The flatMap function in terms of reduce

To understand the power of reduce, we can implement the flatMap function in terms of reduce as well:

func flatMapIntermsOfReduce<Element>(elements: [Element], transform: (Element) -> Element?) -> [Element] { 

  return reduce(elements: elements, initial: []) { 
        guard let transformationResult = transform($1) else { 
            return $0 
        } 
        return $0 + [transformationResult] 
    } 
} 

let anArrayOfNumbers = [1, 3, 5] 
let oneDimensionalArray = flatMapIntermsOfReduce(elements: anArrayOfNumbers) { $0 + 5 } 

Also, we can flatten a two-dimensional array using the reduce method as follows:

func flatMapIntermsOfReduce<Element>(elements: [[Element]], transform: (Element) -> Element) -> [Element] { 

    return elements.reduce([]) { $0 + $1 } 
} 

let aTwoDimArrayOfNumbers = [[1, 3, 5],[2, 4, 6]] 
let flatMappedArray = flatMapIntermsOfReduce(elements: aTwoDimArrayOfNumbers) { $0 } 

The flatMappedArray will be [1, 3, 5, 2, 4, 6].