Updating Dictionaries

Part of the Logic & Flow section of Coddy's Swift journey — lesson 14 of 56.

You write to a dictionary the same way you read from one: with the subscript.

var prices: [String: Int] = ["apple": 1]
prices["bread"] = 3                // adds the key
prices["apple"] = 2                // updates an existing key
prices["apple"] = nil              // removes the key

Two named methods do the same thing more explicitly:

prices.updateValue(5, forKey: "milk") // returns the old value, or nil
prices.removeValue(forKey: "bread")  // returns the removed value, or nil

The named methods return the previous value, which is useful when you need to know whether the key already existed.

The default-subscript pattern from the previous lesson chains perfectly with mutation, this is how you build a counter without writing an if:

var counts: [String: Int] = [:]
for letter in ["a", "b", "a"] {
    counts[letter, default: 0] += 1
}
// counts == ["a": 2, "b": 1]

Challenge

Medium

Read a single line of input: a comma-separated list of operations on a starting dictionary. Each operation is one of:

set:<key>:<value> add or update (value is an integer)
add:<key>:<value> add the value to the existing one (use the default-subscript pattern)
del:<key> remove the key

After applying every operation in order to an initially-empty [String: Int], print the keys in alphabetical order, one per line, in the format <key>=<value>.

For input set:a:5,add:a:3,set:b:1,del:b,add:c:2, the output is:

a=8
c=2

Cheat sheet

Modify a dictionary using subscript or named methods:

prices["bread"] = 3      // add
prices["apple"] = 2      // update
prices["apple"] = nil    // remove

prices.updateValue(5, forKey: "milk") // returns old value or nil
prices.removeValue(forKey: "bread")   // returns removed value or nil

Use the default-subscript pattern to mutate without if:

var counts: [String: Int] = [:]
for letter in ["a", "b", "a"] {
    counts[letter, default: 0] += 1
}
// counts == ["a": 2, "b": 1]

Try it yourself

let ops = readLine()!.components(separatedBy: ",")
var dict: [String: Int] = [:]

// TODO: parse each op and apply set/add/del; print sorted keys with values