CS471 Program Language Structure I
Fall 2000

List processing

Using the list-processing primitives car, cdr, cons and null? (Scheme) or hd, tl, :: and = (ML), write the following functions:

1. filter, that takes a function that has one parameter and a simple list of values as its two arguments, and returns a list consisting only of those elements that return true when the passed function is applied to them. e.g. if there exists a function odd that return true if its argument is an odd number, then:

(filter odd (list 1 2 3 4 5)) or filter odd [1,2,3,4,5]

returns the list (1 3 5) or [1,3,5].

2. accumulate, that takes a function that has two parameters , an initial value and a simple list of values as its three arguments, and returns a single value that is the result of applying the function to the elements of the list, accumulating the value as it goes through the list. e.g.

(accumulate + 0 (list 1 2 3 4 5)) or

accumulate + 0 [1,2,3,4,5]

returns the value 15.

· Points will not be taken off for bad syntax, but the recursive strategy must be clear from the answer you give. It would help if you explain the strategy, rather than simply giving the answer.

Due Date

December 6th. in class.