Determinant

program determinant
    implicit none
    integer, parameter:: m=3, n=3
    integer:: i, j, k
    real:: a(m,n), t, p, c=0.0, det
    open(11,file="in.txt")
    do i=1,m
        read(11,*) (a(i,j), j=1,n)
    end do
    close(11)

    do j=1,m-1

        ! pivoting
        do i=j+1, m
            if(abs(a(i,j)) > abs(a(j,j))) then
                !row interchange
                c=c+1.0
                do k=1,n
                    t=a(j,k)
                    a(j,k)=a(i,k)
                    a(i,k)=t
                end do
            end if
        end do

        if(a(j,j)==0) cycle

        !row echelon
        do i=j+1, m
            p=a(i,j)/a(j,j)
            do k=j,n
                a(i,k)=a(i,k)-p*a(j,k)
            end do
        end do

    end do

    open(12,file="out.txt")

    p=1.0
    do i=1,m
        p=p*a(i,i)
    end do
    det = ((-1)**c)*p
    write(12,*) "Determinat is ",det

    close(12)


end program determinant