### Exercises

#### Exercise 2.17

Define a function last-pair/1 that returns the list that contains only the last element of a given (nonempty) list:

> (last-pair (list 23 72 149 34))
(34)


#### Exercise 2.18

Define a function reverse/1 that takes a list as argument and returns a list of the same elements in reverse order:

> (reverse (list 1 4 9 16 25))
(25 16 9 4 1)


#### Exercise 2.19

Consider the change-counting program of the section Tree Recursion. It would be nice to be able to easily change the currency used by the program, so that we could compute the number of ways to change a British pound, for example. As the program is written, the knowledge of the currency is distributed partly into the function first-denomination/1 and partly into the function count-change/1 (which knows that there are five kinds of U.S. coins). It would be nicer to be able to supply a list of coins to be used for making change.

We want to rewrite the function cc/2 so that its second argument is a list of the values of the coins to use rather than an integer specifying which coins to use. We could then have lists that defined each kind of currency:

> (set us-coins (list 50 25 10 5 1))
(50 25 10 5 1)
> (set uk-coins (list 100 50 20 10 5 2 1 0.5))
(100 50 20 10 5 2 1 0.5)


We could then call cc/2 as follows:

> (cc 100 us-coins)
292


To do this will require changing the program cc/2 somewhat. It will still have the same form, but it will access its second argument differently, as follows:

(defun cc
((0 _) 1)
((amount coin-values)
(if (or (< amount 0) (no-more? coin-values))
0
(+ (cc amount
(except-first-denomination coin-values))
(cc (- amount
(first-denomination coin-values))
coin-values)))))


Define the functions first-denomination/1, except-first-denomination/1, and no-more?/1 in terms of primitive operations on list structures. Does the order of the list coin-values affect the answer produced by cc/2? Why or why not?

#### Exercise 2.20

Removed from the LFE Edition.1

1. In the Scheme 2nd edition, Exercise 2.20 covers a feature in Scheme that is not present in LFE. The analog would require creating a macro with a single parameter that is the body of the code; as we haven't covered quoting or macros yet, this is not a suitable exercise at this point.