# Hashing

<table><thead><tr><th width="118">Method</th><th width="224">Set</th><th width="307">Map</th><th>Big O</th></tr></thead><tbody><tr><td>Init</td><td><code>new Set(['1','2']);</code></td><td><code>new Map([ [0,1],[0,2] ]);</code></td><td></td></tr><tr><td>增加</td><td><code>mySet.add('1')</code></td><td><code>myMap.set(0, 1)</code></td><td>O(1)</td></tr><tr><td>刪除</td><td><code>mySet.delete('1')</code></td><td><code>myMap.delete(0)</code></td><td>O(1)</td></tr><tr><td>清掉</td><td><code>mySet.clear()</code></td><td><code>myMap.clear()</code></td><td></td></tr><tr><td>取值</td><td>-</td><td><code>mySet.get(0) // 1</code></td><td>O(1)</td></tr><tr><td>是否存在</td><td><code>mySet.has('1')</code></td><td><code>myMap.has(0)</code></td><td>O(1)</td></tr><tr><td>長度</td><td><code>mySet.size</code></td><td><code>myMap.size</code></td><td></td></tr><tr><td>轉陣列</td><td><code>[...mySet]</code></td><td><code>[...myMap.values()]</code></td><td></td></tr></tbody></table>

## Set

```javascript
// ES6 Set
let instruments = new Set();
// or 一開始就把值存進去
let instruments = new Set(['1','2']);


instruments.add('piano');
instruments.has('guitar'); // false
instruments.delete('guitar'); // 裡面根本沒有 guitar 所以回傳 false
instruments.size; // 1
[...instruments] //['piano']

// or use Array.from, 
Array.from(instruments); 
```

## Map

```javascript
let myMap= new Map();

// or 一開始就存值，記得是 [[]] double array
let myMap = new Map([[keyString ,'value']]);

var keyString = 'I am string',
    keyObj = {},
    keyFunc = function() {},
    keyNumber = 1

// 增加
myMap.set(keyString , 'string value');
myMap.set(keyObj, {obj: 1});
myMap.set(keyFunc , function(){console.log('I am function')});
myMap.set(keyNumber , 100);

// 有幾個
myMap.size; // 4

// 取值
myMap.get(keyObj); // {obj: 1}

// 看是否存在
myMap.has(keyString ); //  true

// 刪掉
myMap.delete(keyNumber); 
myMap.size; // 3

// 轉陣列
[...myMap.values()] // ["string value", {obj: 1}, ƒ]
```

## 其他

```javascript
if( lookup.has(str) ){
    lookup.set(str, lookup.get(str) + 1 )
} else {
    lookup.set(str, 1 )
}
```

可以改寫更簡短

```javascript
lookup.set(str, (lookup.get(str) ?? 0) + 1 )
```

以下同理

```javascript
if( !lookup.has(sortStr) ){
    lookup.set(sortStr, [str] )
} else {
     lookup.set(sortStr, [...lookup.get(sortStr), str] )
}

// 可以變以下
lookup.set(sortStr, [...(lookup.get(sortStr) ?? []), str] )
```


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