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An n-gram here means a contiguous sequence of n words in the original text. The step extracts all n-grams of a given text, i.e. starting at each individual word in original order. The result is one list of n-grams per input text, where each n-gram is a single text string with individual words separated by spaces (unless configured otherwise). The maximum size and kind of n-grams extracted, as well as how to represent them in the result can be configured via the parameters described below. The step also allows filtering of n-grams based on their frequency in the dataset.

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Usage

The following example shows how the step can be used in a recipe.

Examples

  • Example 1
  • Signature
Using a custom configuration to select the maximum size (n) of the n-grams, and how to represent them:
extract_ngrams(ds.text, ds.lang, {
  "ngrams": {
        "n_max": 4,
        "filters": ["punct", "stops"],
        "attrib": "lower",
        "unigram_lemmas": true,
        "concat": false
    }
}) -> (ds.ngrams)

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Inputs & Outputs

The following are the inputs expected by the step and the outputs it produces. These are generally columns (ds.first_name), datasets (ds or ds[["first_name", "last_name"]]) or models (referenced by name e.g. "churn-clf").
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text
column[text]
required
A text column to extract n-grams from.
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*lang
column[category]
An (optional) column identifying the languages of the corresponding texts. It is used to identify the correct model (spaCy) to use for each text. If the dataset doesn’t contain such a column yet, it can be created using the infer_language step. Ideally, languages should be expressed as two-letter ISO 639-1 language codes, such as “en”, “es” or “de” for English, Spanish or German respectively. We also detect fully spelled out names such as “english”, “German”, “allemande” etc., but it is not guaranteed that we will recognize all possible spellings correctly always, so ISO codes should be preferred.Alternatively, if all texts are in the same language, it can be identified with the language parameter instead.
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ngrams
column[list[category]]
required
A column of lists containing the n-grams extracted from the texts.

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Configuration

The following parameters can be used to configure the behaviour of the step by including them in a json object as the last “input” to the step, i.e. step(..., {"param": "value", ...}) -> (output).

Parameters

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extended_language_support
boolean
default:"false"
Whether to enable support for additional languages. By default, Arabic (“ar”), Catalan (“ca”), Basque (“eu”), and Turkish (“tu”) are not enabled, since they’re supported only by a different class of language models (stanfordNLP’s Stanza) that is much slower than the rest. This parameter can be used to enable them.
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min_language_freq
[number, integer]
default:"0.02"
Minimum number (or proportion) of texts to include a language in processing. Any texts in a language with fewer documents than these will be ignored. Can be useful to speed up processing when there is noise in the input languages, and when ignoring languages with a small number of documents only is acceptable. Values smaller than 1 will be interpreted as a proportion of all texts, and values greater than or equal to 1 as an absolute number of documents.
  • number
  • integer
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{_}
number
number.Values must be in the following range:
0 < {_} < 1
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language
[string, null]
The language of inputs texts. If all texts are in the same language, it can be specified here instead of passing it as an input column. The language will be used to identify the correct spaCy model to parse and analyze the texts. For allowed values, see the comment regarding the lang column above.
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ngrams
object
N-gram configuration. Configure maximum size, which words/tokens to exclude, and how to represent the n-grams in the result.
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n_max
number
default:"4"
N-grams with up to this number of words will be extracted.Values must be in the following range:
1 ≤ n_max ≤ 4
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filters
array[string]
default:"['punct', 'stops']"
Exclude these kinds of tokens from n-grams. For longer n-grams, those containing either extraneous whitespace or any punctuation are automatically excluded. Additionally, if stops is included in filters, n-grams containing stopwords as the first and/or last token are also excluded.
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Item
string
Each item in array.Values must be one of the following:punct stops url digits non_alpha non_ascii
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attrib
string
default:"lower"
Representation of the individual words/tokens to extract. I.e. whether verbatim (text/ortho), lower(-case) or lemmatized. Also see spaCy’s attribute reference in this table for further information.Values must be one of the following:
  • orth
  • lemma
  • lower
  • text
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unigram_lemmas
boolean
default:"true"
Whether unigrams should always be extracted lemmatized, irrespective of the attrib parameter.
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concat
boolean
default:"false"
Whether to separate the words in n-grams with an underscore character instead of a space.
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frequency_filter
object
N-gram frequency filter. Filters n-grams based on the number of texts they occur in.
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min_rows
integer
default:"2"
Minimum number of rows. N-grams not occurring in at least these many rows (texts) will be excluded.Values must be in the following range:
0 ≤ min_rows < inf
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max_rows
number
default:"0.5"
Maximum proportion of rows. N-grams occurring in more than this proportion of rows (texts) will be excluded.Values must be in the following range:
0 ≤ max_rows ≤ 1
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keep_top_n
[integer, null]
Keep n most frequent keywords. Whether to always include the n most frequent n-grams, independent of the other filter parameters. Set to null to ignore.Values must be in the following range:
0 ≤ keep_top_n < inf
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filter_top_n
integer
default:"0"
Exclude n most frequent keywords. Exclude the n most frequent n-grams, even if they passed the other filter conditions.Values must be in the following range:
0 ≤ filter_top_n < inf
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by_lang
boolean
default:"false"
Filter per language. Apply filter conditions separately to texts grouped by language, rather than across all texts.
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