Intro to Scala Lists

Scala Lists are always immutable. This means that a list in Scala, once created, will remain the same.

It is very easy to create a list in Scala. You do not have to give the type of list in Scala. Scala can infer its type from the object it is pointing to.

Here are some examples using the Scala interpreter: Creating Lists You can create an empty list, by coding List() This list contains nothing.

Creating a List of Integers We define the list integerlist using val so we create an immutable list, which is initialized with a new List of integers with the values 1, 2, 3, 4 and 5. Notice the result shows List[Int]. This is because the list contains integers.

To Print the List of Integers

This creates a field named listofstrings, which is initialized with a new List of strings with the values A, B, C, and D. Creating a List of Strings Notice the result shows List[java.lang.String]. This is because you have a list of strings.

Print the List This creates a field named listofstrings which is initialized with a new List of strings with the values A, B, C, and D.

If you want to print the values on a separate line. You could create a method printOut to loop through the list. The method reads in a List of type String. Then in the for loop declaration, we have defined a new variable i of inferred type Int. This variable contains the current value of the list as we loop through until variable i is equal to length of the list. Thus ending the loop.

Reversing a List With Scala, you can also reverse the list of strings and integers by using.reverse. Reverse the list of integers: Reverse the list of strings:

Concatenating a List List has a method named ::: that concatenates a passed List and the List on which ::: was invoked. Here's how you use it to combine the integerlist and listofstrings lists: Notice the result shows List[Any]. This is because you have a list of both integers and strings. This is a catchall type for Scala.

Also, you can use the :: operator to add an element to the beginning of a list. For example to add Turtle to a list of dog names:

Accessing an Element from the List To access an element from the list, you simply list the index for the element you want to access. Scala indexing starts at index 0.

Or to pull element 1 from a list: If you try to access an element out of range you will throw an exception. For example to pull element 3 from a list:

Using the Filter Function Lists can be manipulated very easily with functions that you pass into the list operators. These functions can be built with very short syntax.

This code filters out the numbers that fail the test 1 > n. The {1>} builds a function that takes a single number, compares it to 1 and returns true if it is greater than 1. So the List returns (2, 4,5). For example, say we only want the numbers in the list that are greater than the number 1.

If we only want the even numbers from the list: This function takes a parameter n and returns "n mod 2 equals 0". In this case, we specify the parameter name (but we don't have to specify the type because we've got a list of integers) and we perform operations on that parameter. The result of the operation is used to determine if the element is included in the newly built list.

Using the Map Method with List Map applies a one-parameter function to every element of a List, returning a new List. In this example, we will create a list called listmap and then we will use the map function on the list to increase each value by 2. This will not change listmap, but will return the new values.

The above code adds 2 to each list element and "map" returns the resulting list. Also, you can nest map operations: As you can see the nested map function returns a new list with the elements in list2 increased by the first value(1) from list1 and then a second new list is returned with the elements in list2 increased by the second value(2) in list1.

If you want to make changes to certain elements in the list then you can use filter and map together. In this code, the filter function pulls out elements in the list greater than 0. (-3,-2,-1) The map function then adds 1 to each of those elements returning the new list(-2,-1,0).

Sorting Lists In this example we will create a list sorted by the second character of each element and put the list in descending order. The result is a new list in descending order by the second character of each element. (charAt(1).

Foreach Method foreach method is similar to a loop in that it will process for each element in the list. A list is created with types of fruit followed by how many of each we have. We use foreach on the fruit list to process each element printing the number of fruit, a space and then the name of the fruit.

Another Example using Foreach Put the elements in a list of names in Upper Case format.

List Type Matching Scala has a function match which allows comparison of an expression against a number of different types. The format is similar to the switch statement in Java. The following slide is an example using match against a list to pull out any integer or string types from the list.

This code uses a for loop with variable I which is inferred to be 0. It loops through the list until the end of list. Each item in the list is matched against type each case to see if there is a match. If the list element is an integer then it should match on case b for Integer, if string then case a for strings and print out the message. If the element in the list is neither a String or Int then case other message will be printed.

As you can see Scala Lists has some very useful methods available to make programming with lists very easy to do.