What is the best way to represent N/A if a value is not available?

[thomasd3]

I'm using a binary classifier.

My feature list has a list of columns representing various states in my model. The range of values is -1 to +1. But, in some cases, some states are simply not present, in some rows.

For example:

0, 1, 1, 1, 0, 0, 1
1, 0, 0, 1, 1, 0, 0
1, X, 1, 1, 1, 0, 1
1, 0, 0, 0, 0, 1, 1

Notice the X? which really means, in my model, that there is an absence of data there. The model is supposed to classify a specific situation based on the states of various systems. And sometimes that data is not present, shouldn't be interpolated from neighbors either, it really means that this signal does not exist at that time.

What is the best way to represent this?