Why can't I define the constant A0 (some pin) to something?

Question

What I'm doing wrong or why is it not working:

#define Uin A0;

void setup() {
}

void loop() {
  analogRead(Uin);
}

Results in a Error (expected ')' before ; token.

I thought #define just exists as a place holder.

Answer 1

Because you have a ; at the end of your #define.

#define Uin A0;
analogRead(Uin);

becomes:

analogRead(A0;);

The Arduino website mentions this in their reference. For more in-depth detail about the C pre-processor you can read the (rather large) manual here.