orders to ingredients

Now we can write a function that takes a list of orders and gives you the whole list of ingredients you'll need to complete all of them.

Building a total shopping list from a list of orders

When we call get-morning-orders, we have a list of orders. We will want to combine them all to get one big shopping list so we can gather all of the ingredients we need in one go.

There's a function that's made for taking a list of things and building them up into a single value using a function. It's called reduce.

Let's assume we want to add some numbers. We can use reduce to do that. We pass +, for adding the numbers. And we start with 0. reduce will loop through the list and add each number to the previous result, until all items are added. Then the value is returned. We can use reduce to build up a value using a function, an initial value, and a list of values.

(reduce + 0 [1 4 5 3 90 23 2 32]) ;=> 157

Exercise 9

Write a function orders->ingredients that builds a total ingredients list from a list of orders. You should use for and reduce. Instead of using + with reduce, you can use add-ingredients.

To navigate to this point in the introduction-to-clojure repo:

$CMD git checkout -f 2.17

Watch me

Let's build this up one step at a time. First the signature:

(defn orders->ingredients [orders]

We can use a for to transform the list of orders into a list of shopping lists.

(defn orders->ingredients [orders]
  (for [order orders]
    (order->ingredients order)))

Then we can reduce all of the shopping lists into a single shopping list. We will start with an empty map as an empty shopping list and add each shopping list to it, in sequence.

(defn orders->ingredients [orders]
  (reduce add-ingredients {}
    (for [order orders]
      (order->ingredients order))))

So, there it is.

To navigate to this point in the introduction-to-clojure repo:

$CMD git checkout -f 2.18