Subsetting

Subsetting

• Three subsetting operators.
• Five types of subsetting.
• Extensions to more than 1d.

All basic data structures can be teased apart using the subsetting operators: [, '[[ and $.

1d subsetting

It's easiest to explain subsetting for 1d first, and then show how it generalises to higher dimensions. You can subset by 6 different things:

• blank: return everything
• positive integers: return elements at those positions
• zero: returns nothing
• negative integers: return all elements except at those positions
• character vector: return elements with matching names
• logical vector: return all elements where the corresponding logical value is TRUE

(Note for integers that it's not just subsetting that you can do.)

• lookup tables
• expanding aggregated counts
• ordering
• matching by hand
• logical vs integer, boolean vs sets

nd subsetting

For higher dimensions these are separated by commas.

You can also subset with matrices.

Simplifying vs. preserving subsetting

• [ . Drop argument controls simplification.
• [[ returns an element
• x$y is equivalent to x[["y"]]