Reading unknown size matrix to Fortran [duplicate] - matrix

Question

How can I read the data file containing known number of lines but the number of entries in each line is unknown, e.g. if my data file contain some thing like
1 3 4 5 6 -7 8 -9
1 3 5 6
4 5 6 7 8 3 5 6 7 8 4 5 7 8
i.e. three lines but the data in each line is unknown. At one time I need the data from one line.

Answer 1

One method: read the line into a string, using a string that is at least as long as the longest expected line. Then you go about parsing the string. E.g., if the numbers are always split by spaces, use that to figure out the substring boundaries. Then you can use "internal reads" to read from each sub-string to obtain the numeric values. An internal read uses a string instead of a unit number and obtains the data from the string -- at least you don't have to recreate the conversion of characters to numeric values, the read statement will do that for you. The intrinsic functions provided with Fortran will make the parsing easier.

Answer 2

An implementation based on what M. S. B. pointed out. Quite late, but I guess it could be useful to someone.
Have an array of the type you expect to read ready:
double precision, dimension(MAX_NUM_OF_COLS) :: test_array
Read a line from your file:
READ(reading_unit,'(A)',iostat=io) line
Loop and try to read from the line a maximum quantity of numbers:
do i=1,MAX_NUM_OF_COLS
READ(line, *, iostat=io) test_array(1:i)
if(io==0) exit
enddo
write(*,*) 'number of columns = ', (i-1)
If needed, loop this over all the lines of your file, and keep the maximum or minimum number of columns.
Minimum example:
integer, parameter :: MAX_NUM_OF_COLS=30
integer, parameter :: MAX_LINE_LENGTH=1000
character(len=MAX_LINE_LENGTH) line
integer i, io, reading_unit
double precision, dimension(MAX_NUM_OF_COLS) :: test_array
reading_unit=100
OPEN(reading_unit, file='the_file')
! Get first line of file.
DO
READ(reading_unit,'(A)',iostat=io) line
IF (io/=0) then
write(*,*) "Error reading file."
stop
endif
exit ! Eventually, do not exit and put the DO loop below here.
ENDDO
CLOSE(reading_unit)
do i=1,MAX_NUM_OF_COLS
READ(line,*,iostat=io) test_array(1:i)
if(io==-1) exit
enddo
write(*,*) 'number of columns = ', (i-1)

Answer 3

Assuming that you're okay with padding the array with zeros (specifically referring to the later duplicate question here), this is my idea:
Read the data line by line into a string, then append a number of zeros, finally read each row of the data from this array. Here's an example:
program unknown_numbers
implicit none
integer, parameter :: nrow=3, ncol=14
integer :: data(ncol, nrow)
character(len=2*ncol) :: zeros ! will contain lots of zeros
character(len=10*ncol) :: line ! temporary storage for each line of the file
integer :: i, u
! Write " 0 0 0 0 0 0 0 0" into the string "zeros"
write(zeros, '(*(I2))') [(0, i=1, ncol)]
open(newunit=u, file='data.txt', status='old', action='read')
do i = 1, nrow, 1
! Read the next line into a temporary string array
read(u, '(A)') line
! Append a number of zeros to the temporary string
line = trim(line) // zeros
! Read the row of data from the string.
read(line, *) data(:, i)
end do
close(u)
! For testing purposes, print the data.
print '(14(X, I3))', data
end program unknown_numbers

Answer 4

integer,parameter :: reclen=99999 ! maximum record length
integer,parameter :: undef=-9999 ! undefined value
integer :: actual_reclen ! actual record length
integer,dimension(reclen) :: dummy=undef ! dummy array used for reading
integer,dimension(:),allocatable :: a ! final array
open(unit=10,file='sample.txt',form='formatted',access='sequential')
read(unit=10,fmt=*,end=101)(dummy(i),i=1,reclen)
101 close(unit=10)
actual_reclen=COUNT(dummy/=undef)
allocate(a(actual_reclen))
a=dummy(1:actual_reclen)
end

Answer 5

This is a program that can count numbers in a line (or number of columns) but for a line. If you have many lines, you should change it slightly.
program test12
implicit none
integer n,m,i
integer,allocatable::x(:)
open(10,file='C:\Users\user\Desktop\file.txt')
allocate(x(n))
20 n=n+1
deallocate(x)
allocate(x(n))
read(10,*,iostat=m)(x(i),i=1,n)
if (m==-1)then
goto 30
else
rewind 10
goto 20
end if
30 print*,n-1
end