How to add auto post excerpt function in wordpress

********How to add auto post excerpt function in wordpress from it-bari tutorial **********

//first past this code in function.php to register excerpt function

            /*This Code is For Read More Function*/
                function excerpt($num) {
                $limit = $num+1;
                $excerpt = explode(' ', get_the_excerpt(), $limit);
                array_pop($excerpt);
                $excerpt = implode(" ",$excerpt)." <a href='" .get_permalink($post->ID) ." ' id='".readmore."'>Read More</a>";
                echo $excerpt;
                }
                function wpse_filter_excerpt_length( $length ) {
                    if ( is_front_page() ) {
                        return 55; // change this to number of words you want on homepage.
                    } else {
                        return $length;
                    }
                }
                add_filter( 'excerpt_length', 'wpse_filter_excerpt_length', 999 );



//than use this code to call post content


                <?php echo  excerpt('55'); ?>