Logical indexing in R Programming - Step-by-Step Execution

Concept Flow - Logical indexing

Start with vector

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Create logical condition

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Apply condition to vector

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Select elements where condition is TRUE

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Return filtered vector

Logical indexing uses a TRUE/FALSE condition to pick elements from a vector.

Execution Sample

R Programming

x <- c(5, 10, 15, 20, 25)
cond <- x > 15
x[cond]

Selects elements from x that are greater than 15.

Execution Table

Step	Action	Condition/Expression	Result	Output
1	Create vector x	c(5, 10, 15, 20, 25)	x = [5, 10, 15, 20, 25]
2	Evaluate condition	x > 15	cond = [FALSE, FALSE, FALSE, TRUE, TRUE]
3	Apply logical indexing	x[cond]	Select elements where cond is TRUE	[20, 25]
4	End	No more steps

💡 All elements checked; only those with TRUE in cond are selected.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
x	undefined	[5, 10, 15, 20, 25]	[5, 10, 15, 20, 25]	[5, 10, 15, 20, 25]	[5, 10, 15, 20, 25]
cond	undefined	undefined	[FALSE, FALSE, FALSE, TRUE, TRUE]	[FALSE, FALSE, FALSE, TRUE, TRUE]	[FALSE, FALSE, FALSE, TRUE, TRUE]
output	undefined	undefined	undefined	[20, 25]	[20, 25]

Key Moments - 2 Insights

Why does x[cond] only return some elements, not all?

Because cond is a logical vector with TRUE only for elements greater than 15 (see execution_table step 2 and 3). Only those TRUE positions are selected.

What happens if cond has a different length than x?

In R, logical vectors used for indexing must be the same length as x or will recycle, which can cause unexpected results. Here cond matches x length exactly (execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of cond for the third element?

A NA

B TRUE

C FALSE

D 15

💡 Hint

Check the 'Result' column in step 2 of execution_table; cond is [FALSE, FALSE, FALSE, TRUE, TRUE]

At which step does the program select elements from x using cond?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table; step 3 applies logical indexing.

If cond was all FALSE, what would be the output at step 3?

A [5, 10, 15, 20, 25]

B [] (empty vector)

C [TRUE, TRUE, TRUE, TRUE, TRUE]

D Error

💡 Hint

Logical indexing returns elements where cond is TRUE; if none TRUE, output is empty (see variable_tracker output).

Concept Snapshot

Logical indexing in R:
- Create a logical vector condition (e.g., x > 15)
- Use it inside square brackets: x[condition]
- Returns elements where condition is TRUE
- Logical vector length should match x
- Useful for filtering vectors easily

Full Transcript

Logical indexing in R means using a TRUE/FALSE condition to pick elements from a vector. First, we create a vector x with values 5, 10, 15, 20, 25. Then we create a logical condition cond that checks which elements are greater than 15. This results in cond being FALSE for the first three elements and TRUE for the last two. When we use x[cond], R returns only the elements where cond is TRUE, which are 20 and 25. This process filters the vector based on the condition. It's important that the logical vector cond has the same length as x to avoid unexpected behavior. If cond had all FALSE values, the result would be an empty vector. Logical indexing is a simple and powerful way to select elements from vectors in R.