Fortran 95 source code to compute a simple linear regression line.

regression_line.f95

!****************************************************
! 単回帰直線計算
!
!   date          name            version
!   2018.12.18    mk-mode.com     1.00 新規作成
!
! Copyright(C) 2018 mk-mode.com All Rights Reserved.
!****************************************************
!
module const
  ! SP: 単精度(4), DP: 倍精度(8)
  integer,     parameter :: SP = kind(1.0)
  integer(SP), parameter :: DP = selected_real_kind(2 * precision(1.0_SP))
end module const

module comp
  use const
  implicit none
  private
  public :: calc_reg_line

contains
  ! 単回帰直線計算
  !
  ! :param(in)  real(8) x(:): 説明変数配列
  ! :param(in)  real(8) y(:): 目的変数配列
  ! :param(out) real(8)    a: 切片
  ! :param(out) real(8)    b: 傾き
  subroutine calc_reg_line(x, y, a, b)
    implicit none
    real(DP), intent(in)  :: x(:), y(:)
    real(DP), intent(out) :: a, b
    integer(SP) :: size_x, size_y, i
    real(DP)    :: sum_x, sum_y, sum_xx, sum_xy

    size_x = size(x)
    size_y = size(y)
    if (size_x == 0 .or. size_y == 0) then
      print *, "[ERROR] array size == 0"
      stop
    end if
    if (size_x /= size_y) then
      print *, "[ERROR] size(X) != size(Y)"
      stop
    end if

    sum_x  = sum(x)
    sum_y  = sum(y)
    sum_xx = sum(x * x)
    sum_xy = sum(x * y)
    a = (sum_xx * sum_y - sum_xy * sum_x) &
    & / (size_x * sum_xx - sum_x * sum_x)
    b = (size_x * sum_xy - sum_x * sum_y) &
    & / (size_x * sum_xx - sum_x * sum_x)
  end subroutine calc_reg_line
end module comp

program regression_line
  use const
  use comp
  implicit none
  real(DP)    :: x(10), y(10), a, b

  x = (/107, 336, 233, 82, 61, 378, 129, 313, 142, 428/)
  y = (/286, 851, 589, 389, 158, 1037, 463, 563, 372, 1020/)
  print '(A, 10F8.2, A)', "説明変数 X = (", x, ")"
  print '(A, 10F8.2, A)', "目的変数 Y = (", y, ")"
  print '(A)', "---"
  call calc_reg_line(x, y, a, b)
  print '(A, F12.8)', "切片 a = ", a
  print '(A, F12.8)', "傾き b = ", b
end program regression_line