#
Process text data

This guide shows specific methods for processing text datasets. Learn how to:

- Tokenize a dataset with map().
- Align dataset labels with label ids for NLI datasets.

For a guide on how to process any type of dataset, take a look at the general process guide.

##
Map

The map() function supports processing batches of examples at once which speeds up tokenization.

Load a tokenizer from 🤗 Transformers:

    >>> from transformers import AutoTokenizer

    >>> tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")

Set the `batched` parameter to `True` in the map() function to apply the tokenizer to batches of examples:

    >>> dataset = dataset.map(lambda examples: tokenizer(examples["text"]), batched=True)
    >>> dataset[0]
    {'text': 'the rock is destined to be the 21st century\'s new " conan " and that he\'s going to make a splash even greater than arnold schwarzenegger , jean-claud van damme or steven segal .',
     'label': 1,
     'input_ids': [101, 1996, 2600, 2003, 16036, 2000, 2022, 1996, 7398, 2301, 1005, 1055, 2047, 1000, 16608, 1000, 1998, 2008, 2002, 1005, 1055, 2183, 2000, 2191, 1037, 17624, 2130, 3618, 2084, 7779, 29058, 8625, 13327, 1010, 3744, 1011, 18856, 19513, 3158, 5477, 4168, 2030, 7112, 16562, 2140, 1012, 102],
     'token_type_ids': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
     'attention_mask': [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]}

The map() function converts the returned values to a PyArrow-supported format. But explicitly returning the tensors as NumPy arrays is faster because it is a natively supported PyArrow format. Set `return_tensors="np"` when you tokenize your text:

    >>> dataset = dataset.map(lambda examples: tokenizer(examples["text"], return_tensors="np"), batched=True)

##
Align

The align_labels_with_mapping() function aligns a dataset label id with the label name. Not all 🤗 Transformers models follow the prescribed label mapping of the original dataset, especially for NLI datasets. For example, the MNLI dataset uses the following label mapping:

    >>> label2id = {"entailment": 0, "neutral": 1, "contradiction": 2}

To align the dataset label mapping with the mapping used by a model, create a dictionary of the label name and id to align on:

    >>> label2id = {"contradiction": 0, "neutral": 1, "entailment": 2}

Pass the dictionary of the label mappings to the align_labels_with_mapping() function, and the column to align on:

    >>> from datasets import load_dataset

    >>> mnli = load_dataset("glue", "mnli", split="train")
    >>> mnli_aligned = mnli.align_labels_with_mapping(label2id, "label")

You can also use this function to assign a custom mapping of labels to ids.