Format columns extracted from different input files in a single output file in Fortran90 [duplicate] - loops

Question

I have a program in Fortran that saves the results to a file. At the moment I open the file using
OPEN (1, FILE = 'Output.TXT')
However, I now want to run a loop, and save the results of each iteration to the files 'Output1.TXT', 'Output2.TXT', 'Output3.TXT', and so on.
Is there an easy way in Fortran to constuct filenames from the loop counter i?

Answer 1

you can write to a unit, but you can also write to a string
program foo
character(len=1024) :: filename
write (filename, "(A5,I2)") "hello", 10
print *, trim(filename)
end program
Please note (this is the second trick I was talking about) that you can also build a format string programmatically.
program foo
character(len=1024) :: filename
character(len=1024) :: format_string
integer :: i
do i=1, 10
if (i < 10) then
format_string = "(A5,I1)"
else
format_string = "(A5,I2)"
endif
write (filename,format_string) "hello", i
print *, trim(filename)
enddo
end program

Answer 2

A much easier solution IMHO ...................
character(len=8) :: fmt ! format descriptor
fmt = '(I5.5)' ! an integer of width 5 with zeros at the left
i1= 59
write (x1,fmt) i1 ! converting integer to string using a 'internal file'
filename='output'//trim(x1)//'.dat'
! ====> filename: output00059.dat

Answer 3

Well here is a simple function which will return the left justified string version of an integer:
character(len=20) function str(k)
! "Convert an integer to string."
integer, intent(in) :: k
write (str, *) k
str = adjustl(str)
end function str
And here is a test code:
program x
integer :: i
do i=1, 100
open(11, file='Output'//trim(str(i))//'.txt')
write (11, *) i
close (11)
end do
end program x

Answer 4

I already showed this elsewhere on SO (How to use a variable in the format specifier statement? , not an exact duplicate IMHO), but I think it is worthwhile to place it here. It is possible to use the techniques from other answers for this question to make a simple function
function itoa(i) result(res)
character(:),allocatable :: res
integer,intent(in) :: i
character(range(i)+2) :: tmp
write(tmp,'(i0)') i
res = trim(tmp)
end function
which you can use after without worrying about trimming and left-adjusting and without writing to a temporary variable:
OPEN(1, FILE = 'Output'//itoa(i)//'.TXT')
It requires Fortran 2003 because of the allocatable string.

Answer 5

For a shorten version.
If all the indices are smaller than 10, then use the following:
do i=0,9
fid=100+i
fname='OUTPUT'//NCHAR(i+48) //'.txt'
open(fid, file=fname)
!....
end do
For a general version:
character(len=5) :: charI
do i = 0,100
fid = 100 + i
write(charI,"(A)"), i
fname ='OUTPUT' // trim(charI) // '.txt'
open(fid, file=fname)
end do
That's all.

Answer 6

I've tried #Alejandro and #user2361779 already but it gives me an unsatisfied result such as file 1.txt or file1 .txt instead of file1.txt. However i find the better solution:
...
integer :: i
character(len=5) :: char_i ! use your maximum expected len
character(len=32) :: filename
write(char_i, '(I5)') i ! convert integer to char
write(filename, '("path/to/file/", A, ".dat")') trim(adjustl(char_i))
...
Explanation:
e.g. set i = 10 and write(char_i, '(I5)') i
char_i gives " 10" ! this is original value of char_i
adjustl(char_i) gives "10 " ! adjust char_i to the left
trim(adjustl(char_i)) gives "10" ! adjust char_i to the left then remove blank space on the right
I think this is a simplest solution that give you a dynamical length filename without any legacy blank spaces from integer to string conversion process.

Answer 7

Try the following:
....
character(len=30) :: filename ! length depends on expected names
integer :: inuit
....
do i=1,n
write(filename,'("output",i0,".txt")') i
open(newunit=iunit,file=filename,...)
....
close(iunit)
enddo
....
Where "..." means other appropriate code for your purpose.

Answer 8

To convert an integer to a string:
integer :: i
character* :: s
if (i.LE.9) then
s=char(48+i)
else if (i.GE.10) then
s=char(48+(i/10))// char(48-10*(i/10)+i)
endif

Answer 9

Here is my subroutine approach to this problem. it transforms an integer in the range 0 : 9999 as a character. For example, the INTEGER 123 is transformed into the character 0123. hope it helps.
P.S. - sorry for the comments; they make sense in Romanian :P
subroutine nume_fisier (i,filename_tot)
implicit none
integer :: i
integer :: integer_zeci,rest_zeci,integer_sute,rest_sute,integer_mii,rest_mii
character(1) :: filename1,filename2,filename3,filename4
character(4) :: filename_tot
! Subrutina ce transforma un INTEGER de la 0 la 9999 in o serie de CARACTERE cu acelasi numar
! pentru a fi folosite in numerotarea si denumirea fisierelor de rezultate.
if(i<=9) then
filename1=char(48+0)
filename2=char(48+0)
filename3=char(48+0)
filename4=char(48+i)
elseif(i>=10.and.i<=99) then
integer_zeci=int(i/10)
rest_zeci=mod(i,10)
filename1=char(48+0)
filename2=char(48+0)
filename3=char(48+integer_zeci)
filename4=char(48+rest_zeci)
elseif(i>=100.and.i<=999) then
integer_sute=int(i/100)
rest_sute=mod(i,100)
integer_zeci=int(rest_sute/10)
rest_zeci=mod(rest_sute,10)
filename1=char(48+0)
filename2=char(48+integer_sute)
filename3=char(48+integer_zeci)
filename4=char(48+rest_zeci)
elseif(i>=1000.and.i<=9999) then
integer_mii=int(i/1000)
rest_mii=mod(i,1000)
integer_sute=int(rest_mii/100)
rest_sute=mod(rest_mii,100)
integer_zeci=int(rest_sute/10)
rest_zeci=mod(rest_sute,10)
filename1=char(48+integer_mii)
filename2=char(48+integer_sute)
filename3=char(48+integer_zeci)
filename4=char(48+rest_zeci)
endif
filename_tot=''//filename1//''//filename2//''//filename3//''//filename4//''
return
end subroutine nume_fisier