what the difference of keys and values in dictionary of swift - swift

Question

I'm trying the tutor of swift, and found the question.
I need to use optional values to keys and then unwrap it, however the values don't to be done so.But why?
Don't know why to use "kinds" rather than kind in the closure, but the one before that used number rather than numbers ,and how can I use numbers to get same result?
let interestingnumbers = [
"prime": [2,3,5,7,9,11,21],
"fibonacci": [1,23,5,32,4,123,11],
"dadgadfgh":[1,2,4,5,43,6,576,12],
"square": [1243325,123,455,1111],
]
var thekind:String?
var max=0
for (kinds,numbers) in interestingnumbers{
for kind in kinds{
for number in numbers{
if number>max {
max=number
thekind = kinds // I don't know why here should use "kinds" rather than kind, but the one before that used number rather than numbers ,and how can I use numbers to get same result?
}
}
}
}
print(max,thekind!)
//this is a fault code
let interestingnumbers = [
"prime": [2,3,5,7,9,11,21],
"fibonacci": [1,23,5,32,4,123,11],
"dadgadfgh":[1,2,4,5,43,6,576,12],
"square": [1243325,123,455,1111],
]
var thekind:String
var max=0
for (kinds,numbers) in interestingnumbers{
for kind in kinds{
for number in numbers{
if number>max {
max=number
thekind = kinds
}
}
}
}
print(max,thekind)
//and i tried this also
let interestingnumbers = [
"prime": [2,3,5,7,9,11,21],
"fibonacci": [1,23,5,32,4,123,11],
"dadgadfgh":[1,2,4,5,43,6,576,12],
"square": [1243325,123,455,1111],
]
var thekind:String?
var max:Int? //also [:Int]?
for (kinds,numbers) in interestingnumbers{
for kind in kinds{
for number in numbers{
if number>max {
max=numbers
thekind = kinds
}
}
}
}
print(max,thekind!)

Answer 1

a) "I need to use optional values to keys and then unwrap it, however the values don't to be done so.But why?"
This is not true. You can initialize thekind string variable with an empty string the same way you did with your integer type max setting its initial value to zero.
b) "Don't know why to use "kinds" rather than kind in the closure, but the one before that used number rather than numbers ,and how can I use numbers to get same result?"
You are unnecessarily iterating each character of your dictionary keys. Thats why you are assigning kinds String instead of kind Character.
let interestingnumbers = [
"prime": [2,3,5,7,9,11,21],
"fibonacci": [1,23,5,32,4,123,11],
"dadgadfgh":[1,2,4,5,43,6,576,12],
"square": [1243325,123,455,1111],
]
var thekind: String = ""
var max = 0
for (key, numbers) in interestingnumbers {
for number in numbers {
if number > max {
max = number
thekind = key
}
}
}
print(max, thekind) // "1243325 square\n"
Just for fun the functional approach:
// result is a tuple (kind and max)
// reduce iterates every key value pair
// $0 is the accumulated which in this case it is used to hold the max value and the corresponding key
// $1 is the current key,value pair (kind, numbers)
let (kind, max) = interestingnumbers.reduce(into: ("",0)) {
// here we check if the current value has a maximum value and if it is greater than $0.1 (the resulting tuple maxValue)
if let maxValue = $1.value.max(), maxValue > $0.1 {
// if true assign a new tuple to the result with the new key and the new maximum value
$0 = ($1.key, maxValue)
}
}
print(kind, max) // "square 1243325\n"

Answer 2

Swift's standard library doesn't have this flatMap overload, which makes things easy.
// (key: "square", value: 1243325)
interestingNumbers.flatMap().max(by: \.value)
public extension Dictionary where Value: Sequence {
/// Flatten value sequences,
/// pairing each value element with its original key.
func flatMap() -> [(key: Key, value: Value.Element)] {
flatMap { key, value in value.map { (key, $0) } }
}
}
public extension Sequence {
func max<Comparable: Swift.Comparable>(
by getComparable: (Element) throws -> Comparable
) rethrows -> Element? {
try self.max {
try getComparable($0) < getComparable($1)
}
}
}