How to find the dimension of a matrix? - matrix

Question

I have a matrix that contains both character and reals and I want a program that reads this matrix (finds the dimensions by itself). Here is my code:
! A fortran95 program for G95
Program Project2nd
implicit none
character(len=40), allocatable :: a(:,:)
integer i,j,k,n,m,l,st
character(len=40) d
n=0; m=1; j=1;
open(10,file=&
'/Users/dariakowsari/Documents/Physics/Programming/Fortran95-Projects/Project2nd/input.txt', &
IOstat=st)
do while (st == 0)
read(10,*,IOstat=st) d
n=n+1
end do
st=0
do j=1,m
do while (st == 0)
allocate(a(1,m))
read(10,*,IOstat=st) (a(1,j),j=1,m)
m=m+1
deallocate(a)
end do
print*, n,m
end
Here is my Matrix:
a b 13 15.5 13.2
c d 16 16.75 19
e f 19.2 12.2 18.2
With this code I got (3,2) for the dimensions of my matrix.

Answer 1

There are a few errors in your example code which means it doesn't compile for me but after a few changes I managed to get a similar result to you.
*Update: As noted by #francescalus in the comments to my other (now deleted) answer, that approach involved undefined behaviour and as such is not an appropriate solution. This arose from trying to read more elements from the file than were present.)
Here's an alternative approach, which should avoid this undefined behaviour, but is probably pretty inefficient.
Program Project2nd
implicit none
character(len=40), allocatable :: a(:)
integer, allocatable :: ind(:)
integer, parameter :: maxElements = 100
integer i,j,n,m,st
character(len=40) d
n=0;
open(10,file='mat.txt',IOstat=st)
!Find number of lines
do while (st == 0)
read(10,*,IOstat=st) d
if(st ==0) n=n+1
end do
!Move back to the start of the file
rewind(10)
!Read all of the data
do m=n,maxElements,n
allocate(a(m))
read(10,*,IOstat=st) a
deallocate(a)
rewind(10)
if(st.ne.0) exit
enddo
m = m -n !Need to roll back m by one iteration to get the last which worked.
if(mod(m,n).ne.0) then
print*,"Error: Number of elements not divisible by number of rows."
stop
endif
!Number of columns = n_elements/nrow
m=m/n
print*, n,m
end Program Project2nd
Essentially this uses the same code as you had for counting the number of lines, however note that you only want to increment n when the read was successful (i.e. st==0). Note we do not exit the whilst block as soon as st becomes non-zero, it is only once we reach the end of the whilst block. After that we need to rewind the file so that the next read starts at the start of the file.
In a previous comment you mentioned that you'd rather not have to specify maxElement if you really want to avoid this then replace the second do loop with something like
st = 0 ; m = n
do while (st==0)
allocate(a(m))
read(10,*,IOstat=st) a
deallocate(a)
rewind(10)
if(st.ne.0) then
m = m - n !Go back to value of m that worked
exit
endif
m=m+n
enddo

Answer 2

here is how to do w/o rewinding.
implicit none
character(len=100) wholeline
character(len=20), allocatable :: c(:)
integer iline,io,ni,nums
open(20,file='testin.dat')
iline=0
do while(.true.)
read(20,'(a)',iostat=io)wholeline
if(io.ne.0)exit
iline=iline+1
ni=lineitems(wholeline)
allocate(c(ni))
read(wholeline,*)c
nums=ctnums(c)
write(*,*)'line',iline,' contains ',ni,'items',nums,
$ 'are numbers'
deallocate(c)
enddo
write(*,*)'total lines is ',iline
contains
integer function ctnums(c)
! count the number of items in a character array that are numbers
! this is a template,
! obviously you could assign the numbers to a real array here
character(len=*), allocatable :: c(:)
real f
integer i,io
ctnums=0
do i = 1,size(c)
read(c(i),*,iostat=io)f
if(io.eq.0)ctnums=ctnums+1
enddo
end function
integer function lineitems(line)
! count the number of items in a space delimited string
integer,parameter ::maxitems=100
character(len=*) line
character(len=80) :: c(maxitems)
integer iline,io
lineitems=0
do iline=1,maxitems
read(line,*,iostat=io)c(:iline)
if(io.ne.0)return
lineitems=iline
enddo
if(lineitems.eq.maxitems)write(*,*)'warning maxitems reached'
end function
end
output
line 1 contains 5 items 3 are numbers
line 2 contains 5 items 3 are numbers
total lines is 2