8.12
Lecture 25: Accumulators
This assignment is due on Tuesday, April 9 at 11:59pm. Submit it using Handin as assignment lecture25.
Exercise 1.
Finish designing the following function.
Do not use the built-in function reverse.
; rev : [ListOf X] -> [ListOf X] ; reverses the order of elements in the given list (check-expect (rev empty) empty) (check-expect (rev (list "man" "bites" "dog")) (list "dog" "bites" "man")) (check-expect (rev (list "bites" "dog")) (list "dog" "bites")) (check-expect (rev (list "dog")) (list "dog"))
Hint: fill in the list-processing template using the wishlist method.
Exercise 2.
What’s the cumulative distance to Andrew station?
Define it as a constant named ashmont-andrew.
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Exercise 3.
Finish designing this function using the accumulator:
; sum : [ListOf Number] -> Number ; returns the sum of the given numbers (define (sum lon) (sum/a lon 0)) (check-expect (sum empty) 0) (check-expect (sum (list 10 20 50)) 80) ; sum/a : [ListOf Number] Number -> Number ; returns the sum of the given numbers ; *Accumulator*: total is the sum of numbers seen so far (define (sum/a lon total) ...)
Hint: Write sequences of examples and follow the template for processing a list.
But if your recursive call looks like (sum/a (rest lon) total),
then it is incorrect because it does not update the accumulator
according to the accumulator statement.
Optional: Read Chapters 31–32 and 34 of the textbook.