def sudoku2str(su):
    """
    Converts a sudoku into a string.

    @param      su      sudoku
    @return              string
    """
    s = ''
    for i in range(0, len(su)):
        if i % 3 == 0:
            s += '-' * 35
            s += '\n'
        for j in range(0, len(su[i])):
            if j % 3 == 0:
                s += ' | '
            if su[i][j] > 0:
                s += str(su[i][j]) + ' '
            else:
                s += '_ '
        s += ' |\n'
    s += '-' * 35
    s += '\n'
    return s
