How to deal with feature vector of variable length?

Question

Say you're trying to classify houses based on certain features:

• Total area
• Number of rooms
• Garage area

But not all houses have garages. But when they do, their total area makes for a very discriminating feature. What's a good approach to leverage the information contained in this feature?

Answer 1

You could incorporate a zero/one dummy variable indicating whether there is a garage, as well as the cross-product of the garage area with the dummy (for houses with no garage, set the area to zero).