Reading an array from a text file with Fortran 90/95

May 21, 2010

If you’re used to coding in more modern languages, Fortran I/O can seem a little bizarre. Strings in Fortran are much more difficult to work with, since they are fixed-length rather than null-terminated. The following example illustrates a simple way to read an array of numbers from a text file when the array length is unknown at compile time.

program io_test
      real, dimension(:), allocatable :: x
      integer :: n
      open (unit=99, file='array.txt', status='old', action='read')
      read(99, *), n
      allocate(x(n))
      read(99,*) x
      write(*,*) x
end

Here is the text file that the array is read from. The integer on the first line is the number of elements to read from the next line.

10
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
```---

## Comments

### Comment #1 by Marcel Cobián

[Marcel Cobián](mailto:<sientelaclaridad@yahoo.com>) - Feb 6, 2012

good job!

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#### Comment #2 by Giuseppe

[Giuseppe](mailto:<joematara@hotmail.com>) - Jan 3, 2013

Superb! Let me show (below) a slight modification in order to have the oneline string directly stored in a file. Cheers program biz real, dimension(:), allocatable :: x integer :: n open (unit=89, file='447mono.dat', status='new', action='write') open (unit=99, file='447km.dat', status='old', action='read') read(99, \*), n allocate(x(n)) read(99,\*) x write(89,\*) x end

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#### Comment #3 by Martin

[Martin](http://&lt;www.puntocritico.wordpress.com&gt; "martinpedra&#64;gmail.com") - Mar 2, 2014

Nice work, however I'm not able to use this in ifort... how is your txt file formated? I'm trying to use comma separated files...

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#### Comment #4 by Martin

[Martin](http://&lt;www.puntocritico.wordpress.com&gt; "martinpedra&#64;gmail.com") - Mar 2, 2014

Oh, sorry, I got it... but having to specify the size of the array in the first line of the text won't work for me... but indeed, this is a simple method...

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#### Comment #5 by Ade

[Ade](mailto:<adeajayi.kunle@gmail.com>) - Oct 4, 2014

Hi martin, am having similar problem, it works fine if i specify the size of the array. Please I need to obtain the length of the array without specifing the size at the beginning,please How do i go about it.

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#### Comment #6 by Craig

[Craig](http://amazon.com/author/craig.finch "cfinch&#64;ieee.org") - Oct 0, 2014

Ade, you have to know the length of the data before reading it into an array. This is a fundamental limitation of Fortran. If you have to read files of unknown length, you will have to read each file twice. Read a file once to determine the length, allocate the array, and then read in the data.

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#### Comment #7 by nivas

[nivas](mailto:<spyngjkr@gmail.com>) - Sep 3, 2015

Run through the file once to get the no of lines.

```text
open(99,file='447mono.dat')
n=0
do
  read(99,*, iostat=iostatus)dummyv
  if(iostatus/=0) then ! to avoid end of file error.
    exit
  else
    n=n+1
  end if
end do
allocate (x(n))

Here n contains the no of lines in the data file. Later n can be used in several ways. To read the data in to array now,

open(99,file='447mono.dat')
do i=1,n
  read(99,*)array(i)
end do
```---

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