Pass derived type as array

In Fortran, one can operate on arrays, but how can one treat the indices of a derived type as part of an array too? Code would explain what I want to do best:

type mytype
    integer :: b(3,3)
    real :: c(4)
endtype

integer :: a(3,3)
real :: d(2,4)
type(mytype) :: mat(2)

!do stuff so that 'mat' gets values
....

!usually one does this
a = matmul(mat(1)%b, transpose(mat(2)%b))

!multiplying two 3x3 matrices

!but how does one do this? Note the "array"
d = matmul(mat(:)%c, mat(:)%c)

I assumed that the final line is analogous to a 2x4 matrix being multiplied with itself. However, when I try to compile, gfortran complains

Error: Two or more part references with nonzero rank must not be specified

Is this possible to do in Fortran?

Solution

You want the compiler to regard mat(:)%c as a 2 x 4 matrix? It doesn't work that way. mat and c are different objects and their ranks don't merge into a single array. mat is a user-defined type and c is a real matrix. Just because you are only using the c-component of mat doesn't mean the compiler will promote c to a higher dimensional real array, based on the dimension of mat.

You could create a new array via X = [ mat(1)%c, mat(2)%c ]. You could use reshape to control the shape.