## CSci 1301: Problem Set 2

#### Due: Wednesday, September 17 at 11:59pm by e-mail

Please test all your programs carefully and include all the test cases with your program. You should have at least 3 tests for each function, except the ones that produce an image or a scene.

### Problem 1 (6 points)

Recall the function `first-letter-A?` that we wrote in class. Here is our code:

``````;; (first-letter-A? str) -> boolean
;; str: string
;; consumes a string, returns  true
;; if this string starts with a capital A, otherwise returns false
(define (first-letter-A? str)
(string=? (substring str 0 1) "A"))

(check-expect (first-letter-A? "AbC") true)
(check-expect (first-letter-A? "abC") false)
(check-expect (first-letter-A? "bcde") false)
;(check-expect (first-letter-A? "") false)
(check-expect (first-letter-A? "aaAAaaaa") false)
``````
Working from this example, perform the following steps:
1. Add the case to handle the empty string in the function (it must return `false` for it, as the commented out test case indicates). You can use `cond` or `if` or (better!) combine your conditions using boolean operations, such as `and, or, not` (either approach will get full credit if works correctly).
2. Write a function `first-letter-a-or-A?` that returns `true` if this string starts with "A" or "a" and `false` otherwise. It must return `false` for an empty string. Make sure to write test cases and the description (contract) for your function.

### Problem 2 (2 points)

Modify the version 5 of the landing rocket example (figure 4 in Prologue) so that when the width of the scene is changed, the rocket is placed in the middle of the new scene.

### Problem 3

Exercises 24 (5 points) and 25 (5 points) in Section 2.2.

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