Can I populate required field in before trigger dynamically while uploading from dataloader and not giving it in file(no column for that field)?

For e.g.
I have master relationship(Field C) in Object A to Object B. I want to insert records of Object A via dataloader but I don't have column for Field C in my upload file. I am populating Field C(Master Relationship on Object B) in before trigger. It is showing me Required field Missing "Field C".How can I overcome this?

I want to change component contents(using rendered for a outpanel) based on the value of another component's input field.I am using using both the component in one VF page.

Provide me the solution so that I can rerender(ajax) the component's content based on the value of another component's input field. 

I want to change component contents(using rendered for a outpanel) based on the value of another component's input field.I am using using both the component in one VF page.

Provide me the solution so that I can rerender(ajax) the component's content based on the value of another component's input field.