corpustools (version 0.5.1)

tCorpus$subset: Subset a tCorpus

Description

Returns the subset of a tCorpus. The selection can be made separately (and simultaneously) for the token data (using subset) and the meta data (using subset_meta). The subset arguments work according to the subset.data.table function.

There are two flavours. You can either use subset(tc, ...) or tc$subset(...). The difference is that the second approach changes the tCorpus by reference. In other words, tc$subset() will delete the rows from the tCorpus, instead of creating a new tCorpus. Modifying the tCorpus by reference is more efficient (which becomes important if the tCorpus is large), but the more classic subset(tc, ...) approach is often more obvious.

Subset can also be used to select rows based on token/feature frequences. This is a common step in corpus analysis, where it often makes sense to ignore very rare and/or very frequent tokens. To do so, there are several special functions that can be used within a subset call. The freq_filter() and docfreq_filter() can be used to filter terms based on term frequency and document frequency, respectively. (see examples)

The subset_meta() method is an alternative for using subset(subset_meta = ...), that is added for consistency with the other _meta methods.

Note that you can also use the tCorpus$feature_subset method if you want to filter out low/high frequency tokens, but do not want to delete the rows in the tCorpus.

Usage:

## R6 method for class tCorpus. Use as tc$method (where tc is a tCorpus object).

subset(tc, subset = NULL, subset_meta = NULL, 
       window = NULL)
tc$subset(subset = NULL, subset_meta = NULL,
          window = NULL, copy = F)
tc$subset_meta(subset = NULL, copy = F)

Arguments

subset

logical expression indicating rows to keep in the tokens data.

subset_meta

logical expression indicating rows to keep in the document meta data.

window

If not NULL, an integer specifiying the window to be used to return the subset. For instance, if the subset contains token 10 in a document and window is 5, the subset will contain token 5 to 15. Naturally, this does not apply to subset_meta.

copy

If TRUE, the method returns a new tCorpus object instead of subsetting the current one. This is added for convenience when analyzing a subset of the data. e.g., tc_nyt = tc$subset_meta(medium == "New_York_Times", copy=T)

Examples

tc = create_tcorpus(sotu_texts[1:5,], doc_column = 'id')
tc$n ## original number of tokens

## select only first 20 tokens per document
tc2 = subset(tc, token_id < 20)
tc2$n

## Note that the original is untouched
tc$n

## Now we subset by reference. This doesn't make a copy, but changes tc itself
tc$subset(token_id < 20)
tc$n 

## you can filter on term frequency and document frequency with the freq_filter() and
## docfreq_filter() functions
tc = create_tcorpus(sotu_texts[c(1:5,800:805),], doc_column = 'id')
tc$subset( freq_filter(token, min = 2, max = 4) )
tc$tokens

###### subset can be used for meta data by using the subset_meta argument, or the subset_meta method
tc$n_meta
tc$meta
tc$subset(subset_meta = president == 'Barack Obama')
tc$n_meta