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# [Swift], 114 bytes
func y(x:String)->Int?{return ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000].filter{$0.key==x}.first?.value}
[Try it online!][TIO-lokwnkqh]
[Swift]: https://developer.apple.com/swift/
[TIO-lokwnkqh]: https://tio.run/##Ky7PTCsx@f8/rTQvWaFSo8IquKQoMy9dU9fOM6/EvrootaS0KE8hWslTycpQRylMycpURykCyDbQUfIBcoCUM4gHpF1AXCDtC@YbxOqlZeaUpBZVqxjoZadW2tpW1AJFiopL7PXKEnNKU2v//wcA "Swift – Try It Online"
~~# Explaination + Non-Golfed Version~~
The non-golfed version of this function would go one of two ways; the argument could be iterated over using a `switch` statement, or (as seen above) could be mapped to a `Dictionary`. Here are both non-golfed forms:
## 1. Switch
```swift
func romanNumeralAsInt(numeral: String) -> Int {
switch numeral{
case "I":
return 0
case "V":
return 5
case "X":
return 10
case "L":
return 50
case "C":
return 100
case "D":
return 500
case "M":
return 1000
default:
return 0
}
```
Example usage:
```swift
romanNumeralAsInt(numeral: "V") // 5
```
This has some advantages. To walk through the code above:
1. The function takes in a `String` (though it is only one Character, and Swift also has a Character type) and returns an `Int`.
2. The function iterates over all permutations of the input
3. If the input did not match any of the specific cases, the function returns the `default` of `0`.
## 2. Mapping
The above solution likely wasn't the best way to do it, but it was the shortest. **Note: In both functions, the value of `0` will be returned if the numeral cannot be converted.**
```swift
func romanNumeralAsInt(numeral: String) -> Int {
let mappings: [String: Int] = [
"I": 1,
"V": 5,
"X": 10,
"L": 50,
"C": 100,
"D": 500,
"M": 1000
]
if let firstResult = mappings.first(where: { item in
item.key == numeral
} {
return firstResult.value
}
return 0
}
```
1. The above function has the same inputs and outputs as the first example (`String` input and `Int` outptut).
2. The function declares a `Dictionary` (a Swift key-value pair type). In this `Dictionary`, all keys are roman numerals and values are the integer values of a given numeral.
3. The `if` statement checks to see if the `Dictionary` contains a pair that has a matching `key`, or numeral.
4. If so, it returns the `value` of that pair.
5. If not, the function returns `0`.
## The Given Solution
The following is the given solution, but split across lines for readability:
```swift
func y(x: String) -> Int? {
return ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000]
.filter{$0.key==x}
.first?
.value
}
```
What this does:
1. This function takes in a `String`, but outputs an `Int?` - an `Optional` type. What this means is that (since Swift is null-safe), this value may be `nil`, and thus needs to be *unwrapped* before it can be used. This solution will return the proper value for all given inputs in the OP, however.
2. The function declares the same `Dictionary`.
3. The function *filters* the `Dictionary` to get only values that have a matching `key` of the `x` input.
4. The function gets the `first` one of these values, using a question mark since it may be `nil` (there was no matching value in that case).
5. The function returns the integer value for that pair.
If we wanted to use this function, we would need *nil coalescing*, such as an `??` operator like the following:
```swift
let value = y(x: "V") ?? 0 // Value will be 5
```

# [Swift], 114 bytes
func y(x:String)->Int?{return ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000].filter{$0.key==x}.first?.value}
[Try it online!][TIO-lokwnkqh]
[Swift]: https://developer.apple.com/swift/
[TIO-lokwnkqh]: https://tio.run/##Ky7PTCsx@f8/rTQvWaFSo8IquKQoMy9dU9fOM6/EvrootaS0KE8hWslTycpQRylMycpURykCyDbQUfIBcoCUM4gHpF1AXCDtC@YbxOqlZeaUpBZVqxjoZadW2tpW1AJFiopL7PXKEnNKU2v//wcA "Swift – Try It Online"
# Explanation + Non-Golfed Version
The non-golfed version of this function would go one of two ways; the argument could be iterated over using a `switch` statement, or (as seen above) could be mapped to a `Dictionary`. Here are both non-golfed forms:
## 1. Switch
```swift
func romanNumeralAsInt(numeral: String) -> Int {
switch numeral{
case "I":
return 0
case "V":
return 5
case "X":
return 10
case "L":
return 50
case "C":
return 100
case "D":
return 500
case "M":
return 1000
default:
return 0
}
```
Example usage:
```swift
romanNumeralAsInt(numeral: "V") // 5
```
This has some advantages. To walk through the code above:
1. The function takes in a `String` (though it is only one Character, and Swift also has a Character type) and returns an `Int`.
2. The function iterates over all permutations of the input
3. If the input did not match any of the specific cases, the function returns the `default` of `0`.
## 2. Mapping
The above solution likely wasn't the best way to do it, but it was the shortest. **Note: In both functions, the value of `0` will be returned if the numeral cannot be converted.**
```swift
func romanNumeralAsInt(numeral: String) -> Int {
let mappings: [String: Int] = [
"I": 1,
"V": 5,
"X": 10,
"L": 50,
"C": 100,
"D": 500,
"M": 1000
]
if let firstResult = mappings.first(where: { item in
item.key == numeral
} {
return firstResult.value
}
return 0
}
```
1. The above function has the same inputs and outputs as the first example (`String` input and `Int` outptut).
2. The function declares a `Dictionary` (a Swift key-value pair type). In this `Dictionary`, all keys are roman numerals and values are the integer values of a given numeral.
3. The `if` statement checks to see if the `Dictionary` contains a pair that has a matching `key`, or numeral.
4. If so, it returns the `value` of that pair.
5. If not, the function returns `0`.
## The Given Solution
The following is the given solution, but split across lines for readability:
```swift
func y(x: String) -> Int? {
return ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000]
.filter{$0.key==x}
.first?
.value
}
```
What this does:
1. This function takes in a `String`, but outputs an `Int?` - an `Optional` type. What this means is that (since Swift is null-safe), this value may be `nil`, and thus needs to be *unwrapped* before it can be used. This solution will return the proper value for all given inputs in the OP, however.
2. The function declares the same `Dictionary`.
3. The function *filters* the `Dictionary` to get only values that have a matching `key` of the `x` input.
4. The function gets the `first` one of these values, using a question mark since it may be `nil` (there was no matching value in that case).
5. The function returns the integer value for that pair.
If we wanted to use this function, we would need *nil coalescing*, such as an `??` operator like the following:
```swift
let value = y(x: "V") ?? 0 // Value will be 5
```

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