Convert token sequences into compound tokens

Replace multi-token sequences with a multi-word, or "compound" token. The resulting compound tokens will represent a phrase or multi-word expression, concatenated with concatenator (by default, the "_" character) to form a single "token". This ensures that the sequences will be processed subsequently as single tokens, for instance in constructing a dfm.

tokens_compound(
  x,
  pattern,
  valuetype = c("glob", "regex", "fixed"),
  concatenator = concat(x),
  window = 0L,
  case_insensitive = TRUE,
  join = TRUE,
  apply_if = NULL
)

Arguments

x

an input tokens object

pattern

a character vector, list of character vectors, dictionary, or collocations object. See pattern for details.

valuetype

the type of pattern matching: "glob" for "glob"-style wildcard expressions; "regex" for regular expressions; or "fixed" for exact matching. See valuetype for details.

concatenator

character; the concatenation character that will connect the tokens making up a multi-token sequence.

window

integer; a vector of length 1 or 2 that specifies size of the window of tokens adjacent to pattern that will be compounded with matches to pattern. The window can be asymmetric if two elements are specified, with the first giving the window size before pattern and the second the window size after. If paddings (empty "" tokens) are found, window will be shrunk to exclude them.

case_insensitive

logical; if TRUE, ignore case when matching a pattern or dictionary values

join

logical; if TRUE, join overlapping compounds into a single compound; otherwise, form these separately. See examples.

apply_if

logical vector of length ndoc(x); documents are modified only when corresponding values are TRUE, others are left unchanged.

Value

A tokens object in which the token sequences matching pattern

have been replaced by new compounded "tokens" joined by the concatenator.

Note

Patterns to be compounded (naturally) consist of multi-word sequences, and how these are expected in pattern is very specific. If the elements to be compounded are supplied as space-delimited elements of a character vector, wrap the vector in phrase(). If the elements to be compounded are separate elements of a character vector, supply it as a list where each list element is the sequence of character elements.

See the examples below.

Examples

txt <- "The United Kingdom is leaving the European Union."
toks <- tokens(txt, remove_punct = TRUE)

# character vector - not compounded
tokens_compound(toks, c("United", "Kingdom", "European", "Union"))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"      "United"   "Kingdom"  "is"       "leaving"  "the"      "European"
#> [8] "Union"   
#> 

# elements separated by spaces - not compounded
tokens_compound(toks, c("United Kingdom", "European Union"))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"      "United"   "Kingdom"  "is"       "leaving"  "the"      "European"
#> [8] "Union"   
#> 

# list of characters - is compounded
tokens_compound(toks, list(c("United", "Kingdom"), c("European", "Union")))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"            "United_Kingdom" "is"             "leaving"       
#> [5] "the"            "European_Union"
#> 

# elements separated by spaces, wrapped in phrase() - is compounded
tokens_compound(toks, phrase(c("United Kingdom", "European Union")))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"            "United_Kingdom" "is"             "leaving"       
#> [5] "the"            "European_Union"
#> 

# supplied as values in a dictionary (same as list) - is compounded
# (keys do not matter)
tokens_compound(toks, dictionary(list(key1 = "United Kingdom",
                                      key2 = "European Union")))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"            "United_Kingdom" "is"             "leaving"       
#> [5] "the"            "European_Union"
#> 
# pattern as dictionaries with glob matches
tokens_compound(toks, dictionary(list(key1 = c("U* K*"))), valuetype = "glob")
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"            "United_Kingdom" "is"             "leaving"       
#> [5] "the"            "European"       "Union"         
#> 

# note the differences caused by join = FALSE
compounds <- list(c("the", "European"), c("European", "Union"))
tokens_compound(toks, pattern = compounds, join = TRUE)
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"                "United"             "Kingdom"           
#> [4] "is"                 "leaving"            "the_European_Union"
#> 
tokens_compound(toks, pattern = compounds, join = FALSE)
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "The"            "United"         "Kingdom"        "is"            
#> [5] "leaving"        "the_European"   "European_Union"
#> 

# use window to form ngrams
tokens_remove(toks, pattern = stopwords("en")) |>
    tokens_compound(pattern = "leav*", join = FALSE, window = c(0, 3))
#> Tokens consisting of 1 document.
#> text1 :
#> [1] "United"                 "Kingdom"                "leaving_European_Union"
#>