Segmentation Fault - Jacobi Method

[takafoo]

So I need to write a program to implement the Jacobi Iteration Scheme but I am having some issues with it causing segmentation faults. I am not sure where the issue is occuring. Any help would be great.

PROGRAM 961

IMPLICIT NONE

! Declare Variables

   INTEGER:: i, j, k, n, aux, max_iter, iter

   REAL, ALLOCATABLE :: a(:,:),x(:),xold(:), ea(:)
   REAL(KIND=8) :: tol, suma

   CHARACTER(LEN=20) :: output

! Request File Information

   WRITE(*,*)"Jacobi Iterative Method"
   WRITE(*,*)

   PRINT *, "Please specify output filename: "
   READ (*,*) output

   PRINT *, "Please specify number of variables: "
   READ(*,*) n

   PRINT *, "Please specify maimum number of iterations: "
   READ(*,*) max_iter

   PRINT *, "Please specify tolerance: "
   READ(*,*) tol


   OPEN(unit=20,file=output,status='replace')

   ALLOCATE (a(n+1,n),x(n),ea(n),xold(n))

! Bulk Coding

   DO i=1,n,1
      WRITE(*,*)'xo(i):'
      READ(*,*)xold(i)
   END DO

   x(1:n)=0
   ea(:)=1
   iter=0

   DO i=1,n,1

      DO j=1,n+1,1
         WRITE(*,*) "a(",i,",",j,"):"
         READ(*,*) a(i,j)
      END DO

   END DO

   DO WHILE (iter <= max_iter)
      aux=0

      DO i=1,n,1
         suma=0

         DO k=1,n,1
            IF (i /= k) THEN
               suma=suma+xold(k)*a(k,i)
            END IF
         END DO

         x(i)=(a(n+1,i)-suma)/a(i,i)

         IF (xold(i) /= 0)THEN
            ea(i)=abs(((x(i)-xold(i))/x(i)))
         ELSE
            IF (x(i) == 0)THEN
               ea(i)=0
            END IF
         END IF

         WRITE(*,*) ea(i)
      END DO

      DO i=1,n,1
         WRITE(20,*) j,i,x(i),ea(i)

         IF (ea(i) < tol) THEN
            aux=aux+1
         END IF
      END DO

      IF (aux == n) THEN
         WRITE(*,*) "Number of Iterations: ", iter
         WRITE(*,*)
         WRITE(*,*) " i xi ea"
         WRITE(*,*) "----------------------------"

         DO i=1,n,1
            WRITE(*,*)i,x(i),ea(i)
         END DO
      END IF

      iter=iter+1
      xold(1:n)=x(1:n)
   ENDDO

      CLOSE(20)

      WRITE(*,*) "Solution"
      WRITE(*,*)
      WRITE(*,*) "Number of Iterations: ", j
      WRITE(*,*)
      WRITE(*,*) " i xi ea"
      WRITE(*,*) "----------------------------"

   DO i=1,n,1
      WRITE(*,*)i,x(i),ea(i)
   END DO

END PROGRAM 961

 

[FJacq]

look carefully into that loop :

      DO j=1,n+1,1
         WRITE(*,*) "a(",i,",",j,"):"
         READ(*,*) a(i,j)
      END DO

You are reading the matrix "a" in assuming that its size is a(n,n+1)

Unfortunately, you have allocated it previously with :

   ALLOCATE (a(n+1,n),x(n),ea(n),xold(n))

=> a probable segmentation fault.

To detect this kind of error rapidly, you should activate compiler options detecting "out of bounds" situations.

François Jacq