Question: I'm making web-app using react-native. But stuck on font-size for different screen-size.

How can I change the different font-size for different screen resolution?

Answer: 
Your example code introduces five identifiers, namely `week`, `mon`, `tue`, `day`, and `a`. Each of the first four could reasonably be called "an enum" in a paraphrase of a misunderstood quote. Without a quote, it is difficult to tell to which of these the book refers. However, it is possible to list which identifiers can and cannot have their address taken.

* `&week` -- **not** legal to take the address of a type
* `&mon` -- **not** legal to take the address of an enumerator
* `&tue` -- **not** legal to take the address of an enumerator
* `&day` -- **legal** to take the address of a variable

The upshot of the quote is probably that enumerators – the values of an enumeration type, e.g. `mon`, `tue` – are not variables, so their address cannot be taken. Syntactically, the valid uses of `mon` closely match the valid uses of the numeric literal `1`. In particular, just like one cannot take the address of a numeric literal (no `&1` allowed), one cannot take the address of an enumerator (no `&mon` allowed).
In that phrase - 'the address of an enum', an 'enum' means a symbol you defined as a constant. In your code, `mon` and `tue` are enums. Experiment yourself like this.

```
void *p = &mon;

```