Using flashcards to learn mathematics, computer programming, physics, or anything requiring more creative or analytical thought is not so useful. For these subjects the number of raw facts required is usually smaller, and the thought processes, concepts and problem solving ability are much more important.
My programming flash card usually consists of evaluating pieces of code like:
Question -->
Evaluate the following:
puts "Hello world"
Answer --> Hello world
I cannot memorize the solution as if I memorize the answer to multiplication problems, which is just straight rote memorization. I memorize the process for code evaluation instead. In any case, I do not think anki and similar spaced repetition flash card software are in any way a replacement for actually just writing code and solving lot of different problems. They are always complementary to learning the materials in other ways.