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（空气呼啸）
(air whooshing)

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- 如何对数据集进行切片和切块？
- How to slice and dice the dataset?

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大多数时候，你使用的数据
Most of the time, the data you work with

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不会为训练模型做好充分准备。
won't be perfectly prepared for training models.

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在本视频中，我们将探索各种功能
In this video, we'll explore various features

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数据集库提供的用于清理数据的数据集。
that the datasets library provides to clean up your data.

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数据集库提供了几种内置方法
The datasets library provides several built-in methods

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允许你以各种方式处理数据。
that allow you to wrangle your data in various ways.

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在本视频中，我们将了解如何随机播放
In this video, we'll see how you can shuffle

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并拆分你的数据，选择你感兴趣的行，
and split your data, select the rows you're interested in,

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调整列，
tweak the columns,

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并使用 map 方法应用处理函数。
and apply processing functions with the map method.

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让我们从洗牌开始。
Let's start with shuffling.

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这通常是个好主意
It is generally a good idea

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将改组应用于你的训练集
to apply shuffling to your training set

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这样你的模型就不会学习
so that your model doesn't learn

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任何人工排序数据。
any artificial ordering the data.

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如果你想洗牌整个数据集，
If you wanna shuffle the whole dataset,

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你可以应用适当命名的 shuffle 方法。
you can apply the appropriately named shuffle method.

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你可以在这里看到这个方法的一个例子，
You can see an example of this method in action here,

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我们在哪里下载了训练拆分
where we've downloaded the training split

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小队数据集并随机打乱所有行。
of the squad dataset and shuffled all the rows randomly.

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另一种打乱数据的方法
Another way to shuffle the data

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是创建随机训练和测试拆分。
is to create random train and test splits.

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如果你必须创建，这将很有用
This can be useful if you have to create

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你自己的测试从原始数据中分离出来。
your own test splits from raw data.

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为此，你只需应用 train_test_split 方法
To do this, you just apply the train_test_split method

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并指定测试拆分应该有多大。
and specify how large the test split should be.

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在这个例子中，我们指定测试集应该是 10%
In this example, we specify that the test set should be 10%

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总数据集大小。
of the total dataset size.

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可以看到 train_test_split 方法的输出
You can see that the output of the train_test_split method

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是一个 DatasetDict 对象
is a DatasetDict object

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其键对应于新的拆分。
whose keys correspond to the new splits.

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现在我们知道如何打乱数据集了，
Now that we know how to shuffle the dataset,

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让我们来看看返回我们感兴趣的行。
let's take a look at returning the rows we're interested in.

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最常见的方法是使用 select 方法。
The most common way to do this is with the select method.

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此方法需要一个列表
This method expects a list

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或数据集索引的生成器，
or a generator of the datasets indices,

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然后将返回一个新的数据集对象
and will then return a new dataset object

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只包含那些行。
containing just those rows.

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如果你想创建一个随机的行样本，
If you wanna create a random sample of rows,

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你可以通过链接洗牌来做到这一点
you can do this by chaining the shuffle

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并一起选择方法。
and select methods together.

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在这个例子中，
In this example,

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我们创建了一个包含五个元素的样本
we've created a sample of five elements

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来自小队数据集。
from the squad dataset.

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在数据集中挑选特定行的最后一种方法
The last way to pick out specific rows in a dataset

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是通过应用过滤器方法。
is by applying the filter method.

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此方法检查每一行是否
This method checks whether each row

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是否满足某种条件。
fulfills some condition or not.

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例如，这里我们创建了一个小的 lambda 函数
For example, here we've created a small lambda function

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检查标题是否以字母 L 开头。
that checks whether the title starts with the letter L.

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一旦我们用 filter 方法应用这个函数，
Once we apply this function with the filter method,

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我们得到仅包含这些行的数据子集。
we get a subset of the data just containing these rows.

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到目前为止，我们一直在谈论数据集的行，
So far, we've been talking about the rows of a dataset,

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但是列呢？
but what about the columns?

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数据集库有两个主要方法
The datasets library has two main methods

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用于转换列，
for transforming columns,

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用于更改列名称的 rename_column 方法
a rename_column method to change the name of the column

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以及删除它们的 remove_columns 方法。
and a remove_columns method to delete them.

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你可以在此处查看这两种方法的示例。
You can see examples of both these methods here.

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一些数据集有嵌套的列，
Some datasets have nested columns,

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你可以通过应用展平方法来扩展它们。
and you can expand these by applying the flatten method.

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例如，在小队数据集中，
For example, in the squad dataset,

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answers 列包含文本和 answer_start 字段。
the answers column contains a text and answer_start field.

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如果我们想将它们提升到各自的专栏中，
If we wanna promote them to their own separate columns,

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我们可以应用扁平化，如图所示。
we can apply flatten as shown here.

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当然，现在不讨论数据集库
Now of course, no discussion of the datasets library

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不提著名的 map 方法就完整了。
would be complete without mentioning the famous map method.

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此方法应用自定义处理功能
This method applies a custom processing function

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到数据集中的每一行。
to each row in the dataset.

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比如这里我们先定义
For example, here we first define

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小写标题函数，
a lowercase title function,

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这只是将标题列中的文本小写。
that simply lowercases the text in the title column.

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然后我们将该函数提供给 map 方法，
And then we feed that function to the map method,

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瞧，我们现在有了小写标题。
and voila, we now have lowercase titles.

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map 方法也可用于批量输入行
The map method can also be used to feed batches of rows

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到处理函数。
to the processing function.

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这对于标记化特别有用
This is especially useful for tokenization

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其中分词器由分词器库支持，
where the tokenizer is backed by the Tokenizers library,

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他们可以使用快速多线程
and they can use fast multithreading

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并行处理批次。
to process batches in parallel.

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（空气呼啸）
(air whooshing)

