Brackets

So for this week, I had some problems with the tutorial. Again, let me reiterate, I'm no pro at computer science, but I'm trying!

Specifically, I had problems with question 8.23 in our 'textbook' Picturing Programs. The question was to
make an animation of a picture on a white background (200x200) that would always have the y coordinate of 50 with a changing x variable.

So, I had a little bit of an idea of how to get started.

First I would need to define a function that puts an image on top of another image at a specific coordinate. And given the hint from the tutorial handout, this function would be 'place-image'.

My first attempt on DrRacket was this:

(define picture x-coordinate (place-image pic:hacker
                        x-coordinate 50
                        (rectangle 200 200 "solid" "white")))
  
(big-bang 10
           (check-with number?)
           (on-draw picture x-coordinate)

...And well it failed. The error message was: define: expected only one expression after the variable name picture, but found 1 extra part

You know, I don't know if it's just me but sometimes I wish the computer can talk more explicitly... 

Anyways, I think the problem is that I need to let DrRacket know that I want the x-coordinate to represent, well, the x-coordinate. I went about many different ways and failed. So on Wednesday I went to ask my TA for help.

I didn't bring my attempted solution to the tutorial...and when my TA helped me, the animation worked... I didn't understand why I couldn't get it with my solution when it was essentially the same idea... until I got home and compared the TA's solution to mine and I realized I was missing a bracket. Really, a bracket?

So here's the TA's solution (with just the added brackets...) 

 (define (picture x-coordinate) (place-image pic:hacker
                        x-coordinate 50
                        (rectangle 200 200 "solid" "white")))

 (big-bang 10
           (check-with number?)
           (on-draw picture)
           (on-tick add1 1))

Well  of course, the lesson isn't just that I need to be more careful with brackets but that this bigbang function is quite versatile and that defining a function before usiong bigbang can add much more dimension to the animation.