quanteda (version 0.99)

tokens_lookup: apply a dictionary to a tokens object

Description

Convert tokens into equivalence classes defined by values of a dictionary object.

Usage

tokens_lookup(x, dictionary, levels = 1:5, valuetype = c("glob", "regex",
  "fixed"), case_insensitive = TRUE, capkeys = !exclusive,
  exclusive = TRUE, nomatch = NULL, verbose = quanteda_options("verbose"))

Arguments

x

tokens object to which dictionary or thesaurus will be supplied

dictionary

the dictionary-class object that will be applied to x

levels

integers specifying the levels of entries in a hierarchical dictionary that will be applied. The top level is 1, and subsequent levels describe lower nesting levels. Values may be combined, even if these levels are not contiguous, e.g. `levels = c(1:3)` will collapse the second level into the first, but record the third level (if present) collapsed below the first. (See examples.)

valuetype

how to interpret keyword expressions: "glob" for "glob"-style wildcard expressions; "regex" for regular expressions; or "fixed" for exact matching. See valuetype for details.

case_insensitive

ignore the case of dictionary values if TRUE uppercase to distinguish them from other features

capkeys

if TRUE, convert dictionary keys to uppercase to distinguish them from other features

exclusive

if TRUE, remove all features not in dictionary, otherwise, replace values in dictionary with keys while leaving other features unaffected

nomatch

an optional character naming a new key for tokens that do not matched to a dictionary values If NULL (default), do not record unmatched tokens.

verbose

print status messages if TRUE

Examples

# NOT RUN {
toks <- tokens(data_corpus_inaugural)
dict <- dictionary(list(country = "united states", 
                   law=c('law*', 'constitution'), 
                   freedom=c('free*', 'libert*')))
dfm(tokens_lookup(toks, dict, valuetype='glob', verbose = TRUE))
dfm(tokens_lookup(toks, dict, valuetype='glob', verbose = TRUE, nomatch = 'NONE'))

dict_fix <- dictionary(list(country = "united states", 
                       law = c('law', 'constitution'), 
                       freedom = c('freedom', 'liberty'))) 
# dfm(applyDictionary(toks, dict_fix, valuetype='fixed'))
dfm(tokens_lookup(toks, dict_fix, valuetype='fixed'))

# hierarchical dictionary example
txt <- c(d1 = "The United States has the Atlantic Ocean and the Pacific Ocean.",
         d2 = "Britain and Ireland have the Irish Sea and the English Channel.")
toks <- tokens(txt)
dict <- dictionary(list(US = list(Countries = c("States"), 
                                  oceans = c("Atlantic", "Pacific")),
                        Europe = list(Countries = c("Britain", "Ireland"),
                                      oceans = list(west = "Irish Sea", 
                                                    east = "English Channel"))))
tokens_lookup(toks, dict, levels = 1)
tokens_lookup(toks, dict, levels = 2)
tokens_lookup(toks, dict, levels = 1:2)
tokens_lookup(toks, dict, levels = 3)
tokens_lookup(toks, dict, levels = c(1,3))
tokens_lookup(toks, dict, levels = c(2,3))

# show unmatched tokens
tokens_lookup(toks, dict, nomatch = "_UNMATCHED")

# }