JuniorTheoryCommon

What does the program print?

SWIFT

let stock = ["apples": 3, "pears": 0]
let apples = stock["apples"] ?? -1
let pears = stock["pears"] ?? -1
let plums = stock["plums"] ?? -1
print(apples, pears, plums)

3 0 -1

Expecting a crash, as with an array index past the end. A dictionary lookup of a missing key gives nil, and ?? replaces it with -1.
Expecting -1 for "pears": treating a stored 0 as an empty value. ?? replaces only nil, and the key "pears" exists.

How can you check whether the key "pears" is in the dictionary at all?
What does stock["plums", default: -1] give, and does it add the key?

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JuniorTheoryCommon

What does the program print?

SWIFT

let stock = ["apples": 3, "pears": 0]
let apples = stock["apples"] ?? -1
let pears = stock["pears"] ?? -1
let plums = stock["plums"] ?? -1
print(apples, pears, plums)

3 0 -1

Expecting a crash, as with an array index past the end. A dictionary lookup of a missing key gives nil, and ?? replaces it with -1.
Expecting -1 for "pears": treating a stored 0 as an empty value. ?? replaces only nil, and the key "pears" exists.

How can you check whether the key "pears" is in the dictionary at all?
What does stock["plums", default: -1] give, and does it add the key?

Back to questions Open topic in roadmap →Answer it yourself — Quiz