test.swift

import Foundation

// This example comes from https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Subscripts.html

struct Matrix {
  let rows: Int, columns: Int
  var grid: [Double]
  init(rows: Int, columns: Int) {
    self.rows = rows
    self.columns = columns
    grid = Array(repeating: 0.0, count: rows * columns)
  }
  func indexIsValid(row: Int, column: Int) -> Bool {
    return row >= 0 && row < rows && column >= 0 && column < columns
  }
  subscript(row: Int, column: Int) -> Double {
    get {
      assert(indexIsValid(row: row, column: column), "Index out of range")
      return grid[(row * columns) + column]
    }
    set {
      assert(indexIsValid(row: row, column: column), "Index out of range")
      grid[(row * columns) + column] = newValue
    }
  }
}

var matrix = Matrix(rows: 2, columns: 2)

let x = matrix[0][0] // error: "Cannot convert value of type 'Int' to expected arguemnt type `(Int, Int).