R Lists

How it works

A list can hold values of different types, including other lists and data frames. [ returns a sublist, while [[ extracts one component. $ is convenient for a literal component name, but exact [[ lookup is clearer when a name comes from data. Do not rely on partial-name matching in reusable code.

Run this R example

These examples run locally with R, not PHP, Python or JavaScript. The reviewed baseline is R 4.5.2 on Windows x64/UCRT, using only packages shipped with R. This is a tested version, not a newest-release claim. Install your own R from the official R project; an IDE is optional. Save the complete source as lists.R in a separate learning folder and run Rscript --vanilla lists.R. Keep code and fictional fixtures separate from a real project.

Bhylu has no R browser runner. There is no Try in playground link because the existing Python and JavaScript editors do not execute R. No package installation, account, network dataset, Apache, MySQL or PHP setting is needed to run these base-R sources.

The complete download uses a local scope, restores its temporary options and treats unexpected warnings as errors. Examples that handle an expected warning do so narrowly. Random examples change only their R process’s RNG state. File examples own their temporary output; figure examples leave their named PNG in the current learning folder and refuse to overwrite it. Use a new empty folder to rerun a figure.

Download the complete lists.R · Download all source files with hash checks

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Example

local({
  previous_options <- options(warn = 2, digits = 8, stringsAsFactors = FALSE)
  on.exit(options(previous_options), add = TRUE)
  session <- list(id = "S1", visits = 8L, open = TRUE)
  cat("component:", session[["visits", exact = TRUE]], "\n")
  cat("single bracket class:", class(session["visits"]), "\n")
  cat("missing exact name:", is.null(session[["visit", exact = TRUE]]), "\n")
  invisible(NULL)
})

session <- list(id = "S1", visits = 8L, open = TRUE)
cat("component:", session[["visits", exact = TRUE]], "\n")
cat("single bracket class:", class(session["visits"]), "\n")
cat("missing exact name:", is.null(session[["visit", exact = TRUE]]), "\n")

Expected result

component: 8
single bracket class: list
missing exact name: TRUE

The sublist still wraps its visits element, while double brackets expose the integer value. A missing exact list key returns NULL. Validate required names before treating a missing component as a legitimate empty field.