//- 💫 DOCS > USAGE > VECTORS & SIMILARITY > BASICS

+aside("Training word vectors")
    |  Dense, real valued vectors representing distributional similarity
    |  information are now a cornerstone of practical NLP. The most common way
    |  to train these vectors is the #[+a("https://en.wikipedia.org/wiki/Word2vec") word2vec]
    |  family of algorithms. If you need to train a word2vec model, we recommend
    |  the implementation in the Python library
    |  #[+a("https://radimrehurek.com/gensim/") Gensim].

include ../_spacy-101/_similarity
include ../_spacy-101/_word-vectors

+h(3, "in-context") Similarities in context

p
    |  Aside from spaCy's built-in word vectors, which were trained on a lot of
    |  text with a wide vocabulary, the parsing, tagging and NER models also
    |  rely on vector representations of the #[strong meanings of words in context].
    |  As the  #[+a("/usage/processing-pipelines") processing pipeline] is
    |  applied spaCy encodes a document's internal meaning representations as an
    |  array of floats, also called a tensor. This allows spaCy to make a
    |  reasonable guess at a word's meaning, based on its surrounding words.
    |  Even if a word hasn't been seen before, spaCy will know #[em something]
    |  about it. Because spaCy uses a 4-layer convolutional network, the
    |  tensors are sensitive to up to #[strong four words on either side] of a
    |  word.

p
    |  For example, here are three sentences containing the out-of-vocabulary
    |  word "labrador" in different contexts.

+code.
    doc1 = nlp(u"The labrador barked.")
    doc2 = nlp(u"The labrador swam.")
    doc3 = nlp(u"the labrador people live in canada.")

    for doc in [doc1, doc2, doc3]:
        labrador = doc[1]
        dog = nlp(u"dog")
        print(labrador.similarity(dog))

p
    |  Even though the model has never seen the word "labrador", it can make a
    |  fairly accurate prediction of its similarity to "dog" in different
    |  contexts.

+table(["Context", "labrador.similarity(dog)"])
    +row
        +cell The #[strong labrador] barked.
        +cell #[code 0.56] #[+procon("yes", "similar")]

    +row
        +cell The #[strong labrador] swam.
        +cell #[code 0.48] #[+procon("no", "dissimilar")]

    +row
        +cell the #[strong labrador] people live in canada.
        +cell #[code 0.39] #[+procon("no", "dissimilar")]

p
    |  The same also works for whole documents. Here, the variance of the
    |  similarities is lower, as all words and their order are taken into
    |  account. However, the context-specific similarity is often still
    |  reflected pretty accurately.

+code.
    doc1 = nlp(u"Paris is the largest city in France.")
    doc2 = nlp(u"Vilnius is the capital of Lithuania.")
    doc3 = nlp(u"An emu is a large bird.")

    for doc in [doc1, doc2, doc3]:
        for other_doc in [doc1, doc2, doc3]:
            print(doc.similarity(other_doc))

p
    |  Even though the sentences about Paris and Vilnius consist of different
    |  words and entities, they both describe the same concept and are seen as
    |  more similar than the sentence about emus. In this case, even a misspelled
    |  version of "Vilnius" would still produce very similar results.

+table
    - var examples = {"Paris is the largest city in France.": [1, 0.85, 0.65], "Vilnius is the capital of Lithuania.": [0.85, 1, 0.55], "An emu is a large bird.": [0.65, 0.55, 1]}
    - var counter = 0

    +row
    +row
        +cell
        for _, label in examples
            +cell=label

    each cells, label in examples
        +row(counter ? null : "divider")
            +cell=label
            for cell in cells
                +cell.u-text-center
                    - var result = cell < 0.7 ? ["no", "dissimilar"] : cell != 1 ? ["yes", "similar"] : ["neutral", "identical"]
                    |  #[code=cell.toFixed(2)] #[+procon(...result)]
        - counter++

p
    |  Sentences that consist of the same words in different order will likely
    |  be seen as very similar – but never identical.

+code.
    docs = [nlp(u"dog bites man"), nlp(u"man bites dog"),
            nlp(u"man dog bites"), nlp(u"dog man bites")]

    for doc in docs:
        for other_doc in docs:
            print(doc.similarity(other_doc))

p
    |  Interestingly, "man bites dog" and "man dog bites" are seen as slightly
    |  more similar than "man bites dog" and "dog bites man". This may be a
    |  conincidence – or the result of "man" being interpreted as both sentence's
    |  subject.

+table
    - var examples = {"dog bites man": [1, 0.9, 0.89, 0.92], "man bites dog": [0.9, 1, 0.93, 0.9], "man dog bites": [0.89, 0.93, 1, 0.92], "dog man bites": [0.92, 0.9, 0.92, 1]}
    - var counter = 0

    +row("head")
        +cell
        for _, label in examples
            +cell.u-text-center=label

    each cells, label in examples
        +row(counter ? null : "divider")
            +cell=label
            for cell in cells
                +cell.u-text-center
                    - var result = cell < 0.7 ? ["no", "dissimilar"] : cell != 1 ? ["yes", "similar"] : ["neutral", "identical"]
                    |  #[code=cell.toFixed(2)] #[+procon(...result)]
        - counter++