如何判断元素所属列表？R语言data.table列表名称匹配实现

Solution #

First, let's recap your setup to ensure we're working with the same data:

# Define your category list
catlist <- list(
  c("< 30 days","1-3 months","4-6 months"),
  c("7-12 months"),
  c("1-3 years"),
  c("4-5 years","5+ years","never")
)
names(catlist) <- 1:length(catlist)

# Load data.table and create your temp table
library(data.table)
tmp <- data.table(variable = c("never","1-3 years"))

1. Add new_variable to map values to list names

The most efficient way (especially for large datasets) is to create a lookup table first, then join it to your tmp data.table. This leverages data.table's fast join operations:

# Create a lookup table that maps each category to its list name
lookup <- data.table(
  variable = unlist(catlist),
  new_variable = rep(names(catlist), sapply(catlist, length))
)

# Join the lookup table to tmp to add the new column
tmp <- merge(tmp, lookup, by = "variable", all.x = TRUE)

Alternatively, you can use an update join to modify tmp in place without creating a new object:

tmp[lookup, on = "variable", new_variable := i.new_variable]

After either approach, your tmp table will look like this:

variable new_variable
1: 1-3 years            3
2:     never            4

2. Check if elements are present in catlist

To verify if values in variable exist anywhere in catlist, use the %in% operator with unlist(catlist) (which flattens the list into a single vector):

tmp[, is_present := variable %in% unlist(catlist)]

This adds a logical column where TRUE means the value is found in catlist, and FALSE means it's not. For your sample data, both rows will show TRUE.

Alternative: Using sapply for small datasets

If you're working with a small dataset and prefer a more concise (though less efficient) approach, you can use nested sapply calls to directly map each value to its list name:

tmp[, new_variable := sapply(variable, function(x) {
  names(catlist)[sapply(catlist, function(y) x %in% y)]
})]

This works by checking each value against every element in catlist and returning the corresponding name. However, for large datasets, the lookup table method is significantly faster.

内容的提问来源于stack exchange，提问作者quant