skip certain categories in the loop output

[ahmedkaludi]

Here is a use case of a user: Exclude category from the loop.

I want to loop through all post but skip certain categories in the output. For example:

//If category is "category a" then skip this iteration

or output a specific categroy:

//If category is not "category b" then skip this iteration

REF: https://secure.helpscout.net/conversation/436881172/16403/