Compare two arrays and retrun element with same index - ios

Question

I have 2 inputs adress and street. I need to make 2 arrays size street. One array with odd numbers and another with even numbers. Than i need to reverse odd array. Than I need to check what array contains input adress and to return same index element from another array.
This is error what is show to me:
Fatal error: Unexpectedly found nil while unwrapping an Optional value: file test/solution.swift, line 6
func overTheRoad(address: Int, street: Int) -> Int {
var odd = Array(stride(from: 1, through: street * 2, by: 2))
let even = Array(stride(from: 2, through: street * 2, by: 2))
odd.reverse()
if address % 2 == 0{
let index = even.firstIndex(of: address)!
return odd[index]
}else{
let index = odd.firstIndex(of: address)!
return even[index]
}
}
This is how to suppose work:
You've just moved into a perfectly straight street with exactly n identical houses on either side of the road. Naturally, you would like to find out the house number of the people on the other side of the street. The street looks something like this:
Street
1| |6
3| |4
5| |2
you
Evens increase on the right; odds decrease on the left. House numbers start at 1 and increase without gaps. When n = 3, 1 is opposite 6, 3 opposite 4, and 5 opposite 2.
Example (address, n --> output)
Given your house number address and length of street n, give the house number on the opposite side of the street.
1, 3 --> 6
3, 3 --> 4
2, 3 --> 5
3, 5 --> 8

Answer 1

func overTheRoad(address: Int, street: Int) -> Int? {
var odd = Array(stride(from: 1, through: street * 2, by: 2))
let even = Array(stride(from: 2, through: street * 2, by: 2))
odd.reverse()
if address % 2 == 0{
guard let index = even.firstIndex(of: address) else { return nil }
return odd[index]
}else{
guard let index = odd.firstIndex(of: address) else { return nil }
return even[index]
}
}
Instead of force unwrapping, you should unwrap it safely with a guard let (or if let) statement. In case of no address value is found in the array, you can return nil.

Answer 2

Adding on to Hüsamettin Eyibil's answer, there one big thing that could be improved here: You're generating 2 arrays here, only to query them for a single value and discard the rest. Half the time, each of them isn't even used a single time!
There's actually a pretty simple, closed-form math solution:
func overTheRoad(address: Int, street: Int) -> Int {
let totalHouseCount = street * 2
guard address <= totalHouseCount else {
fatalError("The address number \(address) is too high for a street that's only \(street) houses long!")
}
return totalHouseCount - address + 1
}
// Some test cases:
XCTAssertEqual(overTheRoad(address: 1, street: 3), 6)
XCTAssertEqual(overTheRoad(address: 2, street: 3), 5)
XCTAssertEqual(overTheRoad(address: 3, street: 3), 4)
XCTAssertEqual(overTheRoad(address: 4, street: 3), 3)
XCTAssertEqual(overTheRoad(address: 5, street: 3), 2)
XCTAssertEqual(overTheRoad(address: 6, street: 3), 1)
XCTAssertEqual(overTheRoad(address: 3, street: 5), 8)
Here I chose to fatalError on invalid input, but it might make sense to just return nil instead.