how to set cron job for reindex

How to setup cron job for clear cache and re-indexing in Magento. i don't know how to set the cron for re-index. but i saw some where every day cron runs defaultly in magento. still i am facing re indexing issues in my site. i need to clear cache also.On Magento website here

they said that logcleaning and reindexing is commented out in the code so in which file I can un-comment to setup cron job fo log cleaning and reindexing?

thanks, murali.