S3 and S4 introduction

Last reviewed Jun 1, 2026 Content v20260601

Track mode: server_script
Means: Server runner
Reading: ~1 min
Level: advanced

This lesson

An orientation to the R track—how the compiled playground works, core vocabulary, and what you will practice next.

You need a clear map of the R track so vectors, data frames, factors, and the tidyverse mindset do not feel like magic.

You will apply S3 and S4 introduction in contexts like: Research pipelines, Shiny dashboards, and statistical reporting.

Write R in the editor and click Run on server—the dev runner executes with Rscript; use print() or cat() and base R in playground snippets (tidyverse locally; LEARNING_RUNNER_ENABLED=true). Also read the interview prep blocks.

After basic programming and ideally SQL—pair with Python for ML-heavy roles or stay in R for stats and research pipelines.

R's OOP systems: S3 generic functions (print, summary, plot) dispatch on class attributes; S4 formal classes for stricter contracts (Bioconductor).

S3 class

fit <- lm(y ~ x, data = data.frame(x = 1:3, y = c(1, 2, 2)))
print(class(fit))
print(summary(fit))

Generic functions

summary() behaves differently for lm, data.frame, and numeric—method dispatch via UseMethod internally.

Important interview questions and answers

Q: S3 vs S4?
A: S3 informal attributes; S4 slots and validity—S3 dominates base stats.
Q: class() purpose?
A: Returns class vector driving method dispatch and print formatting.

Self-check

What does class(fit) show for lm?
Name one S3 generic function.

Tip: summary(), print(), and plot() are S3 generics—behavior depends on class.

Interview prep

S3 generic example?: print, summary, plot dispatch on class.

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

Self-reflection (saved on this device)

Not saved yet.

Playground

Runs on the configured server runner (dev: npm run runner with LEARNING_RUNNER_ENABLED=true). Output appears below the editor.

Code runner not available

Server runner is disabled. Set LEARNING_RUNNER_ENABLED=true and LEARNING_RUNNER_URL in .env (see .env.example).

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

S3 method dispatch?
Use S4 when?

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