# What does the pair of vertical lines in empirical entropy formula mean?

I am learning from this post.

$$\alpha$$ is the ratio of the first subset,

$$\alpha=\frac{\left|D_{1}\right|}{\left|D\right|}$$

according to the context and code of the post, $$\left|D\right|$$ means the number of samples?

• What are the pair of vertical lines called?

Is it the L1 norm symbol? It does not seem to be the absolute value symbol.

$$D$$ is a set which is further divided into sets $$D_{1}$$ and $$D_{2}$$. In this context the vertical lines represent the cardinality of the set i.e. the number of elements in the set. Hence, $$\alpha$$ represents the ratio of number of samples of $$D_{1}$$ to $$D$$.