Map
Maps are data structures that store key-value pairs. So they are similar to keyword lists, but with some important differences:
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Performance
Maps are faster at finding values for given keys, especially when the map gets large. If you have a big bunch of key-value pairs and need to frequently lookup values, a map is more efficient. Keyword lists can be slower when they grow bigger because they have to search through the list one item at a time.
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Key Uniqueness
In a map, each key is unique. If you try to add an entry with a key that already exists, it will just update the existing entry. This is useful when you want to ensure there are no duplicates. With keyword lists, you can have the same key multiple times.
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No Key Ordering
Keyword lists keep the order of the elements as you added them, which can be useful in certain situations. Maps, on the other hand, don’t keep track of the insertion order.
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Any Key Type
Maps can have keys of any type, while keyword lists usually have atom keys. This gives maps more flexibility if you need to use different types as keys.
Maps are created using the %{} syntax.
iex(1)> product_prices = %{"Apple" => 0.5, "Orange" => 0.7} (1)
%{"Apple" => 0.5, "Orange" => 0.7}
iex(2)> product_prices["Orange"] (2)
0.7
iex(3)> product_prices["Banana"] (3)
nil
iex(4)> product_prices = %{"Apple" => 0.5, "Apple" => 1, "Orange" => 0.7}
%{"Apple" => 1, "Orange" => 0.7} (4)
warning: key "Apple" will be overridden in map
└─ iex:4| 1 | A new map is created and bound to the variable product_prices. |
| 2 | Value retrieval is straightforward: append the key to the map name within brackets. |
| 3 | If the given key doesn’t exist, it returns nil. |
| 4 | Unlike keyword lists, maps disallow duplicate keys. |
Maps are flexible, allowing any data type to serve as both keys and values:
iex> %{"one" => 1, "two" => "abc", 3 => 7, true => "asdf"}
%{3 => 7, true => "asdf", "one" => 1, "two" => "abc"}| Each key must be unique within a map. If there are duplicates, the last one overwrites the previous values. |
Atom Key
Maps support atom keys, enabling some handy features:
iex> product_prices = %{apple: 0.5, orange: 0.7} (1)
%{apple: 0.5, orange: 0.7}
iex> product_prices.apple (2)
0.5
iex> product_prices.banana (3)
** (KeyError) key :banana not found in: %{apple: 0.5, orange: 0.7}| 1 | This syntax makes reading and typing easier when using atoms as keys. |
| 2 | Atom keys allow the use of the dot operator (.) to access their values. |
| 3 | If an attempt is made to access a nonexistent key with the dot operator, an error is thrown. |
Sure, let’s break it down a little bit more and explain each part in a simpler way.
Map Functions
Elixir’s Map module is equipped with various functions that help to perform different operations on maps.[1] Let’s explore the functions I use the most.
Creating a Map
Create a map named product_prices that stores the prices of various products.
iex> product_prices = %{apple: 0.5, orange: 0.7, coconut: 1}
%{apple: 0.5, orange: 0.7, coconut: 1}
Since Elixir 1.18, small atom-keyed maps print in the order they
were built (insertion order). Older Elixir versions sorted the keys
alphabetically, so older tutorials may show %{apple: 0.5, coconut: 1,
orange: 0.7} for the same map.
Here, we have three items - apple, orange, and coconut - each with their respective prices.
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Converting a Map to a List
The Map.to_list/1 function allows us to convert a map into a keyword list.
iex> Map.to_list(product_prices)
[apple: 0.5, orange: 0.7, coconut: 1]We can see that our map has now been transformed into a keyword list.
Retrieving All Values from a Map
To fetch all the values from a map (in our case, the product prices), we can utilize the Map.values/1 function.
iex> Map.values(product_prices)
[0.5, 0.7, 1]This gives us the prices for all our products.
Retrieving All Keys from a Map
To fetch all the keys from a map, we can utilize the Map.keys/1 function.
iex> Map.keys(product_prices)
[:apple, :orange, :coconut]Splitting a Map
We can split a map into two new maps based on a provided list of keys using the Map.split/2 function.
iex> Map.split(product_prices, [:orange, :apple])
{%{apple: 0.5, orange: 0.7}, %{coconut: 1}}Our original map is divided into two maps: one containing apple and orange, and the other containing coconut.
Removing a Key-Value Pair from a Map
The Map.delete/2 function can be used when we need to remove a specific key-value pair from our map.
iex> a = Map.delete(product_prices, :orange)
%{apple: 0.5, coconut: 1}A new map a is created where the key-value pair for orange is removed.
Removing Multiple Key-Value Pairs from a Map
For removing multiple key-value pairs, we can use the Map.drop/2 function.
iex> b = Map.drop(product_prices, [:apple, :orange])
%{coconut: 1}We have removed apple and orange, leaving only coconut in the new map b.
Merging Two Maps
The Map.merge/2 function enables us to combine two maps.
iex> additional_prices = %{banana: 0.4, pineapple: 1.2}
%{banana: 0.4, pineapple: 1.2}
iex> Map.merge(product_prices, additional_prices)
%{apple: 0.5, orange: 0.7, coconut: 1, banana: 0.4, pineapple: 1.2}A new map is created that contains the items and their prices from both the maps.