for @shibu_tan_i

lesson1.f90

program lesson1
  integer i, j, k, l
  real a, b, c
  real mean, geo_mean
  print *, "Please input 4 integers:"
  read *, i, j, k, l
  print *, "i=", i, "j=", j, "k=", k, "l=", l
  print *, "Please input 3 real numbers:"
  read *, a, b, c
  print *, "a=", a, "b=", b, "c=", c
  mean = (i + j + k + l) / 4.0
  geo_mean = (a * b * c) ** (1.0 / 3.0)
  print *, "mean of i, j, k and l = ", mean
  print *, "geometric mean of a, b and c = ", geo_mean
end program lesson1

lesson2_a.f90

program lesson2_a
  integer M, N
  integer tmp
  print *, "Please input 2 integers:"
  read *, M, N
  if (M < N) then
  call swap(M, N)
  end if
  ! ensure M >= N
  do while (N /= 0)
     tmp = N
     N = mod(M,  N)
     M = tmp
  end do
  print *, M
end program lesson2_a


subroutine swap(x, y)
  integer z, x, y
  z = x
  x = y
  y = z
end subroutine swap

lesson2_b.f90

program lesson2_b
  integer A, B, C, D
  integer AB, CD, ABCD
  
  print *, "Please input 4 integers 100~500:"
  read *, A, B, C, D
  call GCD(A, B, AB)
  call GCD(C, D, CD)
  call GCD(AB, CD, ABCD)
  print *, ABCD
end program lesson2_b

subroutine GCD(I, J, R)
  integer I, J, R
  integer M, N
  integer tmp
  M = I
  N = J
  if (M < N) then
     call swap(M, N)
  end if
  ! ensure M >= N
  do while (N /= 0)
     tmp = N
     N = mod(M,  N)
     M = tmp
  end do
  R = M
end subroutine GCD

subroutine swap(x, y)
  integer z, x, y
  z = x
  x = y
  y = z
end subroutine swap

lesson3.f90

program lesson3
  integer A, B, C, D
  integer, dimension(4) :: array, i_array
  i_array(1) = 1
  i_array(2) = 2
  i_array(3) = 3
  i_array(4) = 4
  print *, "Please input 4 integers"
  read *, A, B, C, D
  array(1) = A
  array(2) = B
  array(3) = C
  array(4) = D
  call qsort(array, i_array, 1, 4)
  print *, i_array
  print *, array
end program lesson3

recursive subroutine qsort(a, b, first, last)
  integer first, last
  integer, dimension(4) :: a
  integer, dimension(4) :: b
  integer i, j
  integer x, t

  x = a((first + last) / 2)     !choose pivot
  i = first
  j = last
  do
     do while (a(i) < x)
        i = i + 1
     end do
     do while (x < a(j))
        j = j - 1
     end do
     if (i >= j) exit
     call swap(a(i), a(j))
     call swap(b(i), b(j))
     i=i+1
     j=j-1
  end do
  if (first < i - 1) then
     call qsort(a, b, first, i - 1)
  end if
  if (j + 1 < last) then
     call qsort(a, b, j + 1, last)
  endif
end subroutine qsort

subroutine swap(x, y)
  integer z, x, y
  z = x
  x = y
  y = z
end subroutine swap

Makefile

all: hello lesson1 lesson2_a lesson2_b lesson3


hello: hello.f90
  gfortran $< -o $@
lesson1: lesson1.f90
	gfortran $< -o $@
lesson2_a: lesson2_a.f90
	gfortran $< -o $@
lesson2_b: lesson2_b.f90
	gfortran $< -o $@
lesson3: lesson3.f90
	gfortran $< -o $@

clean:
	rm -f hello lesson1 lesson2_a lesson2_b lesson3