Programmatically add Magento products to categories

Question

I am using Magento 1.4.0.1. I have over 21000 simple products, each entered into a single category. There are hundreds of categories in my site. Some products belong in mult

Answer 1

I just want to add that you can remove and add with getSingleton category API:

To Remove product from category:

Mage::getSingleton('catalog/category_api')->removeProduct($category->getId(),$p‌​roduct->getId());

To Add Product to Category:

Mage::getSingleton('catalog/category_api')->assignProduct($category->getId(),$p‌​roduct->getId());

This will not overwrite any categories the product is already in