Why is this plugin not working?

Question

I am currently learning WordPress Plugin Development. I was trying to insert data into a custom table by means of a plugin. Such that, when an action hook is fired, the data is inserted. It is a pretty simple code here but not getting executed although on debugging I found that the commented echo statement within the function does get executed.

Here is the code -

<?php
/*
Plugin Name: What the wpdb
Description: Making use of wpdb
Version: 1.0
Author: Navin Nagpal
License: GPLv2
*/
add_action('wp_head','nn_do_dbstuff');

function nn_do_dbstuff(){
global $wpdb;
//echo 'Text';
$values = array(
    'column1'=>'Navin',
    'column2'=>22);
$formats_values = array('%s','%d');
$wpdb->insert($wpdb->custom,$values,$formats_values);
}
?>

My table is named wp_custom and it has three fields including the 'id' field which is the primary key

Answer 1

$wpdb->custom should be "{$wpdb->prefix}custom"

$wpdb->insert( $table, $data, $format ); take three params where 1st is table name. you should avoid wp as its not necessary that prefix will always be wp_ instead use $wpdb->prefix to get table prefix, "{$wpdb->prefix}custom" will output wp_custom (given prefix is wp for your site)