Writing "$B$ is the number of elements in set $A$ that are less than $0$" in symbols

Question

Let

$$A = \{0, 0, -1, 0, -1, 1, 0\}$$

and let $B$ be the number of elements in $A$ that are less than $0$. Is it correct if I write it like this?

$$B = |\{A\}| < 0$$

Thank you very much. :)

Answer 1

No, it is not.

You could write:

$$B=\left\vert\{\;x\in A \;|\; x < 0\;\}\right\vert$$