program distances
   !Subroutine distances(x1,y1,x2,y2,x3,y3,d1,d2,d3)
   !Subroutine to calculate three distances between three points.
   !Variables used are:
   !A(x1,y1),B(x2,y2),C(x3,y3) are the coordinates of three points.
   !d1: the distance between A and B
   !d2: the distance between A and B
   !d3: the distance between A and B
   !Input:The points A,B,C
   !Output:The distances between A and B, B and C,C and A
   !Variable declerations
   real:: x1,x2,x3,y1,y2,y3,d1,d2,d3,a1,a2,a3
   !Read the coordinates of points
   print*, "Enter the coordinates x1,y1,x2,y2,x3,y3"
   read*, x1,y1,x2,y2,x3,y3
   !Calculate d1,d2,d3
    a1=(x2-x1)**2+(y2-y1)**2
    a2=(x3-x2)**2+(y3-y2)**2
    a3=(x3-x1)**2+(y3-y1)**2
    d1=sqrt(a1)
    d2=sqrt(a2)
    d3=sqrt(a3)
   !Calculate and print distances
   if (( d1==d2) .and. ( d1==d3 )) then
       !If the distances are equal, the shape is a equilateral triangle.
       print*, "The distances d1 , d2 , d3 have the same value",d1
   else if (d1==d2 .and. d3<d1+d2) then
       !If two of the sides are equal lenght, the shape is a isosceles triangle.
       print*, "The values of the two distances d1 and d2 are equal and d3 is", &
                              "smaller than d1 + d2 ",d2
   else
   endif
   if (((d1 /= d2) .and. (d1 /= d3)) .or. (d3 < d1+d2)  &
         .or. (d2 < d1+d3) .or. (d1 < d2+d3)) then
       !None of the distances are equal.
       print*, "The values of three distances are different",d3
   else
   endif
end program distances


! Find averages of ten number pairs.
    implicit none
    integer, parameter :: LIMIT = 10
    integer :: Loop
    real :: Average, X, Y
! start program E01
    open (unit = 1, file = "e01.txt", status = "old", action = "read", position = "rewind")
    do Loop = 1, LIMIT
      read (unit = 1, fmt = *) X, Y
      Average = (X +  Y) / 2.0
      write (unit = *, fmt = *) Loop, X, Y, Average
    end do
    stop
  end program E01


! Calculate factorial of Limit.
  program E02
    implicit none
    integer :: Factorial, Limit, Loop
! start program E02
    write (unit = *, fmt = *) " Please enter the number whose factorial is wanted. "
    read (unit = *, fmt = *) Limit
    Factorial = 1
    do Loop = 1, Limit
      Factorial = Factorial * Loop
    end do
    write (unit = *, fmt = *) " Factorial of: ", Limit, " is: ", Factorial
    stop
  end program E02


! Print index value at normal termination.
  program E03
    implicit none
    integer :: I
    real :: Total, Datum
    integer, parameter :: LIMIT = 40
! start program E03
    Total = 0
    open (unit = 1, file = "e03.txt", status = "old", action = "read", position = "rewind")
    do I = 1, LIMIT
      read (unit = 1, fmt = *) Datum
      Total = Total + Datum
    end do
    write (unit = *, fmt = *) I, Total
    stop
  end program E03

! Sum alternating harmonic series until term is small.
  program E06
    implicit none
    integer, parameter :: K = selected_real_kind( 12 )
    real(kind = K) :: Total, Term_Sign, Next_Term
    integer :: N
    real(kind = K), parameter :: TOLERANCE = 1.0e-4
! start program E06
    Total = 0.0_K
    Term_Sign = 1.0_K
    N = 1
    do ! forever
      Next_Term = Term_Sign / real( N, kind = K )
      if (abs( Next_Term ) < TOLERANCE) then
        exit
      end if
      Total = Total + Next_Term
      N = N + 1
      Term_Sign = - Term_Sign
    end do
    write (unit = *, fmt = *) " The sum after: ", N - 1, " terms is: ", Total
    stop
  end program E06


! Compute Fibonacci sequence; exit to avoid overflow.
  program E07
    implicit none
    integer :: FI, FJ, Next
! start program E07
    FI = 1
    FJ = 1
    write (unit = *, fmt = *) FI
    write (unit = *, fmt = *) FJ
    do
      if (FJ > huge (1) - FI) then
        exit
      end if
      Next = FI + FJ
      FI = FJ
      FJ = Next
      write (unit = *, fmt = *) FJ
    end do
    stop
  end program E07


! Sum a power series until next term is small enough.
  program E09
    implicit none
    integer, parameter :: K = selected_real_kind( 12 )
    real(kind = K) :: Total, Next_Term
    real(kind = K), parameter :: TOLERANCE = 1.0e-6_K
! start program E09
    Total = 0.0_K
    Next_Term = 1.0_K
    do
      if (Next_Term < TOLERANCE) then
        exit
      end if
      Total = Total + Next_Term
      Next_Term = 0.5_K * Next_Term
    end do
    write (unit = *, fmt = *) " Sum of terms greater than: ", real(TOLERANCE), " is: ", Total
    stop
  end program E09