Fortran 95 source code to solve simultaneous equations by Gauss-Jordan's method(pivot).

gauss_jordan_pivot.f95

!**************************************************************
! Simultaneous equations solving by Gauss-Jorden(pivot) method
!
!   date          name            version
!   2018.12.04    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 gauss_jordan_pivot
  use const
  implicit none
  private
  public :: solve

contains
  ! 連立方程式を解く
  !
  ! :param(in)    integer(4)     n: 元数
  ! :param(inout) real(8) a(n,n+1): 係数配列
  subroutine solve(n, a)
    implicit none
    integer(SP), intent(in)    :: n
    real(DP),    intent(inout) :: a(n, n + 1)
    integer(SP) :: i, j, s
    real(DP)    :: mx, dummy, p, d

    do j = 1, n
      mx = 0.0_DP
      s  =j 
      do i = j, n
        if (abs(a(i, j)) <= mx) cycle
        mx = abs(a(i, j))
        s = i
      end do
      if (mx == 0.0_DP) then
        print *, "Can't solve!"
        stop
      end if
      do i = 1, n + 1
        dummy = a(j, i)
        a(j, i) = a(s, i)
        a(s, i) = dummy
      end do
      p = a(j, j)
      a(j, j:n+1) = a(j, j:n+1) / p
      do i = 1, n
        if (i == j) cycle
        d = a(i, j)
        a(i, j:n+1) = a(i, j:n+1) - d * a(j, j:n+1)
      end do
    end do
  end subroutine solve
end module gauss_jordan_pivot

program simultaneous_equations
  use const
  use gauss_jordan_pivot
  implicit none
  integer(SP)   :: n, i            ! 元数、ループインデックス
  character(20) :: f               ! 書式文字列
  real(DP), allocatable :: a(:,:)  ! 係数配列

  ! 元数取得
  write (*, '(A)', advance="no") "n ? "
  read (*, *) n

  ! 配列メモリ確保
  allocate(a(n, n + 1))

  ! 係数取得
  do i = 1, n
    write (*, '("row(", I0, ",1:", I0, ") ? ")', advance="no") i, n + 1
    read (*, *) a(i,:)
  end do
  write (*, '(/A)') "Coefficients:"
  write (f, '("(", I0, "(F8.2, 2X)", ")")') n + 1
  do i = 1, n
    write (*, f) a(i,:)
  end do

  ! 連立方程式を解く
  ! (計算後 a の最右列が解)
  call solve(n, a)

  ! 結果出力
  write (*, '(A)') "Answer:"
  write (f, '("(", I0, "(F8.2, 2X)", ")")') n
  write (*, f) a(:, n + 1)

  ! 配列メモリ解放
  deallocate(a)
end program simultaneous_equations