Precision is defined as:

>p = true positives / (true positives + false positives)

What is the value of precision if (true positives + false positives) = 0? Is it just undefined?

Same question for recall:

>r = true positives / (true positives + false negatives)

In this case, what is the value of recall if (true positives + false negatives) = 0?

P.S. This question is very similar to the question http://stats.stackexchange.com/questions/1773/what-are-correct-values-for-precision-and-recall-in-edge-cases.