Prevent changing variables with intent(in) - fortran

Question

so reading the following question (Correct use of FORTRAN INTENT() for large arrays) I learned that defining a variable with intent(in) isn't enough, since when the variable is passed to another subroutine/function, it can be changed again. So how can I avoid this? In the original thread they talked about putting the subroutine into a module, but that doesn't help for me. For example I want to calculate the determinant of a matrix with a LU-factorization. Therefore I use the Lapack function zgetrf, but however this function alters my input matrix and the compiler don't displays any warnings. So what can I do?
module matHelper
implicit none
contains
subroutine initMat(AA)
real*8 :: u
double complex, dimension(:,:), intent(inout) :: AA
integer :: row, col, counter
counter = 1
do row=1,size(AA,1)
do col=1,size(AA,2)
AA(row,col)=cmplx(counter ,0)
counter=counter+1
end do
end do
end subroutine initMat
!subroutine to write a Matrix to file
!Input: AA - double complex matrix
! fid - integer file id
! fname - file name
! stat - integer status =replace[0] or old[1]
subroutine writeMat(AA,fid, fname, stat)
integer :: fid, stat
character(len=*) :: fname
double complex, dimension(:,:), intent(in) :: AA
integer :: row, col
character (len=64) :: fmtString
!opening file with given options
if(fid /= 0) then
if(stat == 0) then
open(unit=fid, file=fname, status='replace', &
action='write')
else if(stat ==1) then
open(unit=fid, file=fname, status='old', &
action='write')
else
print*, 'Error while trying to open file with Id', fid
return
end if
end if
!initializing matrix print format
write(fmtString,'(I0)') size(aa,2)
fmtString = '('// trim(fmtString) //'("{",ES10.3, ",", 1X, ES10.3,"}",:,1X))'
!write(*,*) fmtString
!writing matrix to file by iterating through each row
do row=1,size(aa,1)
write(fid,fmt = fmtString) AA(row,:)
enddo
write(fid,*) ''
end subroutine writeMat
!function to calculate the determinant of the input
!Input: AA - double complex matrix
!Output determinantMat - double complex,
! 0 if AA not a square matrix
function determinantMat(AA)
double complex, dimension(:,:), intent(in) :: AA
double complex :: determinantMat
integer, dimension(min(size(AA,1),size(AA,2)))&
:: ipiv
integer :: ii, info
!check if not square matrix, then set determinant to 0
if(size(AA,1)/= size(AA,2)) then
determinantMat = 0
return
end if
!compute LU facotirzation with LAPACK function
call zgetrf(size(AA,1),size(AA,2), AA,size(AA,1), ipiv,info)
if(info /= 0) then
determinantMat = cmplx(0.D0, 0.D0)
return
end if
determinantMat = cmplx(1.D0, 0.D0)
!determinant of triangular matrix is product of diagonal elements
do ii=1,size(AA,1)
if(ipiv(ii) /= ii) then
!a permutation was done, so a factor of -1
determinantMat = -determinantMat *AA(ii,ii)
else
!no permutation, so no -1
determinantMat = determinantMat*AA(ii,ii)
end if
end do
end function determinantMat
end module matHelper
!***********************************************************************
!module which stores matrix elements, dimension, trace, determinant
program test
use matHelper
implicit none
double complex, dimension(:,:), allocatable :: AA, BB
integer :: n, fid
fid = 0;
allocate(AA(3,3))
call initMat(AA)
call writeMat(AA,0,' ', 0)
print*, 'Determinante: ',determinantMat(AA) !changes AA
call writeMat(AA,0, ' ', 0)
end program test
PS: I am using the ifort compiler v15.0.3 20150407

Answer 1

I do not have ifort at home, but you may want to try compiling with '-check interfaces' and maybe with '-ipo'. You may need the path to 'zgetrf' for the '-check interfaces' to work, and if that is not source then it may not help.
If you declare 'function determinantMat' as 'PURE FUNCTION determinantMat' then I am pretty sure it would complain because 'zgetrf' is not known to be PURE nor ELEMENTAL. Try ^this stuff^ first.
If LAPACK has a module, then zgetrf could be known to be, or not be, PURE/ELEMENTAL. https://software.intel.com/en-us/articles/blas-and-lapack-fortran95-mod-files
I would suggest you add to your compile line:
-check interfaces -ipo
During initial build I like (Take it out for speed once it works):
-check all -warn all
Making a temporary array is one way around it. (I have not compiled this, so it is only a conceptual exemplar.)
PURE FUNCTION determinantMat(AA)
USE LAPACK95 !--New Line--!
IMPLICIT NONE !--New Line--!
double complex, dimension(:,:) , intent(IN ) :: AA
double complex :: determinantMat !<- output
!--internals--
integer, dimension(min(size(AA,1),size(AA,2))) :: ipiv
!!--Next line is new--
double complex, dimension(size(AA,1),size(AA,2)) :: AA_Temp !!<- I have no idea if this will work, you may need an allocatable??
integer :: ii, info
!check if not square matrix, then set determinant to 0
if(size(AA,1)/= size(AA,2)) then
determinantMat = 0
return
end if
!compute LU factorization with LAPACK function
!!--Next line is new--
AA_Temp = AA !--Initialise AA_Temp to be the same as AA--!
call zgetrf(size(AA_temp,1),size(AA_Temp,2), AA_Temp,size(AA_Temp,1), ipiv,info)
if(info /= 0) then
determinantMat = cmplx(0.D0, 0.D0)
return
end if
determinantMat = cmplx(1.D0, 0.D0)
!determinant of triangular matrix is product of diagonal elements
do ii=1,size(AA_Temp,1)
if(ipiv(ii) /= ii) then
!a permutation was done, so a factor of -1
determinantMat = -determinantMat *AA_Temp(ii,ii)
else
!no permutation, so no -1
determinantMat = determinantMat*AA_Temp(ii,ii)
end if
end do
end function determinantMat
With the 'USE LAPACK95' you probably do not need PURE, but if you wanted it to be PURE then you want to explicitly say so.