Create menu for bootstrap in wordpress

create a file of name wp_bootstrap_navwalker.php  and put this code in it



class wp_bootstrap_navwalker extends Walker_Nav_Menu {
 public function start_lvl( &$output, $depth = 0, $args = array() ) {
  $indent = str_repeat( "\t", $depth );
  $output .= "\n$indent<ul role=\"menu\" class=\" dropdown-menu\">\n";
 }
 public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
  $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

  if ( strcasecmp( $item->attr_title, 'divider' ) == 0 && $depth === 1 ) {
   $output .= $indent . '<li role="presentation" class="divider">';
  } else if ( strcasecmp( $item->title, 'divider') == 0 && $depth === 1 ) {
   $output .= $indent . '<li role="presentation" class="divider">';
  } else if ( strcasecmp( $item->attr_title, 'dropdown-header') == 0 && $depth === 1 ) {
   $output .= $indent . '<li role="presentation" class="dropdown-header">' . esc_attr( $item->title );
  } else if ( strcasecmp($item->attr_title, 'disabled' ) == 0 ) {
   $output .= $indent . '<li role="presentation" class="disabled"><a href="#">' . esc_attr( $item->title ) . '</a>';
  } else {
   $class_names = $value = '';
   $classes = empty( $item->classes ) ? array() : (array) $item->classes;
   $classes[] = 'menu-item-' . $item->ID;
   $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
   if ( $args->has_children )
    $class_names .= ' dropdown';
   if ( in_array( 'current-menu-item', $classes ) )
    $class_names .= ' active';
   $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
   $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
   $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
   $output .= $indent . '<li' . $id . $value . $class_names .'>';
   $atts = array();
   $atts['title']  = ! empty( $item->title ) ? $item->title : '';
   $atts['target'] = ! empty( $item->target ) ? $item->target : '';
   $atts['rel']    = ! empty( $item->xfn )  ? $item->xfn : '';
   // If item has_children add atts to a.
   if ( $args->has_children && $depth === 0 ) {
    $atts['href']     = '#';
    $atts['data-toggle'] = 'dropdown';
    $atts['class']   = 'dropdown-toggle';
    $atts['aria-haspopup'] = 'true';
   } else {
    $atts['href'] = ! empty( $item->url ) ? $item->url : '';
   }
   $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );
   $attributes = '';
   foreach ( $atts as $attr => $value ) {
    if ( ! empty( $value ) ) {
     $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
     $attributes .= ' ' . $attr . '="' . $value . '"';
    }
   }
   $item_output = $args->before;
  
   if ( ! empty( $item->attr_title ) )
    $item_output .= '<a'. $attributes .'><span class="glyphicon ' . esc_attr( $item->attr_title ) . '"></span>&nbsp;';
   else
    $item_output .= '<a'. $attributes .'>';
   $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
   $item_output .= ( $args->has_children && 0 === $depth ) ? ' <span class="caret"></span></a>' : '</a>';
   $item_output .= $args->after;
   $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
  }
 }
 public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
        if ( ! $element )
            return;
        $id_field = $this->db_fields['id'];
        // Display this element.
        if ( is_object( $args[0] ) )
           $args[0]->has_children = ! empty( $children_elements[ $element->$id_field ] );
        parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
 public static function fallback( $args ) {
  if ( current_user_can( 'manage_options' ) ) {
   extract( $args );
   $fb_output = null;
   if ( $container ) {
    $fb_output = '<' . $container;
    if ( $container_id )
     $fb_output .= ' id="' . $container_id . '"';
    if ( $container_class )
     $fb_output .= ' class="' . $container_class . '"';
    $fb_output .= '>';
   }
   $fb_output .= '<ul';
   if ( $menu_id )
    $fb_output .= ' id="' . $menu_id . '"';
   if ( $menu_class )
    $fb_output .= ' class="' . $menu_class . '"';
   $fb_output .= '>';
   $fb_output .= '<li><a href="' . admin_url( 'nav-menus.php' ) . '">Add a menu</a></li>';
   $fb_output .= '</ul>';
   if ( $container )
    $fb_output .= '</' . $container . '>';
   echo $fb_output;
  }
 }
}
and add this line in your theme functions.php
require_once('wp_bootstrap_navwalker.php');