CODEWITHSUNDEEP
swiftstructswift3
user6451264
user6451264

Reputation:

access struct member by index

If I have a struct like 

struct Point4D
{
    let x:Double, 
        y:Double, 
        z:Double, 
        w:Double
}

is there an efficient way to access the members of the struct by integer index? Meaning

let index:Int = 2
let point:Point4D = Point4D(x: 1, y: 5, z: 3, w: 8)
let component:Double = point [index] // 3

I suppose this is possible by defining the subscript() method on Point4D and using a switch statement but this doesn’t seem very efficient. In C I believe you can increment a pointer to the first struct member since they are all of type Double, but Swift doesn’t allow this…

Upvotes: 3

Views: 866

Answers (2)

Jim Conroy
Jim Conroy

Reputation: 341

Here's one approach to get the value of a component. It should be efficient. It'd be interesting to performance test against the subscript/switch alternative to see the speed difference.

func componentValue(_ pointer: UnsafePointer<Point4D>, at index: Int) -> Double {
    return pointer.withMemoryRebound(to: Double.self, capacity: 4) { ptr -> Double in
        return (ptr+index).pointee
    }
}

struct Point4D
{
    let x:Double,
    y:Double,
    z:Double,
    w:Double
}

let index:Int = 2
var point:Point4D = Point4D(x: 1, y: 5, z: 3, w: 8)
let component:Double = componentValue(&point, at: index)    // 3

Upvotes: 0

CRD
CRD

Reputation: 53000

Think what you are asking for in the context of Swift, a strongly (primarily) statically typed language.

What would the type of the nth member of a struct be where n is not known till compile time?

(Note that in referencing tuples elements, e.g. (1, true, 3.4).1 -> true, the selector is a literal value and not variable so the type is always known.)

The Swift designers could have chosen to include a special case for structures all of whose elements are the same type, but that would probably be rather confusing.

You should consider your design and why you need this operation and named access to components. If it is important consider a custom type with appropriate operations, maybe based on an array for storage.

Upvotes: 0

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