R Indexing and Subsetting

Vector Indexing

Vectors can be indexed using numeric positions, negative indices (to exclude), or logical vectors.

List Indexing

Use [[]] or $ for direct access to list elements.

Matrix Indexing

Use [row, column] format to extract values or slices from matrices.

Data Frame Subsetting

Data frames can be subset by position or logical condition.

Logical Indexing

Use logical comparisons to filter values dynamically.

nums <- c(5, 10, 15, 20)

nums[nums > 10]    # 15, 20
nums[nums %% 2 == 0]  # even numbers

Dropping Dimensions

Prevent R from simplifying output (e.g., to vector) by setting drop=FALSE.

m[1, , drop=FALSE]    # Keeps first row as a matrix
df[ , "age", drop=FALSE]  # Keeps column as data frame

Best Practices

• Always verify the result of subsetting using str() or class()
• Use which() to get positions of TRUE values
• Be explicit with row/column subsetting to avoid ambiguity

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