In data compression this equation describes the 'average length' of the code. Can someone please explain?

Question

$$\bar{l}(\mathcal{P},\mathcal{L}) = \sum_{j=1}^n p_j l_j$$

I have just started studying this module and it describes the equation as the the average length of the code.

Can you please break down what each part of the equation means?

Answer 1

$l_j$ is the length (say, in bits) of code word $j$. $p_j$ is the probability that word $j$ occurs. There is nothing magic about data compression here, it is the usual way of taking a weighted average.