Set a default Selected radio button with gform_pre_render

<?php
// http://www.gravityhelp.com/forums/topic/explicitly-set-default-radio-choice-using-hook
// change 29 here to your form ID
add_filter('gform_pre_render_29', 'set_default_dynamically');
function set_default_dynamically($form) {
    foreach( $form['fields'] as &$field ){
        // change 2 here to your field ID
        if( $field['id'] == 2 ){
            foreach($field['choices'] as &$choice) {
                // I want to set the radio button choice with the value of 31 to selected
                if($choice['value'] == '31'){
                    $choice['isSelected'] = true;
                }
                // unset any default which was set in the form builder
                else {
                    $choice['isSelected'] = '';
                }
           }
        }
    }
    // return the modified $form object
    return $form;
}