Subsetting & Indexing

Subsetting is the act of pulling specific elements out of an R object using square brackets, with [ ] keeping a subset, [[ ]] extracting one element, and $ reaching a named element of a list.

Learn Subsetting & Indexing in our free R course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free R course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end you'll confidently index by position, by negative position, by logical condition, and by name, and you'll know exactly when to use [ ] versus [[ ]] versus $ on vectors and lists.

What You'll Learn in This Lesson

1️⃣ Indexing a Vector: Four Ways

Single brackets accept four kinds of index. A positive number selects by position, a vector of numbers selects several, a negative number drops positions, and a name (if your vector is named) selects by label. R indexes from 1 .

2️⃣ Logical Indexing & which()

The most powerful form: pass a logical vector and keep the elements where it is TRUE . The comparison itself ( nums > 10 ) produces that logical vector. Use which() when you want the positions instead of the values, and combine tests with & (and) / | (or).

3️⃣ Lists: [ ] vs [[ ]] vs $

Lists hold elements of different types, so the bracket you choose matters. [ ] keeps the list wrapper and returns a shorter list. [[ ]] reaches inside and returns the bare element. $ is a friendly shorthand for [[ ]] by name. Once you have the element with [[ ]] or $ , you can index into it normally.

Memorise the one-liner: [ ] keeps the box, [[ ]] and $ open the box.

Your turn. Fill in the ___ blanks, run it, and compare with the expected output.

Mini-Challenge: Dig Into a List

This challenge forces you to switch between $ , [[ ]] , and [ ] on the same object — exactly the muscle memory you need for real data work.

Common Errors (and the fix)

• ❌ x[0] returns an empty result. ✅ R indexes from 1 — the first element is x[1] .
• ❌ Doing math on person[1] fails (it's still a list). ✅ Use person[[1]] or person$name .
• ❌ Mixing signs: x[c(1, -2)] errors. ✅ Use all-positive or all-negative indices.
• ❌ Using && for element-wise filtering. ✅ Use single & / | inside brackets — && only compares the first element.

Pro Tips

• 💡 Assign into a subset: x[x < 0] <- 0 replaces every negative value with 0 in one line.
• 💡 %in% for membership: x[x %in% c(2, 4)] keeps values found in a set.
• 💡 head()/tail(): head(x, 3) grabs the first three without writing the indices.

📋 Quick Reference — Subsetting

Task

R Syntax

By position

x[2]

Drop a position

x[-1]

By condition

x[x > 10]

Positions that match

which(x > 10)

List element (value)

lst[[1]] / lst$name

List subset (wrapped)

lst[1]

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Index a vector by position , negative position , condition , or name
• ✅ x[x > 10] filters; which() returns the positions
• ✅ [ ] keeps the box , [[ ]] and $ open it
• ✅ Combine conditions with single & and |
• ✅ Next lesson: Missing Data — handling NA with is.na() and na.rm

Practice quiz

Continue this course

• Previous: Data Types & Classes
• Next: Missing Data (NA)