Your task
First part
Write a function called tally-item as follows. The function should take two parameters, prices and line-item.
line-item is two-element vector representing an item on an invoice:
- the first member is a keyword value representing an item
- the second member is an integer quantity
prices is a map of items (keyword values) to the price for a single quantity of that item.
The function should return the cost of the line item, meaning the price of the specified item multiplied by the quantity.
You can test your tally-item function using the following example prices map:
(def fruit-prices
{:apple 0.75,
:orange 0.80,
:pomegranate 2.50,
:banana 0.50,
:plum 1.20,
:peach 1.00,
:persimmon 1.75,
:lime 0.60})Here is part of a REPL session showing invocations of tally-item using the fruit-prices map. Try these and make sure your function produces the same results:
user=> (tally-item fruit-prices [:persimmon 2])
3.5
user=> (tally-item fruit-prices [:orange 3])
2.4000000000000004
user=> (tally-item fruit-prices [:peach 1])
1.0
user=> (tally-item fruit-prices [:banana 0])
0.0
(Note that the strange result for three oranges is due to floating-point imprecision. In production software, it’s a bad idea to use floating point values to represent exact quantities such as money.)
Second part
An “invoice” is a sequence of line items. For example:
(def yummy-fruit
[[:persimmon 2]
[:orange 3]
[:peach 1]
[:plum 10]
[:pomegranate 5]])It would be nice to have a way to apply your tally-item function to an invoice to compute a total price for all of the line items.
Here’s how to do it! Well, sort of.
The built-in map function applies a one-parameter function to each item in a sequence, returning a sequence with the results of each application. (Note that there are more complicated ways to use map, but we won’t get into that right now.) We have a bit of a problem, though: tally-item expects two parameters, a prices map and a line item vector.
Not to worry: we can create an anonymous function that takes a single parameter (a line item vector) and applies tally-item to it, passing whichever prices map we want to use. (You encountered anonymous functions in the Intro to Functions problem on 4clojure.)
Try this (which assumes you’ve evaluated the yummy-fruit invoice shown above):
(map (fn [line-item] (tally-item fruit-prices line-item))
yummy-fruit)What happens when you do this? (Ok, it’s not really computing a total price, but perhaps it’s doing something that might be useful for computing the total price?)
Third part
Come up with a way to use the map function (as shown above) to compute a single price for an invoice. Hint: research the built-in reduce function.
When you get this working, show me what you did.
Solution
When you are done, you can compare your solution to mine:
