22a. More Recursion

1. sum-cubes: number(n) -> number. Finds the sum of the first n perfect cubes.

2. vblock: number(n) image(img) -> image. Makes a vertical stack of n copies of img.

3. hblock: number(n) image(img) -> image. Make a horizontal stack of n copies of img.

4. grid: number(rows) number(columns) image -> image. Make a grid that is columns wide and rows tall of the image repeated.

5. add-sqrt: number(start) number(end) -> number. Add up (sqrt n) for every integer n with start <= n <= end If no numbers fit this description, the sum should be zero.

   Example: when start=4 and end=5, the answer is (+ (sqrt 4) (sqrt 5)), which is about 4.236.

    (check-expect (add-sqrt 4 3) 0)
    (check-expect (add-sqrt 4 4) 2)
    (check-within (add-sqrt 4 5) 4.23 0.01)
    (check-within (add-sqrt 4 8) 12.15 0.01)
   

6. lots-of-hyphens: string(word) -> string. Insert a hyphen after every letter in the word.

    (lots-of-hyphens "grape") ==> "g-r-a-p-e-"
   

   Extra: get "g-r-a-p-e" instead.

7. ten-circle: number(start) number(end) -> image. Produces concentric circles starting at radius start and drawing every 10 units until the radius is at least end. Assume start < end.