Reading data file in Fortran with known number of lines but unknown number of entries in each line

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

Answer 1

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)