Array 陣列

JavaScript 中，array 並不是 primitive data type，當我們有需要將用途或性質相近的數據存儲在一起， array 即可派上用場

let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];

具有以下核心特徵

1. 可調整大小的，並且可以包含不同資料類型的混合

   let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
   // array的值被稱為元素(element)，目前有5元素，長度5
   
   let anotherArray = [null, false, "Darren", 3.14, undefined];
   

2. 元素必須使用非負整數作為 index 來訪問(= string index)

   let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
   
   console.log(friends[0]);  // -> Grace
   

3. 第一個元素在 index 0 處，第二個在 index 1 處，依此類推。最後一個元素在 Array 的長度減 1 處

4. 會複製 reference

   let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
   friends[0] = "Michael";
   console.log(friends);  // -> ['Michael', 'Mike', 'Darren', 'Amanda', 'Candy']
   

   let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
   let anotherVariable = friends;
   anotherVariable[0] = "Michael";
   
   console.log("friends array變成是:");
   console.log(friends);
   // -> ['Michael', 'Mike', 'Darren', 'Amanda', 'Candy']
   //因為copy by reference的關係，friends的值也被更動
   
   console.log("anotherVariable array變成是:");
   console.log(anotherVariable);  // -> ['Michael', 'Mike', 'Darren', 'Amanda', 'Candy']
   

Array properties

length

let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
console.log(friends.length);

Array methods

• push(element) - 將一個或多個元素添加到陣列的末尾，並return陣列的新長度

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  friends.push("Jared");
  console.log(friends);  // -> ['Grace', 'Mike', 'Darren', 'Amanda', 'Candy', 'Jared']
  

  array push

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  let returnValue = friends.push("Jared");
  console.log(returnValue);  // -> 6，return value 是陣列的新長度
  

• pop() - 從陣列中刪除 最後一個元素並返回該元素

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  friends.pop();
  console.log(friends);  // -> ['Grace', 'Mike', 'Darren', 'Amanda']
  

• shift() - 從陣列中刪除 第一個元素並返回刪除的元素

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  friends.shift();
  console.log(friends);  // -> ['Mike', 'Darren', 'Amanda', 'Candy']
  

  知道刪去誰

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  let myFriend = friends.shift();
  console.log(myFriend);  // -> Grace
  

• unshift(element) - 將一個或多個元素添加到陣列的開頭，並return陣列的新長度

  let friends = ["Grace", "Mike", "Darren", "Amanda", "Candy"];
  friends.unshift("Wee");
  console.log(friends);  // -> ['Wee', 'Grace', 'Mike', 'Darren', 'Amanda', 'Candy']
  

當Array內部的元素還有Array時，就被稱為是array of arrays，這樣的語法是可行的

let arrs = ["Phoebe", ["fb","ig", [[], [], ["Darren"]]], false, []];

取值方法

let myArr = [
  ["name", "address", "age"],
  ["Phoebe", "Taiwan", 23],
  ["Darren", "America", 25],
];

console.log(myArr[1][0]);  // -> Phoebe

primitive data type V.S reference data type

區分之的原因

• 通常 primitive data type 使用的 RAM 比較小，但 reference data type 很占 RAM
• 用 copy by reference，當遇到物件導向程式設計 OOP 會事半功倍

primitive data type

• copy by value

  let deposit = 500;
  let another = deposit;
  another = 600;  // -> deposit 500, another 600
  

reference data type

• copy by 引用 reference

  let friends = ["a", "b", "c"];
  let another = friends;
  another[0] = "d";  // -> friends ["d", "b", "c"], another ["d", "b", "c"]
  

reference data type比較 需注意的事

let array1 = [1, 2, 3];
let array2 = [1, 2, 3];

console.log(array1 == array2);  // -> false，因為程式看的是RAM的位置

let array1 = [1, 2, 3];
let array2 = array1;  // assignment operation

console.log(array1 == array2);  // -> true

下一篇文章來學習函數的時間複雜度。