# FORTRAN 90: Peculiar bug (?)

1. Apr 8, 2010

### rizzodex

Hi all,
I encountered a problem while running a FORTRAN 90 program, which appears (to me) to be quite surprising. Can someone please explain the following things:
1. Why the (4,1) element of the beta matrix is 5.0 instead of being 2.0 ??
2. Why the (5,8) element of the beta matrix is non-zero instead of being zero ??

Here comes the program with the results at the end:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PROGRAM hamiltonian
IMPLICIT NONE
INTEGER :: N,i,j,k
REAL, ALLOCATABLE, DIMENSION (:,:) :: alpha,beta
COMPLEX, ALLOCATABLE, DIMENSION (:,:) :: hk

N=8

ALLOCATE (alpha(N,N),beta(N,N),hk(N,N))

alpha=0.
beta=0.
hk=(0.,0.)

DO k=0,(N-4)/4

alpha(4*k+1,4*k+1-3)=5.
alpha(4*k+1,4*k+1+1)=5.
alpha(4*k+2,4*k+2-1)=5.
alpha(4*k+2,4*k+2+1)=5.
alpha(4*k+2,4*k+2+3)=5.
alpha(4*k+3,4*k+3-1)=5.
alpha(4*k+3,4*k+3+1)=5.
alpha(4*k+3,4*k+3+5)=5.
alpha(4*k+4,4*k+4-5)=5.
alpha(4*k+4,4*k+4-1)=5.

beta(4*k+4,4*k+4-3)=2.

END DO

DO i=1,N
OPEN (UNIT=10,FILE='alpha.dat')
WRITE(10,100)(alpha(i,j),j=1,N)
100 FORMAT(8(3X,F3.1))
OPEN (UNIT=20,FILE='beta.dat')
WRITE(20,200)(beta(i,j),j=1,N)
200 FORMAT(8(3X,F3.1))
END DO

CLOSE (10)
CLOSE (20)

END PROGRAM

============================================================
~ RESULTS ~

alpha.dat :

0.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0
5.0 0.0 5.0 0.0 5.0 0.0 0.0 0.0
0.0 5.0 0.0 5.0 0.0 0.0 0.0 5.0
0.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0
0.0 5.0 0.0 0.0 0.0 5.0 0.0 0.0
0.0 0.0 0.0 0.0 5.0 0.0 5.0 0.0
0.0 0.0 0.0 0.0 0.0 5.0 0.0 5.0
0.0 0.0 5.0 0.0 0.0 0.0 5.0 0.0

beta.dat :

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 5.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

2. Apr 8, 2010

### rizzodex

For some reason the above post doesn't have the last row of the beta matrix which is:

0.0 0.0 0.0 0.0 2.0 0.0 0.0 0.0

So, the total form of the beta matrix is :

beta.dat :

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 5.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 2.0 0.0 0.0 0.0

3. Apr 8, 2010

### kanato

This line is probably a problem:

alpha(4*k+3,4*k+3+5)=5.

When k = N-4/4 then the second index is 4*k+8 = N+4 which is out of bounds of alpha. So you are writing to something else in the heap which is probably beta. Generally writing out of bounds is a very bad thing, and a variety of strange things can happen as a result.

You're getting a value at row 8, column 5 in your beta matrix because of this one:
beta(4*k+4,4*k+4-3)=2.
When N = 1 the indices evaluate to beta(8,5).

You could make your code a bit easier to read by defining a quantity like index = 4*k+1 inside your loop.