Collections and pipelines

List, Map and Set are values like any other, so a second name is a copy and a change needs a var.

Goal#

At the end of this page you can build and change a collection, loop over it, and turn it into another one with a pipeline.

Build and change#

var numbers = [1, 2, 3]
var ages = ["Ada": 36, "Grace": 45]
var seen: Set<String> = []

numbers.append 4
numbers[0] = 10
ages["Alan"] = 41
seen.insert "Ada"

print numbers
print ages
print seen
// prints [10, 2, 3, 4]
// prints ["Ada": 36, "Grace": 45, "Alan": 41]
// prints {"Ada"}

[1, 2, 3] is a List<Int> and ["Ada": 36] is a Map<String, Int>. An empty collection is [], or [:] for a map, with the type on the binding. Each collection has its own word for adding: a list appends, a set inserts, a map sets. All of them remove.

Every change has a twin that returns a changed copy and works on a const:

const numbers = [1, 2, 3]
const more = numbers.appended(4).removed(1)
print "{numbers} {more}"
// prints [1, 2, 3] [2, 3, 4]

Loop over it#

const ages = ["Ada": 36, "Grace": 45]
for (name, age) in ages {
  print "{name} is {age}"
}
// prints Ada is 36
// prints Grace is 45

for goes through a list value by value, and through a map as (key, value) pairs.

Pipelines#

type Employee {
  name: String
  department: String
  age: Int
}

const employees = [
  Employee("Ada", "Engineering", 36),
  Employee("Grace", "Engineering", 45),
  Employee("Linus", "Operations", 28),
]

const seniors = employees.filter({ _.age >= 40 }).map({ _.name })
print seniors.toList()
print employees.filter({ _.department == "Engineering" }).count()
// prints ["Grace"]
// prints 2

filter and map are steps of a pipeline. They are lazy: nothing runs until a last step such as toList() or count() pulls the values through, and no list is built in between. sorted, take and flatMap are steps as well; fold, find and count end a pipeline.

Next#

• Control flow and your own constructs - if, loops, and control structures you write yourself.
• Pipelines - every step, and how to end a pipeline.
• The collection traits - List, Map, Set, Stack and Queue in full.