Can not understand this line of a popular deep Q learning program

Question

https://github.com/yenchenlin/DeepLearningFlappyBird/blob/master/deep_q_network.py#L82

I have spend a lot of time to understand it.

Why use tf.multiply?

I can not find the math that support this multiply operation.

Answer 1

Every action has a Q_value.

And the action input a is one-hot.

So this line is to choose the 'hot' Q_value.