Swift: Using try? to catch Index out of range - ios

Question

so i am making game and there is a grid and you can only place if any of the adjacent blocks are a source block, well if you are trying to place a block in the top grid there's no block above it so an Index out of range error will be thrown. I am trying to catch it but xcode seems to think no error is possible. Am i doing this wrong? I guess I could go to each case and add a catch but I feel like this should work.
copy friendly text if you would like to try it out in your own IDE:
func isValidPlacement(row:Int,col:Int) -> Bool {
if let t = try? tiles[row-1][col], t.isSource {
return true
}
else if let t = try? tiles[row][col-1], t.isSource {
return true
}
else if let t = try? tiles[row+1][col], t.isSource {
return true
}
else if let t = try? tiles[row][col+1], t.isSource {
return true
}
else {
return false
}
}

Answer 1

Runtime errors and returning bools is the Objective-C way! Throw throw throw! 🙀
public extension Collection {
/// - Returns: same as subscript, if index is in bounds
/// - Throws: CollectionIndexingError
func element(at index: Index) throws -> Element {
guard indices.contains(index)
else { throw CollectionIndexingError() }
return self[index]
}
}
extension Collection where Element: Collection {
/// - Returns: same as subscripting, if indices are in bounds
/// - Throws: CollectionIndexingError
func element( at indices: (Index, Element.Index) ) throws -> Element.Element {
try element(at: indices.0).element(at: indices.1)
}
}
/// Thrown when `getElement` is called with an invalid index.
public struct CollectionIndexingError: Error { }
extension Tile {
enum PlacementError: Error {
case invalid
case noSources
}
}
extension Collection where
Index == Int,
Element: Collection, Element.Index == Int, Element.Element == Tile
{
func validatePlacement(row: Index, column: Element.Index) throws {
let tiles = [(-1, 0), (0, -1), (1, 0), (0, 1)].compactMap {
try? element( at: (row + $0.0, column + $0.1) )
}
guard !tiles.isEmpty
else { throw Tile.PlacementError.invalid }
guard tiles.contains(where: \.isSource)
else { throw Tile.PlacementError.noSources }
}
}

Answer 2

In Swift. you can only catch instances of Error that are thrown by code, you can't catch runtime exceptions such as an array bounds violation
You can create your own safeAccess function. You don't say what type is in your array so I will use SomeType as an example
func safeAccess(row:Int, col:Int) -> SomeType? {
guard row >= 0, row < tiles.count else {
return nil
}
guard col >= 0, col < tiles[row].count else {
return nil
}
return tiles[row][col]
}
func isValidPlacement(row:Int,col:Int) -> Bool {
if let t = tiles.safeAccess(row:row-1,col:col), t.isSource {
return true
}
if let t = tiles.safeAccess(row:row,col:col-1),, t.isSource {
return true
}
if let t = tiles.safeAccess(row:row+1,col:col), t.isSource {
return true
}
if let t = tiles.safeAccess(row:row,col:col+1), t.isSource {
return true
}
return false
}
You could also define an extension on Array
extension Array {
func element(at index: Int) -> Element? {
if index >= 0 && index < self.count {
return self[index]
}
return nil
}
}
And to use it:
func isValidPlacement(row:Int,col:Int) -> Bool {
if let tiles = tiles.element(at:row-1), let t = tiles.element(at:col), t.isSource {
return true
}
else if tiles.element(at:row), let t = tiles.element(at:col-1), t.isSource {
return true
}
else if let tiles = tiles.element(at:row+1), let t = tiles.element(at:col), t.isSource {
return true
}
else if let tiles = tiles.element(at:row), let t = tiles.element(at:col+1), t.isSource {
return true
}
else {
return false
}
}

Answer 3

Accessing an index of an array which doesn't exist results in a run time exception. You can't use try here because the array is not throwing anything.
Try adding these two lines at the start of the func and get rid of all of try?
guard row <= tiles.count - 1 else {
return false
}
guard col <= tiles[row].count - 1 else {
return false
}
Also i am assuming, whatever the type array stores here is an optional. if not you can remove if lets too
func isValidPlacement(row:Int,col:Int) -> Bool {
guard row <= tiles.count - 1 else {
return false
}
guard col <= tiles[row].count - 1 else {
return false
}
if let t = tiles[row-1][col], t.isSource {
return true
}
else if let t = tiles[row][col-1], t.isSource {
return true
}
else if let t = tiles[row+1][col], t.isSource {
return true
}
else if let t = tiles[row][col+1], t.isSource {
return true
}
else {
return false
}
}