I want to find out about 8-bit integers (kind = 1) in gfortran.

Apparently they seem to be signed and stored as "two's complement". When you give the integer a value, you can view the bits like so:

integer(kind = 1) I

integer numbit(8) ! stores the bit values as 0 or 1's

I = 5

do j= 1,8

numbit(i) = ibits(I,j-1,1)

enddo

write(*,*) 'bits: ', numbit

...where here IBITS is extracting 1 bit from I at position j-1 and returning it as an integer with the bit value right-justified and the rest of the bits 0. So "1" at position j-1 is returned by ibits(I,j-1,1) as "00000001" which has integer value 1.

Alternatively, I could do this:

integer(kind = 1) I

integer numbit(8) ! stores the bit values as 0 or 1's

I = 5

do j= 1,8

numbit(i) = btest(I,j-1)

enddo

write(*,*) 'bits: ', numbit

...where BTEST returns .true. if the bit in position j-1 is "1", and .false. otherwise. This is implicitly converted to an integer by the gfortran compiler (but not without warnings at compile time), where .true. => 1 and .false. => 0.

Both of these return:

bits: 1 0 1 0 0 0 0 0

I guess the take-home message is that when you write out a bit-string, eg.

5 = [0101]

then the right-most bit is addressed as bit 0, and in general, bit number 1 is the second bit as you move left, etc.

So if you start with all bits set to zero, you can use IBSET to set certain bits to 1 and get whatever number you fancy...

I = 0 ! [00000000]

I = ibset(I,0) ! [00000001]

I = ibset(I,2) ! [00000101]

write(*,*) 'I = ', I

... and we get I = 5, as intended.

I guess this is a stupid post, but it wasn't obvious to me that the bit string would be numbered from the right starting at index 0. The default index for matrices in Fortran 95 starts at 1. Ho hum.

## Wednesday, 10 December 2008

## Tuesday, 9 December 2008

### Incompatible ranks?!

Here's a fun problem that I have encountered. It might be a bug, or it might not (I've run out of patience with google-ing it now I have my program working).

I have a two dimensional matrix. I sum it down one dimension and then find the position of the maximum entry.

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself

integer, intent(in) :: x_dim, y_dim

integer, intent(in) :: subframe(x_dim,y_dim)

integer, intent(out) :: irow, icol(4) !ion positions

! the code follows...*********************

irow = maxloc(sum(subframe,1))

write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

...but this causes my compiler (gfortran) to shriek:

irow = maxloc(sum(subframe,1))

1

Error: Incompatible ranks 0 and 1 in assignment at (1)

Huuuurrrmmm. Seems a bit silly, but the following kludge fixes things:

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself

integer, intent(in) :: x_dim, y_dim

integer, intent(in) :: subframe(x_dim,y_dim)

integer, intent(out) :: irow, icol(4) !ion positions

! define other datum within function

integer tmp(1)

! the code follows...*********************

tmp = maxloc(sum(subframe,1))

irow = tmp(1)

write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

Well well well. I should probably investigate the cause further but I'm not going to because I have physics to do, not Fortran to fix.

I have a two dimensional matrix. I sum it down one dimension and then find the position of the maximum entry.

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself

integer, intent(in) :: x_dim, y_dim

integer, intent(in) :: subframe(x_dim,y_dim)

integer, intent(out) :: irow, icol(4) !ion positions

! the code follows...*********************

irow = maxloc(sum(subframe,1))

write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

...but this causes my compiler (gfortran) to shriek:

irow = maxloc(sum(subframe,1))

1

Error: Incompatible ranks 0 and 1 in assignment at (1)

Huuuurrrmmm. Seems a bit silly, but the following kludge fixes things:

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself

integer, intent(in) :: x_dim, y_dim

integer, intent(in) :: subframe(x_dim,y_dim)

integer, intent(out) :: irow, icol(4) !ion positions

! define other datum within function

integer tmp(1)

! the code follows...*********************

tmp = maxloc(sum(subframe,1))

irow = tmp(1)

write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

Well well well. I should probably investigate the cause further but I'm not going to because I have physics to do, not Fortran to fix.

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