Writing multiple output files in Fortran - fortran

Question

Dear All, I am writing a code that writes the out put in multiple files named as 1.dat, 2.dat, ..... Here is my code but it gives some unusual output. May you tell me what is wrong in my code please? Basically I could not get the correct syntax to open multiple files, write on them and close before the next file is opened. Thank you. My Code:
implicit double precision (a-h,o-z),integer(i-n)
dimension b(3300,78805),bb(78805)
character*70,fn
character*80,fnw
nf = 3600 ! NUMBER OF FILES
nj = 360 ! Number of rows in file.
do j = 1, nj
bb(j) = 0.0
end do
c-------!Body program-----------------------------------------------
iout = 0 ! Output Files upto "ns" no.
DO i= 1,nf ! LOOP FOR THE NUMBER OF FILES
if(mod(i,180).eq.0.0) then
open(unit = iout, file = 'formatted')
x = 0.0
do j = 1, nj
bb(j) = sin(x)
write(iout,11) int(x),bb(j)
x = x + 1.0
end do
close(iout)
iout = iout + 1
end if
END DO
11 format(i0,'.dat')
END

Answer 1

So there are a few things not immediately clear about your code, but I think here the most relevant bits are that you want to specify the filename with file = in the open statement, not the formatting, and looping over units with iout is problematic because you'll eventually hit system-defined units for stdin and stdout. Also, with that format line it looks like you're getting ready to create the filename, but you never actually use it.
I'm not sure where you're; going with the mod test, etc, but below is a stripped down version of above which just creates the files ina loop:
program manyfiles
implicit none
character(len=70) :: fn
integer, parameter :: numfiles=40
integer, parameter :: outunit=44
integer :: filenum, j
do filenum=1,numfiles
! build filename -- i.dat
write(fn,fmt='(i0,a)') filenum, '.dat'
! open it with a fixed unit number
open(unit=outunit,file=fn, form='formatted')
! write something
write(outunit, *) filenum
! close it
close(outunit)
enddo
end program manyfiles

Answer 2

In my case, I want the file name have an prefix likedyn_
program manyfiles
implicit none
character(len=70) :: filename
integer, parameter :: numfiles=40
integer, parameter :: outunit=44
integer :: filenum, j
do filenum=1,numfiles
write(filename,'("dyn_",i0,".dat")') filenum
open(unit=outunit,file=filename, form='formatted')
write(outunit, *) filenum
close(outunit)
enddo
end program manyfiles