We’ve covered how to manipulate our dictionary and we can certainly print out the entire dictionary, but what if we want to access a single key-value pair?

After all, we shouldn’t have to read through an entire dictionary to find a single definition (or scroll through all of our contacts to find a single phone number).

We can use subscript syntax to access the specific value associated with a key. For example, suppose we had the following dictionary fruitNames:

var fruitNames = [
  "mango": "Mangifera indica",
  "banana": "Musa paradisicum",
  "apple": "Pyrus malus"
]

If we wanted to extract the value associated with the key "apple" and assign it to a new variable called appleScientific, our code would look like this:

var appleScientific = fruitNames["apple"]

If we tried to print the value of (appleScientific), we would get an output of:

Optional("Pyrus malus")

Swift returns an optional. The optional type is used in Swift when a value may not exist. For dictionaries, we might be trying to access a key that doesn’t exist, so as a precaution we first get an optional instead of getting the exact value.

There are two methods for extracting values from optionals.

If we aren’t sure if a key exists within a dictionary, we can use an if-let statement:

if let appleScientific = fruitNames["apple"] {
  print(appleScientific)
} else {
  print("This key does not exist.")
}

if-let statements are used to check if a real value exists inside of an optional. If the value exists, the optional will be unwrapped and assigned to a variable.

If we are absolutely positive a key exists within a dictionary, we can add ! at the end of the statement like so:

var appleScientific = fruitNames["apple"]!

Using ! forces the compiler to unwrap an optional value and interpret the value as its appropriate data type; however, it should be noted that errors can occur when using ! on a value that does not exist.

To read more about unwrapping optionals, check out Swift’s Language Guide.