How efficiently to convert one dimensional array to two dimensional array in swift3

Question

What is the efficient way to convert an array of pixelValues [UInt8] into two dimensional array of pixelValues rows - [[UInt8]]

You want to append arrays into an multidimensional one [[UInt8]] or split the original array [UInt8] into smaller arrays that represent rows? if so, what is the rule to split it ? — Jans
– Jans, Commented Sep 9, 2016 at 22:21
@xhamr I want to make (from an array of pixelxValues [UInt8] of Image) a two-dimensional array where each element is pixelRow (i.e. [UInt8] ) from array of pixelxValues [UInt8] of Image. — VYT
– VYT, Commented Sep 9, 2016 at 22:27

OOPer · Accepted Answer · 2016-09-09 22:32:03Z

2

You can write something like this:

var pixels: [UInt8] = [0,1,2,3, 4,5,6,7, 8,9,10,11, 12,13,14,15]
let bytesPerRow = 4
assert(pixels.count % bytesPerRow == 0)
let pixels2d: [[UInt8]] = stride(from: 0, to: pixels.count, by: bytesPerRow).map {
    Array(pixels[$0..<$0+bytesPerRow])
}

But with the value semantics of Swift Arrays, all attempt to create new nested Array requires copying the content, so may not be "efficient" enough for your purpose.

Re-consider if you really need such nested Array.

answered Sep 9, 2016 at 22:32

OOPer

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3 Comments

VYT Over a year ago

I have an array of pixelValues [UInt8] and have tried convert it using map, and end up with [ArraySlice<UInt8>] instead of [[UInt8]]. Does it make sense to use ArraySlice<UInt8>?

OOPer Over a year ago

ArraySlice is also a value type, but its copy-on-write feature would save some copying cost in some cases, all the same in some other cases. Depends on how you use that "2d-array".

OOPer Over a year ago

@VYT, I need to note that indices of ArraySlice does not always start with 0. That may not be what you want.

Faris Abu Saleem · Accepted Answer · 2020-05-22 02:17:11Z

This should work

private func convert1Dto2DArray(oneDArray:[String], stringsPerRow:Int)->[[String]]?{
       var target = oneDArray
       var outOfIndexArray:[String] = [String]()

       let reminder = oneDArray.count % stringsPerRow

       if reminder > 0 && reminder <= stringsPerRow{
           let suffix = oneDArray.suffix(reminder)
           let list = oneDArray.prefix(oneDArray.count - reminder)
           target = Array(list)
           outOfIndexArray = Array(suffix)
       }

       var array2D: [[String]] = stride(from: 0, to: target.count, by: stringsPerRow).map {
           Array(target[$0..<$0+stringsPerRow])}

       if !outOfIndexArray.isEmpty{
           array2D.append(outOfIndexArray)
       }

       return array2D
   }

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