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See here for more information about sequences and generators. This nice application made me actually understand how you can create your own sequences.

See here for more information about sequences and generators. This nice application made me actually understand how you can create your own sequences.

Update: Due to major changes in the Swift programming language, the above code does not compile anymore with the current Swift 2.1/Xcode 7. Here is an updated version fore anybody's convenience:

struct FibonacciSequence : SequenceType {
 let upperBound : Int
 
 func generate() -> AnyGenerator<Int> {
 var current = 1
 var next = 1
 return anyGenerator {
 if current > self.upperBound {
 return nil
 }
 let result = current
 current = next
 next += result
 return result
 };
 }
}
let fibseq = FibonacciSequence(upperBound: 4_000_000).lazy
let sum = fibseq.filter { 0ドル % 2 == 0 }.reduce(0) { 0ドル + 1ドル }

The following is similar to Flambino's approach, but creates a Swift SequenceType so that you can use the Swift library functions filter() and reduce() to iterate over the elements:

struct FibonacciSequence : SequenceType {
 let upperBound : Int
 
 func generate() -> GeneratorOf<Int> {
 var current = 1
 var next = 1
 return GeneratorOf<Int>() {
 if current > self.upperBound {
 return nil
 }
 let result = current
 current = next
 next += result
 return result
 };
 }
}
let fibseq = FibonacciSequence(upperBound: 4_000_000)
let evenFibseq = LazySequence(fibseq).filter { 0ドル % 2 == 0 }
let sum = reduce(evenFibseq, 0) { 0ドル + 1ドル }

See here for more information about sequences and generators. This nice application made me actually understand how you can create your own sequences.

LazySequence is used here because its filter() method returns the elements "on demand", e.g. as needed in the summation (Kudos to @jtbandes for his suggestion).

The following is similar to Flambino's approach, but creates a Swift SequenceType so that you can use the Swift library functions filter() and reduce() to iterate over the elements:

struct FibonacciSequence : SequenceType {
 let upperBound : Int
 
 func generate() -> GeneratorOf<Int> {
 var current = 1
 var next = 1
 return GeneratorOf<Int>() {
 if current > self.upperBound {
 return nil
 }
 let result = current
 current = next
 next += result
 return result
 };
 }
}
let fibseq = lazy(FibonacciSequence(upperBound: 4_000_000))
let sum = reduce(fibseq.filter { 0ドル % 2 == 0 }, 0) { 0ドル + 1ドル }

See here for more information about sequences and generators. This nice application made me actually understand how you can create your own sequences.

lazy() creates a LazySequence, whose filter() method returns the elements "on demand", e.g. as needed in the summation. The filter() function would return an array of all even Fibonacci numbers before starting the summation. (Kudos to @jtbandes for his suggestion).