Fortran90 program seems to be rounding numbers without my permission and it's messing up my output

Question

I'm in the process of writing a Fortran90 program that will take an input file containing points to be graphed, sort those points, calculate the minimum and maximum x and y values, print all of this to another file, and then create a graph and print that as well.

So far, I'm up to calculating the min and max values, and I've got it all printing to a file.
The structure of the program is as such:

Please enter an input file:

GRAPH3.txt

Please enter an output file:

output.txt

And then it'll pull the data from GRAPH3.txt, do stuff, and print it.

GRAPH3.txt looks like this:

11
8.3       8.3
12.0      12.0
2.0       2.0
4.0       4.0
1.0       1.0
4.5       4.5
12.1      12.1
4.6       4.6
3.0       3.0
7.2       7.2
9.0       9.0

The first number indicates how many points there are. The rest are just the pairs of numbers.

When I run my program, this is my output:

 presort
  8.30   8.30
 12.00  12.00
  2.00   2.00
  4.00   4.00
  1.00   1.00
  4.50   4.50
 12.10  12.10
  4.60   4.60
  3.00   3.00
  7.20   7.20
  9.00   9.00
 postsort
  1.00   1.00
  2.00   2.00
  3.00   3.00
  4.00   4.00
  4.00   4.00
  4.00   4.00
  7.20   7.20
  8.00   8.00
  9.00   9.00
 12.00  12.00
 12.00  12.00
Xmin:    1.00
Xmax:   12.00
Ymin:    0.00
ymax:   12.00

Obviously, ymin should be 1.00, but it's showing up at 0.00. Do you have any idea why it would be doing this?

Here's my program:

        PROGRAM G6P4
        implicit none
        character(len=30) :: infile,outfile
        logical inexist, outexist,more,quit
        integer i,j,n,overwrite
        real x(100),y(100),xmax,xmin,ymax,ymin
        more=.true.
        inexist = .false.
        outexist = .false.
        overwrite=2
        do while (.not.inexist)
            print *, 'Please enter an input filename'
            read *, infile
            INQUIRE(FILE=infile, EXIST=inexist)
            if (.not.inexist) then
                print *, 'Invalid Input File'
            end if
        end do
        do while (more.or.infile.eq.outfile)
            print *, 'Please enter an output filename'
            read *, outfile
            INQUIRE(FILE=outfile, EXIST=outexist)
            if (infile.eq.outfile) then
                print *, 'Outfile cannot equal infile.'
            else
                if (outexist) then
                    print *, 'File already exists'
                    print *, '1 to enter overwrite, 2 to enter new file, 0 to quit'
                    read *,overwrite
                    select case (overwrite)
                    case (1)
                        more =.false.
                    case (2)
                        more = .true.
                    case (0)
                        more = .false.
                        quit = .true.
                    case default
                        print *,'NOPE'
                        more = .true.
                    end select
                else
                    more=.false.
                end if
            end if
        end do
        if (quit.eqv..false.) then
            OPEN(1, FILE=infile)
            OPEN(2, FILE=outfile)
            READ(1,*)n
            if (n.le.100.and.n.gt.0) then
                do i = 1, n
                    read (1,*) x(i),y(i)
                end do
                write (2,*) 'presort'
                do i = 1, n
                    write (2,'(2(F6.2,X))') x(i),y(i)
                end do
                call sort(x,y,n)
                write (2,*) 'postsort'
                do i = 1, n
                    write (2,'(2(F6.2,X))') x(i),y(i)
                end do
                xmin = x(1)
                xmax = x(n)
                ymax = y(1)
                ymax = y(n)
                do i = 2, n
                    if (ymin.GT.y(i)) then
                        ymin=y(i)
                    end if
                    if (ymax.lt.y(i)) then
                        ymax=y(i)
                    end if
                enddo
                write (2,'(A,X,F6.2)') 'Xmin: ',xmin
                write (2,'(A,X,F6.2)') 'Xmax: ',xmax
                write (2,'(A,X,F6.2)') 'Ymin: ',ymin
                write (2,'(A,X,F6.2)') 'ymax: ',ymax
            else
                print *,'File has invalid number of data points'
            end if
        end if

        end PROGRAM

        subroutine sort(x,y,n)
            real, intent(out),dimension(100) :: x,y
            integer ::  n
            integer :: end,j,t
            logical :: more
            end = n-1
            do while (more)
                more = .false.
                do j = 1, end
                    if (x(j).gt.x(j+1)) then
                        write (*,'(A,2(F6.1,X))')'Pre: ',x(j),x(j+1)
                        t = x(j)
                        x(j)=x(j+1)
                        x(j+1)=t
                        t=y(j)
                        y(j) = y(j+1)
                        y(j+1)=t
                        more = .true.
                        write (*,'(A,2(F6.1,X))')'Post: ',x(j),x(j+1)
                    end if
                enddo
                end = end-1
            enddo
        end subroutine

Another problem I've discovered is that if I remove the write statements from the sort subroutine, for some reason, the program doesn't sort at all. Any clue why it would be doing that?

Answer 1

You are not initializing ymin. You are instead initializing ymax twice. Therefore ymin gets a default initialization of zero, which happens to be the lowest number considered, and is reported as the minimum.