# What does setkey mean in R?

## What does setkey mean in R?

Description. setkey sorts a data. table and marks it as sorted with an attribute sorted . The sorted columns are the key. The key can be any number of columns.

## What is R Key?

Description. Key is a vector which goal is to provide information about rows in reference data frame. Its length should always be equal to number of rows in data frame. Keys are stored as tibble in attribute “keys” and so one data frame can have multiple keys.

**How do you join data tables in R?**

If you want to join by multiple variables, then you need to specify a vector of variable names: by = c(“var1”, “var2”, “var3”) . Here all three columns must match in both tables. If you want to use all variables that appear in both tables, then you can leave the by argument blank.

### What is a data table key?

The sorted columns are the key. The key can be any number of columns. The columns are always sorted in ascending order. The table is changed by reference and setkey is very memory efficient.

### What does .n mean in R?

\n is specifically used to move to a new line, while \r is used for a carriage return, which moves the cursor back to the beginning of the current line.

**How do I subset data in R?**

So, to recap, here are 5 ways we can subset a data frame in R:

- Subset using brackets by extracting the rows and columns we want.
- Subset using brackets by omitting the rows and columns we don’t want.
- Subset using brackets in combination with the which() function and the %in% operator.
- Subset using the subset() function.

#### What is data table in R?

Data. table is an extension of data. frame package in R. It is widely used for fast aggregation of large datasets, low latency add/update/remove of columns, quicker ordered joins, and a fast file reader. It is an ideal package for dataset handing in R.

#### What does setkey (…) do?

This has some practical implications: for one thing if you set the key in a table with setkey(…)and then change any of the values in the key column, data.table merely declares the table to be no longer sorted (by turning off the sortedattribute); it does notdynamically re-index to maintain the proper sort order (as would happen in a database).

**How to avoid copy when using setkeyv in R?**

The <- method copies the whole table and we know of no way to avoid that copy without a change in R itself. Please use the set* functions instead, which make no copy at all. setkey accepts unquoted column names for convenience, whilst setkeyv accepts one vector of column names.

## Does setkey (…) create an index?

Re: sorting vs. indexing- Based on the answer to this question, it appears that setkey(…)does in fact rearrange the columns in the table (e.g., a physical sort), and does not create an index in the database sense.

## How do I sort data using setkey?

setkey sorts a data.table and marks it as sorted with an attribute “sorted”. The sorted columns are the key. The key can be any number of columns. The data is always sorted in ascending order with NA s (if any) always first.