Elixir Bootcamp - Enumerable Interface
Introduction
Enum is a very useful module that provides a set of algorithms for working with enumerables. It offers:
-
sorting (
sort/2,sort_by/2), -
filtering (
filter/2), -
grouping (
group_by/3), -
counting (
count/2) -
searching (
find/3), -
finding min/max values (
min/3,max/3), -
reducing (
reduce/3,reduce_while/3),
And much more! Refer to the Enum module documentation for a full list.
Enumerable
In general, an enumerable is any data that can be iterated over, a collection. In Elixir, an enumerable is any data type that implements the Enumerable protocol. Those are:
Don’t worry if you don’t know them all yet.
Anyone can implement the Enumerable protocol for their own custom data structure.
Map
Enum.map/2 allows you to replace every element in an enumerable with another element. The second argument to Enum.map/2 is a function that accepts the original element and returns its replacement.
Enum.map([1, 2, 3], fn x -> x * 2 end)
# => [2, 4, 6]
Enum.map([a: 1, b: 2], fn {k, v} -> {k, -v} end)
# => [a: -1, b: -2]Reduce
Enum.reduce/2 allows you to reduce the whole enumerable to a single value. To achieve this, a special variable called the accumulator is used. The accumulator carries the intermediate state of the reduction between iterations. This makes it one of the most powerful functions for enumerables. Many other specialized functions could be replaced by the more general reduce.
Finding the maximum value:
Enum.max([4, 20, 31, 9, 2])
# => 31
Enum.reduce([4, 20, 31, 9, 2], nil, fn x, acc ->
cond do
acc == nil -> x
x > acc -> x
x <= acc -> acc
end
end)
# => 31Working with maps
When using maps with Enum functions, the map gets automatically converted to a list of 2 {key, value} tuples. To transform it back to a map, use Enum.into/2.
%{a: 1, b: 2, e: 3}
|> Enum.map(fn {key, value} -> {key, value * 10} end)
|> Enum.into(%{})Exercise - Boutique Inventory
You are running an online fashion boutique. Your big annual sale is coming up, so you need to take stock of your inventory to make sure you’re ready.
A single item in the inventory is represented by a map, and the whole inventory is a list of such maps.
%{
name: "White Shirt",
price: 40,
quantity_by_size: %{s: 3, m: 7, l: 8, xl: 4}
}1. Sort items by price
Implement the sort_by_price/1 function. It should take the inventory and return it sorted by item price, ascending.
BoutiqueInventory.sort_by_price([
%{price: 65, name: "Maxi Brown Dress", quantity_by_size: %{}},
%{price: 50, name: "Red Short Skirt", quantity_by_size: %{}},
%{price: 50, name: "Black Short Skirt", quantity_by_size: %{}},
%{price: 20, name: "Bamboo Socks Cats", quantity_by_size: %{}}
])
# => [
# %{price: 20, name: "Bamboo Socks Cats", quantity_by_size: %{}},
# %{price: 50, name: "Red Short Skirt", quantity_by_size: %{}},
# %{price: 50, name: "Black Short Skirt", quantity_by_size: %{}},
# %{price: 65, name: "Maxi Brown Dress", price: 65, quantity_by_size: %{}}
# ]2. Find all items with missing prices
After sorting your inventory by price, you noticed that you must have made a mistake when you were taking stock and forgot to fill out prices for a few items.
Implement the with_missing_price/1 function. It should take the inventory and return a list of items that do not have prices.
BoutiqueInventory.with_missing_price([
%{price: 40, name: "Black T-shirt", quantity_by_size: %{}},
%{price: nil, name: "Denim Pants", quantity_by_size: %{}},
%{price: nil, name: "Denim Skirt", quantity_by_size: %{}},
%{price: 40, name: "Orange T-shirt", quantity_by_size: %{}}
])
# => [
# %{price: nil, name: "Denim Pants", quantity_by_size: %{}},
# %{price: nil, name: "Denim Skirt", quantity_by_size: %{}}
# ]3. Update item names
You noticed that some item names have a word that you don’t like to use anymore. Now you need to update all the item names with that word.
Implement the update_names/3 function. It should take the inventory, the old word that you want to remove, and a new word that you want to use instead. It should return a list of items with updated names.
BoutiqueInventory.update_names(
[
%{price: 40, name: "Black T-shirt", quantity_by_size: %{}},
%{price: 70, name: "Denim Pants", quantity_by_size: %{}},
%{price: 65, name: "Denim Skirt", quantity_by_size: %{}},
%{price: 40, name: "Orange T-shirt", quantity_by_size: %{}}
],
"T-shirt",
"Tee"
)
# => [
# %{price: 40, name: "Black Tee", quantity_by_size: %{}},
# %{price: 70, name: "Denim Pants", quantity_by_size: %{}},
# %{price: 65, name: "Denim Skirt", quantity_by_size: %{}},
# %{price: 40, name: "Orange Tee", quantity_by_size: %{}}
# ]4. Increment the item’s quantity
Some items were selling especially well, so you ordered more, in all sizes.
Implement the increase_quantity/2 function. It should take a single item and a number n, and return that item with the quantity for each size increased by n.
BoutiqueInventory.increase_quantity(
%{
name: "Polka Dot Skirt",
price: 68,
quantity_by_size: %{s: 3, m: 5, l: 3, xl: 4}
},
6
)
# => %{
# name: "Polka Dot Skirt",
# price: 68,
# quantity_by_size: %{l: 9, m: 11, s: 9, xl: 10}
# }5. Calculate the item’s total quantity
To know how much space you need in your storage, you need to know how many of each item you have in total.
Implement the total_quantity/1 function. It should take a single item and return how many pieces you have in total, in any size.
BoutiqueInventory.total_quantity(%{
name: "Red Shirt",
price: 62,
quantity_by_size: %{s: 3, m: 6, l: 5, xl: 2}
})
# => 16Implementation
defmodule BoutiqueInventory do
def sort_by_price(inventory) do
# Please implement the sort_by_price/1 function
end
def with_missing_price(inventory) do
# Please implement the with_missing_price/1 function
end
def update_names(inventory, old_word, new_word) do
# Please implement the update_names/3 function
end
def increase_quantity(item, count) do
# Please implement the increase_quantity/2 function
end
def total_quantity(item) do
# Please implement the total_quantity/1 function
end
endTests
ExUnit.start(autorun: false)
defmodule BoutiqueInventoryTest do
use ExUnit.Case
describe "sort_by_price/1" do
@tag task_id: 1
test "works for an empty inventory" do
assert BoutiqueInventory.sort_by_price([]) == []
end
@tag task_id: 1
test "sorts items by price" do
assert BoutiqueInventory.sort_by_price([
%{price: 65, name: "Maxi Yellow Summer Dress", quantity_by_size: %{}},
%{price: 60, name: "Cream Linen Pants", quantity_by_size: %{}},
%{price: 33, name: "Straw Hat", quantity_by_size: %{}}
]) == [
%{price: 33, name: "Straw Hat", quantity_by_size: %{}},
%{price: 60, name: "Cream Linen Pants", quantity_by_size: %{}},
%{price: 65, name: "Maxi Yellow Summer Dress", quantity_by_size: %{}}
]
end
@tag task_id: 1
test "the order of items of equal price is preserved" do
assert BoutiqueInventory.sort_by_price([
%{price: 65, name: "Maxi Yellow Summer Dress", quantity_by_size: %{}},
%{price: 60, name: "Cream Linen Pants", quantity_by_size: %{}},
%{price: 33, name: "Straw Hat", quantity_by_size: %{}},
%{price: 60, name: "Brown Linen Pants", quantity_by_size: %{}}
]) == [
%{price: 33, name: "Straw Hat", quantity_by_size: %{}},
%{price: 60, name: "Cream Linen Pants", quantity_by_size: %{}},
%{price: 60, name: "Brown Linen Pants", quantity_by_size: %{}},
%{price: 65, name: "Maxi Yellow Summer Dress", quantity_by_size: %{}}
]
end
end
describe "with_missing_price/1" do
@tag task_id: 2
test "works for an empty inventory" do
assert BoutiqueInventory.with_missing_price([]) == []
end
@tag task_id: 2
test "filters out items that do have a price" do
assert BoutiqueInventory.with_missing_price([
%{name: "Red Flowery Top", price: 50, quantity_by_size: %{}},
%{name: "Purple Flowery Top", price: nil, quantity_by_size: %{}},
%{name: "Bamboo Socks Avocado", price: 10, quantity_by_size: %{}},
%{name: "Bamboo Socks Palm Trees", price: 10, quantity_by_size: %{}},
%{name: "Bamboo Socks Kittens", price: nil, quantity_by_size: %{}}
]) == [
%{name: "Purple Flowery Top", price: nil, quantity_by_size: %{}},
%{name: "Bamboo Socks Kittens", price: nil, quantity_by_size: %{}}
]
end
end
describe "update_names/3" do
@tag task_id: 3
test "works for an empty inventory" do
assert BoutiqueInventory.update_names([], "T-Shirt", "Tee") == []
end
@tag task_id: 3
test "replaces the word in all the names" do
assert BoutiqueInventory.update_names(
[
%{name: "Bambo Socks Avocado", price: 10, quantity_by_size: %{}},
%{name: "3x Bambo Socks Palm Trees", price: 26, quantity_by_size: %{}},
%{name: "Red Sequin Top", price: 87, quantity_by_size: %{}}
],
"Bambo",
"Bamboo"
) == [
%{name: "Bamboo Socks Avocado", price: 10, quantity_by_size: %{}},
%{name: "3x Bamboo Socks Palm Trees", price: 26, quantity_by_size: %{}},
%{name: "Red Sequin Top", price: 87, quantity_by_size: %{}}
]
end
@tag task_id: 3
test "replaces all the instances of the word within one name" do
assert BoutiqueInventory.update_names(
[
%{name: "GO! GO! GO! Tee", price: 8, quantity_by_size: %{}}
],
"GO!",
"Go!"
) == [
%{name: "Go! Go! Go! Tee", price: 8, quantity_by_size: %{}}
]
end
end
describe "increase_quantity/2" do
@tag task_id: 4
test "works for an empty quantity map" do
assert BoutiqueInventory.increase_quantity(
%{
name: "Long Black Evening Dress",
price: 105,
quantity_by_size: %{}
},
1
) == %{
name: "Long Black Evening Dress",
price: 105,
quantity_by_size: %{}
}
end
@tag task_id: 4
test "increases quantity of an item" do
assert BoutiqueInventory.increase_quantity(
%{
name: "Green Swimming Shorts",
price: 46,
quantity_by_size: %{s: 1, m: 2, l: 4, xl: 1}
},
3
) == %{
name: "Green Swimming Shorts",
price: 46,
quantity_by_size: %{s: 4, m: 5, l: 7, xl: 4}
}
end
end
describe "total_quantity/1" do
@tag task_id: 5
test "works for an empty quantity map" do
assert BoutiqueInventory.total_quantity(%{
name: "Red Denim Pants",
price: 77,
quantity_by_size: %{}
}) == 0
end
@tag task_id: 5
test "sums up total quantity" do
assert BoutiqueInventory.total_quantity(%{
name: "Black Denim Skirt",
price: 50,
quantity_by_size: %{s: 4, m: 11, l: 6, xl: 8}
}) == 29
end
end
end
ExUnit.run()
