A-M baby

Task/Matrix-exponentiation-operator/Fortran/matrix-exponentiation-operator.f

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+module matmod
+  implicit none
+
+! Overloading the ** operator does not work because the compiler cannot
+! differentiate between matrix exponentiation and the elementwise raising
+! of an array to a power therefore we define a new operator
+  interface operator (.matpow.)
+    module procedure matrix_exp
+  end interface
+
+contains
+
+function matrix_exp(m, n) result (res)
+  real, intent(in)  :: m(:,:)
+  integer, intent(in)  :: n
+  real :: res(size(m,1),size(m,2))
+  integer :: i
+
+  if(n == 0) then
+    res = 0
+    do i = 1, size(m,1)
+      res(i,i) = 1
+    end do
+    return
+  end if
+
+  res = m
+  do i = 2, n
+    res = matmul(res, m)
+  end do
+
+end function matrix_exp
+end module matmod
+
+program Matrix_exponentiation
+  use matmod
+  implicit none
+
+  integer, parameter :: n = 3
+  real, dimension(n,n) :: m1, m2
+  integer :: i, j
+
+  m1 = reshape((/ (i, i = 1, n*n) /), (/ n, n /), order = (/ 2, 1 /))
+
+  do i = 0, 4
+    m2 = m1 .matpow. i
+    do j = 1, size(m2,1)
+      write(*,*) m2(j,:)
+    end do
+    write(*,*)
+  end do
+
+end program Matrix_exponentiation