How do I….list categories an author has wrote in on author.php

<?php
//get all posts for an author, then collect all categories
//for those posts, then display those categories
$cat_array = array();
$args=array(
 'author' => 4,
 'showposts'=>-1,
 'caller_get_posts'=>1
);
$author_posts = get_posts($args);
if( $author_posts ) {
  foreach ($author_posts as $author_post ) {
    foreach(get_the_category($author_post->ID) as $category) {
      $cat_array[$category->term_id] =  $category->term_id;
    }
  }
}
$cat_ids = implode(',', $cat_array);
wp_list_categories('include='.$cat_ids.'&title_li=Author Categories');
?>

There’s no need for any of that, what do you currently have in your author.php file? (aka the author template).

Template files (index.php, archive.php etc..) already know what to do when query vars exist, the only time they can’t intercept and use those queried variables is when you over-ride the query, ie. you’re setting the query_posts line.

If you can show me the code being used in the author template file, i can suggest changes or additions to allow the file to do what it should actually already be doing.

A query like..

query_posts('author=1&cat=1');

should provide a list of posts in that category and by that author only, there should be nothing else required other then to ensure you are querying that page in a fashion that places the correct query vars into the query.

Ok i think i misunderstood before.

However, you could use this method, since it’s a little faster..

<?php
$categories = $wpdb->get_results("
	SELECT DISTINCT(terms.term_id) as ID, terms.name, terms.slug
	FROM $wpdb->posts as posts
	LEFT JOIN $wpdb->term_relationships as relationships ON posts.ID = relationships.object_ID
	LEFT JOIN $wpdb->term_taxonomy as tax ON relationships.term_taxonomy_id = tax.term_taxonomy_id
	LEFT JOIN $wpdb->terms as terms ON tax.term_id = terms.term_id
	WHERE 1=1 AND (
		posts.post_status = 'publish' AND
		posts.post_author = '4' AND
		tax.taxonomy = 'category' )
	ORDER BY terms.name ASC
");
?>
<ul>
	<?php foreach($categories as $category) : ?>
	<li>
		<a href="<?php echo get_category_link( $category->ID ); ?>" title="<?php echo $category->name ?>"><?php echo $category->name ?></a>
	</li>
	<?php endforeach; ?>
</ul>

If it’s a small site and you’ve not got thousands of posts, you’ll likely not notice a difference though… but for the sake of following through … 😉

If you want to find out the which author has been queried then it’s a simple case of..

$author = get_query_var('author'); // Queried by ID

or

$author = get_query_var('author_name'); // Queried by name

Add the variable into the other code, and you are essentially querying for categories for the current author being queried..

This line..

posts.post_author = '4'

..would become..

posts.post_author = '$author'