I added a playground for the 2D array

Author: Thecmar7

Array2D/Array2D.playground/Contents.swift

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+/*
+Two-dimensional array with a fixed number of rows and columns.
+This is mostly handy for games that are played on a grid, such as chess.
+Performance is always O(1).
+*/
+public struct Array2D<T> {
+    public let columns: Int
+    public let rows: Int
+    private var array: [T]
+    
+    public init(columns: Int, rows: Int, initialValue: T) {
+        self.columns = columns
+        self.rows = rows
+        array = .init(count: rows*columns, repeatedValue: initialValue)
+    }
+    
+    public subscript(column: Int, row: Int)  -> T {
+        get {
+            return array[row*columns + column]
+        }
+        set {
+            array[row*columns + column] = newValue
+        }
+    }
+
+}
+
+// initialization
+var Array2DNumbers = Array2D(columns: 3, rows: 5, initialValue: 0)
+
+// makes an array of rows * columns elements all filled with zero
+print(Array2DNumbers.array)
+
+//setting numbers
+Array2DNumbers[0, 0] = 1
+Array2DNumbers[1, 0] = 2
+
+Array2DNumbers[0, 1] = 3
+Array2DNumbers[1, 1] = 4
+
+Array2DNumbers[0, 2] = 5
+Array2DNumbers[1, 2] = 6
+
+Array2DNumbers[0, 3] = 7
+Array2DNumbers[1, 3] = 8
+Array2DNumbers[2, 3] = 9
+
+// now the numbers are set in the array
+print(Array2DNumbers.array)
+
+// printint out the 2D array with a reference around the grid [x, y]
+print("  ", terminator: "")
+for k in 0..<Array2DNumbers.columns {
+    print(" \(k)", terminator:"")
+    if (k == Array2DNumbers.columns - 1){
+        print(" x")
+    }
+}
+
+for i in 0..<Array2DNumbers.rows {
+    print("\(i) [", terminator: "");
+    for j in 0..<Array2DNumbers.columns {
+        if (j == Array2DNumbers.columns - 1) {
+            print("\(Array2DNumbers[j, i])", terminator: "");
+        } else {
+            print("\(Array2DNumbers[j, i]) ", terminator: "");
+        }
+    }
+    print("]");
+}
+print("y")
+
+

Array2D/Array2D.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

Array2D/Array2D.playground/timeline.xctimeline

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+<?xml version="1.0" encoding="UTF-8"?>
+<Timeline
+   version = "3.0">
+   <TimelineItems>
+   </TimelineItems>
+</Timeline>