Array2D

Array2D

In C and Objective-C, you can write the following line,

int cookies[9][7];

to make a 9x7 grid of cookies. This creates a two-dimensional array of 63 elements. To find the cookie at column 3 and row 6, you can write:

myCookie = cookies[3][6];

This statement is not acceptable in Swift. To create a multi-dimensional array in Swift, you can write:

var cookies = [[Int]]()
for _ in 1...9 {
  var row = [Int]()
  for _ in 1...7 {
    row.append(0)
  }
  cookies.append(row)
}

Then, to find a cookie, you can write:

let myCookie = cookies[3][6]

You can also create the array in a single line of code:

var cookies = [[Int]](repeating: [Int](repeating: 0, count: 7), count: 9)

This looks complicated, but can simplfy it in a helper function:

func dim<T>(_ count: Int, _ value: T) -> [T] {
  return [T](repeating: value, count: count)
}

Then, you can create the array:

var cookies = dim(9, dim(7, 0))

Swift infers that the datatype of the array must be Int because you specified 0 as the default value of the array elements. To use a string instead, you can write:

var cookies = dim(9, dim(7, "yum"))

The dim() function makes it easy to go into even more dimensions:

var threeDimensions = dim(2, dim(3, dim(4, 0)))

The downside of using multi-dimensional arrays or multiple nested arrays in this way is to lose track of what dimension represents what.

Instead, you can create your own type that acts like a 2-D array which is more convenient to use:

public struct Array2D<T> {
  public let columns: Int
  public let rows: Int
  fileprivate var array: [T]
  
  public init(columns: Int, rows: Int, initialValue: T) {
    self.columns = columns
    self.rows = rows
    array = .init(repeating: initialValue, count: rows*columns)
  }
  
  public subscript(column: Int, row: Int) -> T {
    get {
      precondition(column < columns, "Column \(column) Index is out of range. Array<T>(columns: \(columns), rows:\(rows))")
      precondition(row < rows, "Row \(row) Index is out of range. Array<T>(columns: \(columns), rows:\(rows))")
      return array[row*columns + column]
    }
    set {
      precondition(column < columns, "Column \(column) Index is out of range. Array<T>(columns: \(columns), rows:\(rows))")
      precondition(row < rows, "Row \(row) Index is out of range. Array<T>(columns: \(columns), rows:\(rows))")
      array[row*columns + column] = newValue
    }
  }
}

Array2D is a generic type, so it can hold any kind of object, not just numbers.

To create an instance of Array2D, you can write:

var cookies = Array2D(columns: 9, rows: 7, initialValue: 0)

By using the subscript function, you can retrieve an object from the array:

let myCookie = cookies[column, row]

Or, you can change it:

cookies[column, row] = newCookie

Internally, Array2D uses a single one-dimensional array to store the data. The index of an object in that array is given by (row x numberOfColumns) + column, but as a user of Array2D, you only need to think in terms of "column" and "row", and the details will be done by Array2D. This is the advantage of wrapping primitive types into a wrapper class or struct.

Written for Swift Algorithm Club by Matthijs Hollemans