How can I properly name and open multiple files in Fortran based on user input?

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  • Thread starter Thread starter LucasCampos
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Discussion Overview

The discussion revolves around how to dynamically name and open multiple files in Fortran based on user input. Participants explore various methods for file handling, including writing to and reading from files, as well as issues related to managing file unit numbers and array dimensions.

Discussion Character

  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant suggests using a loop to open files with names based on user input, but encounters issues with the naming convention.
  • Another participant proposes a subroutine that attempts to read and write file names, questioning whether the reading process will yield an array of file names or a single concatenated string.
  • A different approach is mentioned, where the WRITE command is used to concatenate strings for file names, allowing for dynamic file creation.
  • Concerns are raised about the limitations of using fixed-size arrays for file names, with a suggestion to consider allocatable arrays for flexibility.
  • One participant points out that opening multiple files with the same unit number is not feasible and suggests closing and reopening files as needed.
  • Another participant expresses confusion about the use of unit numbers and the potential issues with exceeding certain limits, particularly above 99.
  • There are discussions about the behavior of loops and the need for proper definitions and changes to variables within the loops.

Areas of Agreement / Disagreement

Participants express differing views on the best practices for file handling in Fortran, particularly regarding the use of unit numbers and array dimensions. There is no consensus on the most effective method to implement the file opening and reading process.

Contextual Notes

Some participants highlight limitations related to fixed-size arrays and the need for proper variable management within loops. There are also unresolved questions about the behavior of file operations when using dynamic naming conventions.

LucasCampos
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Hello there. I've doing a program that I do not know how many files will be needed, because it'll depend on user's choice. So, how to name them?

I tought that a command like, where n is a user-input number

Code: 
Do While (i .LE. n)
Open (Unit=i, File=i'.dat')
i=i+1
EndDo

Would be enough, but I can't name my files this way. Any ideas?
 
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I've been thinking about this program (many thanks to emes) and tought thet I could do something like this

Code: 
Subroutine Opener (n,i)
Implicit None
Integer n,i
Character (LEN=15) :: files
Open (unit=500,File='Files1.dat')
Open (unit=501,File='Files2.dat')
i=1
Do While (i .LE. n)
	Write (500,*) i,'.dat'
	Write (501,*) 'mov',i,'.dat'
	i=i+1
End
i=1
Do While (i .LE. n)
	Read (500,*) files
	Open (Unit=i,File=files)
	Read (501,*) 
	Open (Unit=i+n,File=files)
	i=i+1
EndDo
End

But would it read line after line of the files?

Hehe. I've turned this Open problem into a Read one.
 
I have a post somewhere on here as a way to do it. Basically, you can use the WRITE command to concatenate a string with a variable. You can then OPEN the file with the filename as the variable. It's something like this:

Code: 
DO i=1,10
  WRITE(fname,'I2,A4') i,'.dat'
  OPEN(10,file=filename,form='formatted')
  ...
END DO
 
When I use

Code: 
i=1
Do While (i .LE. n)
	Write (500,*) i,'.dat'
	Write (501,*) 'mov',i,'.dat'
	i=i+1
End

I'll have a file like this

Code: 
1.dat
2.dat
3.dat
.
.
.
n.dat

When I read it with

Code: 
i=1
Do While (i .LE. n)
	Read (500,*) files(i)
	Open (Unit=i,File=files)
	i=i+1
EndDo

Will I get a array with like 1.dat, on index 1, 2.dat, on index 2, ... n.dat, on index n, or I'll receive '1.dat 2.dat 3.dat ... n.dat' in every index?
 
I have solved many issues on this program already, but new ones keep rising.

On this 'subprogram', when it goes into the third loop it can't get out. But it does open all files it should open

Code: 
Program Lennard2
Implicit None
Double Precision dt,e,sigma,x(20),y(20),vx(20),vy(20),tet(20),omeg(20),&
rad(20),mass(20)
Integer n,i,show
Character (LEN=15) :: files(20),files2(20)
Call Inserter (dt,e,sigma,x,y,vx,vy,tet,omeg,rad,mass,n,i,show)
Call Opener (n,i,files,files2)
End

Subroutine Opener (n,i,files,files2)
Implicit None
Integer n,i
Character (LEN=15) :: files(20),files2(20)
i=01
10 format (i2,A4)
20 format (i2,A7)
Open (Unit=500,File="uia.dat",Action='Write')
Open (Unit=501,File="duia.dat",Action='Write')
Do While (i .LE. n)
	Write (500,10) i,'.dat'
	Write (501,20) i,'mov.dat'
	Write (*,*) 'First Loop'
	i=i+1
EndDo
Close (Unit=500)
Close (Unit=501)
i=1
Open (Unit=500,File="uia.dat",Action='Read')
Open (Unit=501,File="duia.dat",Action='Read')
Do While (i .LE. n)
	Read (500,*) files(i)
	Read (501,*) files2(i)
	Write (*,*) 'Second Loop'
	i=i+1
EndDo
i=1
Do While (i .LE.n)
	Open (Unit=i,File=files(i))
	Open (Unit=i+n,File=files2(i))
	Write (*,*) 'Third Loop'
	i=i+1
EndDo
i=1
Do While (i .LE. n)
	Write (i,*) i
	Write (i+n,*) i+n
	i=i+1
	Write (*,*) 'Forth Loop'
EndDo
Write (*,*) 'Opener'
End

Subroutine Inserter (dt,e,sigma,x,y,vx,vy,tet,omeg,rad,mass,n,i,show)
Implicit None
Double Precision dt,e,sigma,x(20),y(20),vx(20),vy(20),tet(20),omeg(20),&
rad(20),mass(20)
Integer n,i,show
Write (*,*) 'Insert time step'
Read (*,*) dt
Write (*,*) 'Insert sigma (distance at which the inter-particle &
potential is zero)'
Read (*,*) sigma
Write (*,*) 'Insert epislon (depth of the potential well)'
Read (*,*) e
Write (*,*) 'Insert steps between output'
Read (*,*) show
200 Write (*,*) 'Insert how many particles (2~20)'
Read (*,*) n
If (n .GT. 20) then
Write (*,*) 'Invalid Input'
Go to 200
End If
If (n .LT. 2) then
Write (*,*) 'Invalid Input'
Go to 200
End If
i=1
Do While (i .LE. n)
Write (*,*) "Insert ",i," disk's mass" 
Read (*,*) mass(i)
Write (*,*) "Insert ",i," disk's radius"
Read (*,*) rad(i)
Write (*,*) "Insert ",i," disk's initial vertical speed"
Read (*,*) vy(i)
Write (*,*) "Insert ",i," disk's initial horizontal speed"
Read (*,*) vx(i)
Write (*,*) "Insert ",i," disk's initial vertical position"
Read (*,*) y(i)
Write (*,*) "Insert ",i," disk's initial horizontal positon"
Read (*,*) x(i)
Write (*,*) "Insert ",i," disk's initial angular speed"
Read (*,*) omeg(i)
Write (*,*) "Insert ",i," disk's initial angular position" 
Read (*,*) tet(i)
i=i+1
Write(*,*) 'Inserter'
EndDo
End

Strangely, when I use up to 4 particles, it works ok. But anything above that won't do the job.
 
You cannot open an array of files in one unit number. Each unit number must correspond to a single file that its operating on. Reading the files in as you described won't work...for several reasons. You can use the same unit number. Just close the file at the end of the do-loop and reopen it after you've changed the filename. After you write out the files, the read would look like:
Code: 
DO i=1,n
  !-- writing routines
END DO

DO i=1,n
  WRITE(fname,'A3,I2,A4') 'mov',i,'.dat'
  OPEN(10,file=fname,form='formatted')
  !--Read the file here
  CLOSE(10)
END DO

Be careful using unit numbers above 99. I wasn't even sure that you could do that. I'm fairly sure that many compilers will have issues with it.

Looking through your code it appears you have some issues. Firstly, you have a DO loop where i < n, but n doesn't appear to be defined, nor does it change. Your arrays that hold the data are only dimensioned to 20, but your program is written so that a general number, n, can be called. If you want this, you should change your arrays to ALLOCATABLE.
 
I fail to see where I was trying to open twice using the same unit. Could you show me more detailed? On the subject of the array of 20, I've put a few Ifs on Inserter to deal with that, but I'll make some research on Allocatables Arrays, it seems quite interesting.

Code: 
.
.
.
200 Write (*,*) 'Insert how many particles (2~20)'
Read (*,*) n
If (n .GT. 20) then
Write (*,*) 'Invalid Input'
Go to 200
End If
If (n .LT. 2) then
Write (*,*) 'Invalid Input'
Go to 200
End If
.
.
.
 
I didn't say that you were opening two files with the same unit number, I said that in your example, you were trying to open an array of files, quote:
Code: 
i=1
Do While (i .LE. n)
	Read (500,*) files(i)
	[b]Open (Unit=i,File=files)[/b]
	i=i+1
EndDo
files here is defined as an array. This implicitly means that you're attempting to open up all files, (files(i),i=1,imax).
 

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