CSC151.01 2015S, Review Session Week 11: Vectors and Beyond
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_Overview_

* You ask questions, I attempt to answer.
* We talk about the forthcoming quiz.

Your Questions
--------------

_Can we make the turtle choose its color based on its position?_

> Um ...

        (turtle-set-color! turtle (irgb-complement (image-get-pixel (turtle-x turtle) (turtle-y turtle))))

> Here's some code that almost works

        (define turtle-oppose!
          (lambda (turtle)
            (turtle-set-color! 
             turtle
             (irgb-complement
              (image-get-pixel (turtle-world turtle)
                               (turtle-x turtle)
                               (turtle-y turtle))))))

_How would one tilt the area of an ellipse?_

> Wait until tonight when Sam distributes the code he wrote this
  afternoon because he was predicting this question.

_Can I select something more complex using a turtle?_

> image-select-polygon (in the tips) shows a technique, including
  using a turtle.

> Once you've selected, you can also use gimp-selection-grow and
  gimp-selection-feather, both in the PDB list.

_Can I draw offset nested circles?_

> Here's a start.

        (define center-circle
          (lambda (image x y radius)
            (image-select-ellipse! image REPLACE
                                   (- x radius)
                                   (- y radius)
                                   (+ radius radius)
                                   (+ radius radius))
            (image-stroke! image)
            (image-select-nothing! image)))
        
        
        (define off-center-circle
          (lambda (image x y radius percent angle)
            (let ([hyp (* percent radius)])
              (center-circle image
                             (+ x (* hyp (sin angle)))
                             (+ y (* hyp (cos angle)))
                             radius))))

About the Quiz
--------------

Main topics:

* Vectors.
* Randomness.

Likely quiz questions:

* Interpret _this code_.
* Finish writing _this code_ that is supposed to accomplish _this task_.
* Find the bugs in _this code_ that is supposed to accomplish _this task_.

Sample

        (define vector-sum
          (lambda (vec)
            (let kernel ([sum-so-far 0]
                         [index 0])
              (if (< index (vector-length vec))
                  (kernel (+ sum-so-far (vector-ref vec index))
                          (+ index 1))
                  sum-so-far))))

* I might leave out parts
* I might swap or change the inequality
* I might swap consequent and alternate
* I might misspell `sum-so-var`.