本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

# \$1ltrim
<a name="ltrim"></a>

4.0 版的新功能。

Elastic 叢集不支援。

Amazon DocumentDB 中的`$ltrim`運算子用於從字串中移除前置字元。根據預設，它會移除前導空格字元，但您也可以透過傳遞字元引數來指定要移除的一組字元。

**參數**
+ `input`：要從中移除前導空格字元的輸入字串。
+ `chars`：（選用） 移除特定字元。

## 範例 (MongoDB Shell)
<a name="ltrim-examples"></a>

下列範例示範 的使用方式`$ltrim`，以從字串開頭移除指定的字元 (" \$1")。

**建立範例文件**

```
db.collection.insertMany([
  { name: " *John Doe", age: 30 },
  { name: "Jane Doe*", age: 25 },
  { name: "  Bob Smith  ", age: 35 }
]);
```

**查詢範例**

```
db.collection.aggregate([
  {
    $project: {
      _id: 0,
      name: {
        $ltrim: { input: "$name", chars: " *" }  
      },
      age: 1
    }
  }
]);
```

**輸出**

```
[
  { "name": "John Doe", "age": 30 },
  { "name": "Jane Doe ", "age": 25 },
  { "name": "Bob Smith  ", "age": 35 }
]
```

## 程式碼範例
<a name="ltrim-code"></a>

若要檢視使用 `$ltrim`命令的程式碼範例，請選擇您要使用的語言標籤：

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');

  try {
    await client.connect();

    const db = client.db('test');
    const collection = db.collection('collection');

    const pipeline = [
      {
        $project: {
          _id: 0,
          name: {
            $ltrim: {
              input: '$name',
              chars: ' *'
            }
          },
          age: 1
        }
      }
    ];

    const result = await collection.aggregate(pipeline).toArray();

    console.dir(result, { depth: null });

  } finally {
    await client.close();
  }
}

example().catch(console.error);
```

------
#### [ Python ]

```
from pymongo import MongoClient

def example():
    try:
        client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')

        db = client.test
        collection = db.collection

        pipeline = [
            {
                "$project": {
                    "_id": 0,
                    "name": {
                        "$ltrim": {
                            "input": "$name",
                            "chars": " *"
                        }
                    },
                    "age": 1
                }
            }
        ]

        results = collection.aggregate(pipeline)

        for doc in results:
            print(doc)

    except Exception as e:
        print(f"An error occurred: {e}")
    
    finally:
        client.close()

example()
```

------