(3, 3) :: A real, dimension(:), pointer :: p ! 1 pointer; ! points to an array p => pivot_row(A) END PROGRAM

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     Example Program: (Demo above code)                        

     • Prog file: (Function returning a pointer to a dyn array) click here

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     NOTE:

     Notice the difference between returning a pointer to a slice of an array and returning a pointer to a dynamic array The dynamic array is a "real" copy

     Returning a pointer to a slice of the input array:

     FUNCTION pivot_row( A ) real, dimension(:, :), target :: A real, dimension( : ), pointer :: pivot_row integer, dimension ( 2 ) :: location location = MAXLOC( A ) !! Find row + column index of MAX elem. pivot_row => A( location(1), : ) !! **** The pointer points to a row of the input array END FUNCTION

     This is not a copy

     Updates to the returned row will change the original matrix

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     Example Program: (Demo above code)                        

     • Prog file: click here