Jacobi and Gauss-Seidal System solving using Fortran

input.txt

0. 0. 0. 0. 0.
 4.  1.  1.  0.  1. 6.
-1. -3.  1.  1.  0. 6.
 2.  1.  5. -1. -1. 6.
-1. -1. -1.  4.  0. 6.
 0.  2. -1.  1.  4. 6.

jacobi-seidal.f90

program jacobi-seidal
    implicit none
    integer, parameter :: n = 5
    integer :: i, j, k
    real :: a(n,n), x(n), b(n), dom, tb1(0:50,50), tb2(0:50,50), s(n), tol
    logical :: stp

    open(unit = 2, file= 'input.txt')

    tol = 0.00001

    read(2,*)x(:)
    do i = 1, n
        read(2,*)(a(i,j), j = 1, n), b(i)
        tb1(0,i)=x(i)
    end do

    do i = 1, n
        dom = 0
        do j = 1, n
            if(i /= j) then
                dom = dom + abs(a(i,j))
            end if
        end do

        if(dom > abs(a(i,i))) then
            write(*,*)"Matrix A is not Dominant"
            stop
        end if
    end do

    write(*,*)"Matrix A is Dominant"

    k = 0
    do
        k = k + 1
        do i = 1, n
            s(i) = 0
            do j = 1,n
                if(i /= j) then
                    s(i) = s(i) + x(j)*a(i,j)
                end if
            end do
        end do

        do i = 1, n
            x(i) = (b(i)-s(i))/a(i,i)
            tb1(k,i) = x(i)
        end do

        stp=.true.
        do i = 1, n
            if(abs(tb1(k,i)-tb1(k-1,i)) > tol) then
                stp=.false.; exit
            end if
        end do

        if(stp)exit
    end do

    write(*,*)"Jacobi Iterative process stop after ",k,"th iteration."
    write(*,*)"  Inter. No. ","      X1     ","         X2     ","          X3    ","          X4     ","         X5     "
    write(*,*)"  ----------","  ----------- ","    ----------- ","     -----------","      -----------","    -----------"
    do i=0,k
        write(*,*)i,(tb1(i,j),j=1,n)
    end do

    do i=1,n    !! for initialization
        x(i)=tb1(0,i)
        tb2(0,i)=tb1(0,i)
    end do

    k = 0
    do
        k = k + 1
        do i = 1, n
            s(i) = 0
            do j = 1,n
                if(i /= j) then
                    s(i) = s(i) + x(j)*a(i,j)
                    x(i) = (b(i)-s(i))/a(i,i)
                    tb2(k,i) = x(i)
                end if
            end do
        end do

        stp=.true.
        do i = 1, n
            if(abs(tb2(k,i)-tb2(k-1,i)) > tol) then
                stp=.false.; exit
            end if
        end do

        if(stp)exit
    end do

    write(*,*)"Jacobi Iterative process stop after ",k,"th iteration."
    write(*,*)"  Inter. No. ","      X1     ","         X2     ","          X3    ","          X4     ","         X5     "
    write(*,*)"  ----------","  ----------- ","    ----------- ","     -----------","      -----------","    -----------"
    do i=0,k
        write(*,*)i,(tb2(i,j),j=1,n)
    end do

end program