(Draft) Fortran solution for code golf challenge: https://codegolf.stackexchange.com/questions/182363/rotate-a-column

rotcol.f

! source file: "rotcol.f"
! compile command:
! gfortran -std=gnu -Wextra -Wall -pedantic -ffree-form -fcheck=all -fbacktrace rotcol.f -o rotcol.app
! executable: "rotcol.app"

! read from file "rotateme.txt":
! K : column to rotate
! Z : number of rows to input

program main
implicit none
integer z,i,k
character(128), allocatable :: A(:)
character(1) :: r,s

! read data
open(unit=8,file='rotateme.txt')
read(8,*) k,z
allocate(A(z))

! loop over Z
do i=1,z
  read(8,'(a)') A(i)
  r=A(i)(k:k)
  if (r.ne.'' .or. r.ne.' ') then
    A(i)(k:k)=s
    s=r
  endif
enddo
A(1)(k:k)=s
print '(A)',A
end

requires input file rotateme.txt with following format:

20 8
A line with more than k characters.
A longer line with more than k character.
A short line.
Rotate here: ------v--
This is long enough.

This is not enough.
Wrapping around to the first line.