I'm new to Fortran, and sorry for this noobish question, I didn't find an answer for it.
In the code:
integer ( kind = 4 ) k
integer ( kind = 4 ) v(k)
integer ( kind = 4 ) list(*)
What does (k) and (*) do in the second, third line?
Thanks
The first integer, k is a scalar. The second integer v(k) is an array v with k elements. The last integer list(*) an assumed size array that is a dummy argument to a procedure. Its length (number of elements) will be determined by the actual argument passed to the procedure.
Note that kind = 4 is not portable and you should instead use the intrinsics kind() or selected_int_kind() to specify the size of your integers.
Complementing the answer of #casey:
The definition of
INTEGER(KIND=4) list(*)
is only valid as definition of a dummy argument. Though, you can define this list with the help of a constant as a named constant (specified by the PARAMETER keyword):
INTEGER(KIND=4), PARAMETER :: list(*) = [1,2,3,4,5]
In this case, this is called an implied-shape array (5.3.8.6) which gets its length implicitely from the constant array.
Related
Is there possibility to use indexing directly on a function's return value? Something like this:
readStr()(2:5)
where readStr() is a function which returns a character string or an array. In many other languages it is quite possible, but what about Fortran? The syntax in my example of course does not compile. Is there any other syntax to be used?
No, that is not possible in Fortran. You could, however, alter your function to take an additional index array that determines which elements are returned. This example illustrates this possibility using an interface to allow for an optional specification of the indices (simplified greatly thanks to the comment by IanH):
module test_mod
implicit none
contains
function squareOpt( arr, idx ) result(res)
real, intent(in) :: arr(:)
integer, intent(in), optional :: idx(:)
real,allocatable :: res( : )
real :: res_( size(arr) )
integer :: stat
! Calculate as before
res_ = arr*arr
if ( present(idx) ) then
! Take the sub-set
allocate( res(size(idx)), stat=stat )
if ( stat /= 0 ) stop 'Cannot allocate memory!'
res = res_(idx)
else
! Take the the whole array
allocate( res(size(arr)), stat=stat )
if ( stat /= 0 ) stop 'Cannot allocate memory!'
res = res_
endif
end function
end module
program test
use test_mod
implicit none
real :: arr(4)
integer :: idx(2)
arr = [ 1., 2., 3., 4. ]
idx = [ 2, 3]
print *, 'w/o indices',squareOpt(arr)
print *, 'w/ indices',squareOpt(arr, idx)
end program
No.
But if it bothers you, you can write your own user defined functions and operators to achieve a similar outcome without having to store the result of the function reference in a separate variable.
You can avoid declaring another variable if you use associate. Whether it is any better or clearer than a temporary variable must be decided by the user. The result has to be stored somewhere anyway.
associate(str=>readStr())
print *, str(2:5)
end associate
It will not be very useful for this specific case with a potentially long string but might be more useful for other similar cases that get linked here as duplicates.
I would like to access the the elements of an array in a an arrayed derived type using the subroutine sum_real. That is: sum over first entry in the weight for all people.
type my_type
real, dimension(:), allocatable :: weight
real :: total_weight
end type my_type
type (my_type), dimension (:), allocatable :: people
type (my_type) :: answer
allocate (people (2))
allocate (people (1)%weight(2))
allocate (people (2)%weight(2))
people (1) % weight(1) = 1
people (2) % weight(1) = 1
people (1) % weight(2) = 3
people (2) % weight(2) = 3
call sum_real ( people (:) % weight(1), answer % total_weight )
What I want to do is similar to the answer found in Array of derived type: select entry, except for the fact that I have an allocated array in an arrayed derived type instead of an single element.
But, I get a compiler error:
error #7828: The part-name to the right of a part-ref with nonzero rank has the ALLOCATABLE attribute (6.1.2). [WEIGHT]
What you try is not possible if your component is allocatable. The reference (6.1.2) is actually a reference to the official standard documents, which prohibits this.
The reason is simple, the allocatable components (scalar or arrays) are stored in a different part of memory than the derived type itself. Therefore if you write
sum(people%total_weight)
or
people%total_weight = 0
it is no problem, total_weight is not allocatable, it is stored within the derived type and the compiler just goes in a simple loop and sets one field after another to zero. You can know the address of each %totalweight beforehand.
On the other hand
sum(people%weight)
or
people%weight = 0
each %weight is stored elsewhere and you don't have any simple formula to compute where is each %weight(i).
The solution is either, to fix the size of the array, if possible
real, dimension(2) :: weight
or use a do loop
s = 0
do i = 1, size(people)
S = S + sum(people(i)%weight)
end do
If you have a F2003 compiler, and the bounds of the component array are the same for a particular parent array object, a third approach to the size specified by constant expression/use a do loop methods specified by VladimirF is to parameterize the type.
type my_type(n) ! This type has one parameter - n
integer, len :: n ! The parameter n is a length parameter.
real :: weight(n) ! This component depends on that parameter.
end type my_type
type (my_type(:)), dimension(:), allocatable :: people
! This integer is the size of the people component. Because
! people is allocatable it can be determined at runtime.
number_of_people = 2
! This integer is the size of the weight component that we want
! in the array people. Other arrays and scalars of type
! my_type can have different sizes for that component.
! Because people is allocatable this can be determined at
! runtime.
number_of_weights = 2
allocate( my_type(number_of_weights) :: people(number_of_people) )
! Define people%weight here.
people(1)%weight(1) = 1
...
! Using sum intrinsic for the sake of example
do i = 1, people%n
! The argument to sum is an array section.
print *, sum(people%weight(i))
! ^ ^ Reference to an element of a component
! | Reference to the entire people array
end do
Every element in an array of parameterized type has the same type parameters, hence every weight component in people has the same bounds, hence references such as people%weight become "regular".
Code using this approach (or the constant component size specification approach) still has to follow the restriction for component references that only one part of the reference can have non-zero rank (you can't work with people%weight as a whole as both people and the weight component have rank one).
In the allocatable component case some components in some elements might not be allocated and where they are allocated the component might have different bounds, making a "regular" reference to the data in the component across the elements of the array conceptually difficult.
I am given this code:
...
IMPLICIT REAL*8(a-h,o-z)
DIMENSION L1(L), L2(M), L3(N)
...
I want to use IMPLICIT NONE but I don't know how to declare variable type using DIMENSION and maintain one line declaration of L1 to L3.
Something like:
INTEGER, DIMENSION :: L1(L), L2(M), L3(N) !(this doesn't work)
The syntax for the dimension statement differs from that of specifying the dimension attribute in a declaration statement.
So, whereas
dimension i(4) ! Implicitly typed
gives i array nature of size 4,
integer, dimension(4) :: i
is the way to go.
Now to come to your question about declaring multiple arrays in one line:
integer i(4), j(5), k(6)
Finally, one can still use
integer, dimension(4) :: i, j(5), k(6), l
making i and l arrays of size 4 and j and k arrays of size 5 and 6.
Is there a way in Fortran to access many elements of an array without using a loop?
For example given array of 100 elements
real(100) :: a
can I do something like this to access elements 1,4,7,54,81 that do not follow a regular step?
a(1,4,7,54,81)= 3.21423
you could use a vector subscript: a( (/1,4,7,54,81/) )= 3.21423
As noted before, an array may be used as the indexes of an array. This is a so-called vector subscript.
A([1,4,7,54,81]) = 3.21423
sets the elements given to that value. (This is the same as the earlier answer but using the Fortran 2003+/modern array constructor notation.)
The array can be any rank-1 array, such as a variable or expression:
integer :: idx(5)=[1,4,7,54,81]
A(idx) = 3.21423
A(idx-1+1) = 3.21423
Of course, vector subscripts are of use in other settings, such as referencing:
print *, A(idx)
call sub(A(idx))
A(idx) = A(idx+1) + 5
However, array sections with vector subscripts are subject to various restrictions, such as:
not always may they be arguments to a procedure;
a pointer may not point to them;
not all such sections may be assigned to.
In the third case, if the same index appears more than once in the subscript we can't define it. So
print *, A([1,5,1])
is allowed, but
A([1,5,1]) = 3.
is not.
RESHAPE and WHERE are worth looking at.
If you are determining which elements to 'pull out' then maybe ALLOCATE a new variable and stuff the elements of A into B.
Maybe something like this:
REAL, DIMENSION(100) :: A
LOGICAL, DIMENSION(100) :: A_Mask
INTEGER :: SizeB
REAL, DIMENSION(:), ALLOCATABLE :: B
!...
A_Mask = .FALSE.
WHERE(A > 1.0)
A_Mask = .TRUE.
ENDWHERE
SizeB = SUM(A_Mask)
!... then allocate B and fill it.
Let A and B be matrices of size 1 times n and n times 1, respectively.
Then the multiplication of A with B is a 1 times 1 matrix.
Which is the better way to assign the value of MATMUL(A,B) to a real number x?
I would like to write:
x=MATMUL(A,B) ! <<--- but this is wrong.
The above expression is wrong because I'm trying to assign a 1 times 1 matrix to a real number.
My solution is to define a 1 times 1 matrix C and with this:
C=MATMUL(A,B)
x=C(1,1) ! <--- this solution is ok, but is too long
But, there exists a better way to assign MATMUL(A,B) to the real number x?
The entire code with my question is as follow:
PROGRAM testing
!
IMPLICIT NONE
REAL :: A(1,2),B(2,1),C(1,1),x
!
A(1,1)=1.0; A(1,2)=3.5
B(1,1)=2.0; B(2,1)=5.0
C=MATMUL(A,B) ! it is ok
x=MATMUL(A,B) ! it is wrong
x=C(1,1) ! it is ok <--- exists a better way ??
!
END PROGRAM testing
You have noticed that it is not possible to do intrinsic assignment of an array to a scalar (and C is a rank-2 array of size 1). x=C(1,1) is the correct way to do such assignment from the single element of C to the scalar x.
There are other ways to abstract that correct assignment statement, but probably little value in doing so.
In your specific case, however, there is alternative. Rather than matmul, consider dot_product.
x = DOT_PRODUCT(A(1,:), B(:,1)) ! Scalar result, intrinsic assignment allowed.
per my comment, you can write a very simple function to extract the first element of an array:
real function first(matrix) !return the (1,1,1,..) element of an array
real, intent(in) :: matrix(*)
first=matrix(1)
end function
simply use as:
real :: a(1,2),b(2,1),x
...
x=first(matmul(a,b))
note if you want to make sure this is only used for a dimension(1,1) array you need to use an explicit interface and do:
real function first(matrix)
real, intent(in) :: matrix(:,:)
if(.not.all(shape(matrix).eq.[1,1]))reporterror()
first=matrix(1,1)
end function