subtitles/en/15_subword-based-tokenizers.srt

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- Let's take a look at
subword based tokenization.

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Understanding why subword
based tokenization is

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interesting requires
understanding the flaws

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of word based and corrector
based tokenization.

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If you haven't seen the first videos

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on word based and character
based tokenization

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we recommend you check them

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out before looking at this video.

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Subword based tokenization
lies in between character based

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and word based tokenization algorithms.

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The idea is to find a middle ground

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between very large vocabularies

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a large quantity of out vocabulary tokens

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and a loss of meaning
across very similar words

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for word based tokenizers
and very long sequences

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as well as less meaningful
individual tokens.

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For character based tokenizers.

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These algorithms rely on
the following principle.

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Frequently used words should not be split

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into smaller subwords

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while rare words should be decomposed

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into meaningful subwords.

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An example is the word dog.

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We would like to have our
tokenizer to have a single ID

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for the word dog rather

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than splitting it into
correctors D O and G.

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However, when encountering the word dogs

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we would like our tokenize to
understand that at the root

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this is still the word dog.

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With an added S, that
slightly changes the meaning

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while keeping the original idea.

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Another example is a complex
word like tokenization

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which can be split into
meaningful subwords.

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The root

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of the word is token and
-ization completes the root

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to give it a slightly different meaning.

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It makes sense to split the word

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into two, token as the root of the word,

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labeled as the start of the word

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and ization as additional
information labeled

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as a completion of the word.

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In turn, the model will
now be able to make sense

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of token in different situations.

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It will understand that the
word's token, tokens, tokenizing

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and tokenization have a
similar meaning and are linked.

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It's will also understand that
tokenization, modernization

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and immunization, which
all have the same suffixes

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are probably used in the
same syntactic situations.

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Subword based tokenizers
generally have a way to

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identify which tokens are a start of word

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and which tokens complete start of words.

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So here token as the start

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of a ward and hash hash
ization as completion of award.

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Here, the hash hash prefix
indicates that ization is part

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of award rather than the beginning of it.

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The hash hash comes

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from the BERT tokenizer based
on the word piece algorithm.

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Other tokenizes use other
prefixes which can be

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placed to indicate part of words

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like in here or start of words instead.

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There are a lot

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of different algorithms that can be used

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for subword tokenization

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and most models obtaining
state-of-the-art results

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in English today

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use some kind of subword
tokenization algorithms.

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These approaches help in
reducing the vocabulary sizes

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by sharing information
across different words

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having the ability to have prefixes

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and suffixes understood as such.

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They keep meaning across
very similar words

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by recognizing similar
tokens, making them up.