how to use do_shortcode in wordpress code example

Example 1: wordpress do shortcode

<?php echo do_shortcode('[name_of_shortcode parameters=""]'); ?>

Example 2: wordpress shortcode

function wp_demo_shortcode() { 

//Turn on output buffering
ob_start();
$code = 'Hello World';
ob_get_clean();

 // Output needs to be return
return $code;
} 

// register shortcode
add_shortcode('helloworld', 'wp_demo_shortcode');