Optional Lookup
Part of the Logic & Flow section of Coddy's Swift journey — lesson 13 of 56.
Dictionary subscripts always return an Optional: the value when the key exists, nil when it doesn't. Swift forces you to handle both cases.
let prices: [String: Int] = ["apple": 1, "bread": 3]
let p = prices["apple"]
print(p) // Optional(1)Three idiomatic ways to deal with the optional. Pick the one that fits the situation.
1. if let: handle both branches explicitly:
if let p = prices["apple"] {
print("apple costs \(p)")
} else {
print("unknown")
}2. nil-coalescing ??: provide a fallback inline:
let p = prices["olive"] ?? 0 // 03. subscript with default: identical result, but the default also stays put if you write back to that key:
var counts = ["a": 2]
counts["b", default: 0] += 1 // counts == ["a": 2, "b": 1]
Challenge
EasyThe dictionary prices is given. Read a single line of input: a comma-separated list of items the user wants to buy.
For each item, in input order, look it up in prices. Print one line per item:
<item> @ <price>when the item is in the dictionary<item> not soldotherwise
After listing every item, print one final line: Total: <sum>. Treat missing items as costing 0 using the nil-coalescing operator.
For input apple,bread,olive,milk, the output is:
apple @ 1
bread @ 3
olive not sold
milk @ 4
Total: 8Cheat sheet
Dictionary subscripts return an Optional — the value if the key exists, nil if not.
1. if let — handle both branches:
if let p = prices["apple"] {
print("apple costs \(p)")
} else {
print("unknown")
}2. Nil-coalescing ?? — provide a fallback:
let p = prices["olive"] ?? 0 // 03. Subscript with default — fallback that also works on write:
var counts = ["a": 2]
counts["b", default: 0] += 1 // ["a": 2, "b": 1]Try it yourself
let prices: [String: Int] = ["apple": 1, "bread": 3, "milk": 4]
let items = readLine()!.components(separatedBy: ",")
// TODO: per-item lookup; print '@ price' or 'not sold'; sum with ?? 0
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Logic & Flow
1Strings In Depth
Count and IndicesCase and TrimSearching in StringsSplitting and JoiningReplacing SubstringsRecap - Username Check
3Dictionaries
Declaring DictionariesOptional LookupUpdating DictionariesIterating DictionariesGrouping ValuesRecap - Inventory