Scala Map Examples

Use Maps to look up values with keys. Create a Map and invoke Map functions.

Map. These lands are all the same. Each hill, each stream and tree—the terrain repeats itself. With a landmark, you could map the area you have covered.

Data type. In Scala, a map is the core dictionary type. We link values (places) from keys (or landmarks). Usually we have Strings and Ints, but any type can be used.

Initial example. Here we build a String map. It has String keys and Int values. We initialize it with just two pairs. Map is immutable so a new map would be needed to add elements.String

Get: To get a value from a key in the "colors" map, we access the colors like a function. We pass the key as the argument.

Scala program that creates Map // Use simple map initialization syntax. val weights = Map("cat" -> 10, "elephant" -> 200000) // Look up animal weights. val weight = weights("elephant") println(weight) Output 200000

Another example. We can initialize a Map with a different syntax based on a list of pairs. Here we use a String to String map. We look up two animal Strings.

Scala program that creates Map with pair syntax // Create map of animals to colors. // ... Has string keys and string values. val colors = Map(("bird", "blue"), ("fox", "red")) // Get value for this key. val result1 = colors("bird") println(result1) val result2 = colors("fox") println(result2) Output blue red

Add. A Map is immutable so we cannot add an element to an existing map. But we can add a key-value pair to a new, copied map. Here we add a zebra in a new map creation statement.

Scala program that adds key, value to map // Create an immutable map. val zoo = Map("frog" -> 1, "lion" -> 1) // Add a pair to the map. // ... This creates a new map. val all = zoo + ("zebra" -> 1) println(all) Output Map(frog -> 1, lion -> 1, zebra -> 1)

Keys, values. Here we create a map with String keys and Int values. We then access the keys and values properties to get collections of those elements.

Keys: We use the foreach function on the keys collection (which is an iterable) and use println on each key.

Values: Here we use an imperative for-loop to call println on each value in the Map.

For

Scala program that gets keys, values // Create map of String keys and Int values. val ids = Map(("abc", 10), ("def", 20)) // Use foreach function to print each key. ids.keys.foreach(println(_)) // Use for-loop to iterate over all values. for (value <- ids.values) { println(value) } Output abc def 10 20

Get, getOrElse. With get, an option is returned. If the value exists in the Map, isDefined will return true on that option. We can then call get() on the option.Option

Get: This returns an option, not the value type of the Map. We must always test the option.

GetOrElse: This lets us provide a default value that is returned if the key does not exist in the Map. Here we use 0 as a default.

Scala program that uses get, getOrElse // Create a String, Int map. val sizes = Map(("Medium", 2), ("Large", 4)) // This key does not exist, so the option is not defined. val result1 = sizes.get("Small") if (!result1.isDefined) { println("Not defined") } // This key exists. // ... Get and print the option's internal value. val result2 = sizes.get("Large") if (result2.isDefined) { println(result2) val number = result2.get println(number) } // Use a default value if the key does not exist. val result3 = sizes.getOrElse("Small", 0) if (result3 == 0) { println("Zero, else value") } Output Not defined Some(4) 4 Zero, else value

Equals. With this method we test for structural equality of two maps. The order of the entries in the maps is not important. But the keys and values must be the same.

Here: We find map1 and map2 are equal because their key-value pairs are the same. But map3 has a different value so it is not equal.

Scala program that uses equals // Create three maps. // ... The first two are equal but have different orders. // The third one has different entries. val map1 = Map((10, true), (20, false)) val map2 = Map((20, false), (10, true)) val map3 = Map((20, true), (10, true)) // These two maps are structurally equal. if (map1.equals(map2)) { println("Maps equal") } // Not equal. if (!map1.equals(map3)) { println("Maps not equal") } Output Maps equal Maps not equal

WithDefaultValue. A map can have a default value or function. We call withDefaultValue to specify a default value—in this program we use a default integer of -1.

So: When we do a lookup on the map that does not find an existing key, the default value is returned.

WithDefault: This method returns a default value based on a function. We can pass a lambda expression to the withDefault method.

Here: The map does not contain the key "bear" so the final look up in the program returns the value -1.

Scala program that uses withDefaultValue val animalMap = Map(("cat", 10), ("bird", 5)) // The default value is now negative 1. val animalMapDefault = animalMap.withDefaultValue(-1); // Use map with default. val result1 = animalMapDefault("cat") println(result1) val result2 = animalMapDefault("bear") println(result2) Output 10 -1

Map performance. A Map is a good optimization. Here we have three elements "abc" and we store them in a Map and a List. We search for one element in the collections.

Version 1: In this version of the code we search the Map with a string argument. We repeat this operation in a for-loop.

Version 2: Here we try to find the element in a List. We use a for-loop to search. The operation is repeated many times.

List

Result: Finding element "c" in the Map is faster than in the List. The fast lookup time helps this benchmark.

Scala program that times Map and List // Map and list used. val lookup = Map(("a", 1), ("b", 1), ("c", 1)) val list = List("a", "b", "c") var total = 0 val t1 = System.currentTimeMillis() // Version 1: look up element in a map. for (i <- 0 until 10000000) { val result = lookup("c") total += result } val t2 = System.currentTimeMillis() // Version 2: search for element in a list. for (i <- 0 until 10000000) { var result = 0 for (v <- list) { if (v == "c") { result = 1 } } total += result } val t3 = System.currentTimeMillis() // Results. println(total) println(t2 - t1) println(t3 - t2) Output 20000000 77 ms, Map 140 ms, List

Immutable, research. Immutable types, such as scala.collection.immutable.Map, are key in Scala. This Map is a generic trait. It indicates an immutable map.

Note: For this Map, elements cannot be added, but new maps can be created. We add elements by creating new maps.

Quote: A generic trait for immutable maps. Concrete classes have to provide functionality for the abstract methods in Map (Scala Standard Library).

To summarize: we use the Map type to associate keys with values. This provides fast, hashed lookups. Maps solve many performance problems, and can even simplify code by removing duplicates.

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Scala Map Examples

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