# 【Indexed】

`indexed` 作為 `event` 的參數綴詞是用於在呼叫 `event` 時我們可以用 `indexed` 來作為過濾，找到我們要找的參數。

需要注意的是最多只有三個 `event` 參數可以被宣告 `indexed`。

舉例來說有一個事件為：

```solidity
event Transfer(address indexed _from, address indexed _to, uint256 _value)
```

這代表前端可以有效率的查出 token 的交易資訊：

* 從何地址送出：`tokenContract.Transfer({_from: senderAddress})`
* 被何地址接收：`tokenContract.Transfer({_to: receiverAddress})`
* 從一個地址送往另一個特定地址：`tokenContract.Transfer({_from: senderAddress, _to: receiverAddress})`

只有當我們想要這個 `event` 的參數可以被搜尋的到時才使用 `indexed`。

使用或不使用 indexed 的 gas 花費比較：

```solidity
event NewEvent(address addr)
event NewEvent(address indexed addr) 
```


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