Hi,

There are a number of problems with the code you've written:
1) void loop() should be void draw().  The loop function controls whether the sketch is repeatedly redrawn or not.  What you've done is redefine it in a way that will never be called.
2) x < width as a statement on it's own won't do anything.  It needs to be inside an if(something == true) {do this} block.
3) You've created a local copy of the float a inside what should be your draw function so that the change you make to a isn't stored.
4) The output of sin() will never go beyond -1 and +1 so the way that you're using map() doesn't entirely make sense
5) Please try to properly indent your code, it makes it much easier for you to read and for others to understand when they're trying to help you.

I've had a go at getting closer to what I think you were intending to do:

1. float x = 0;
2. float a = 0;
3. 
   
4. void setup() {
5.   size(500, 300);
6.   stroke(random(255), random(255), random(255));
7. }
8. 
   
9. void draw() {
10.   if (x < width) {
11.     a = map(sin(a), -2, 2, 50, 200);
12.   }
13. 
    
14.   point(x, a);
15.   x = x + 1; // increase too fast
16.   a = a + 0.5;
17. }