Coding⏱️ 4 min read📅 2026-06-04

How to Fix: Sort rows in data.table in decreasing order on string key `order(-x,v)` gives error on data.table 1.9.4 or earlier

R data.table sort error fix

Quick Answer: Use the `-` operator before `x` to negate the order, e.g., DT[order(-x,-v)]

📋 Table of Contents

  1. 💡 Why You Are Getting This Error
  2. 🔧 Proven Troubleshooting Steps
  3. 💡 Conclusion

The error 'invalid argument to unary operator' in data.table 1.9.4 or earlier occurs when attempting to sort rows using the `order(-x,v)` syntax. This issue affects users who rely on this specific syntax for sorting their data. The frustration comes from having to manually adjust column names or use alternative methods, which can be time-consuming and error-prone.

This error is particularly frustrating because it prevents users from easily sorting their data in the desired order. By understanding the root cause of this issue, we can provide a reliable solution that eliminates the need for manual adjustments.

💡 Why You Are Getting This Error

  • The primary reason for this error lies in the way data.table handles negative signs within column names. In earlier versions (1.9.4 or earlier), data.table interprets the negative sign as a literal character rather than a mathematical operator. This means that when `-x` is encountered, data.table attempts to use `x` as a string value instead of performing the intended arithmetic operation.
  • Alternatively, another reason for this error could be due to the fact that some older versions of data.table may not support the use of negative signs in column names.

🔧 Proven Troubleshooting Steps

Using the `setorder()` function

  1. Step 1: First, create a new variable that represents the desired order by multiplying the original column values by -1. This can be done using the `*=-1` syntax.
  2. Step 2: Next, use the `setorder()` function to reorder the data based on the new variable. The correct syntax is `DT[setorder(x,-x)]`. Note that we need to specify both columns in the order we want them to appear in the sorted output.
  3. Step 3: By using this method, users can easily sort their data in decreasing order without relying on the deprecated `order(-x,v)` syntax.

Using a workaround with `order()`

  1. Step 1: One possible workaround for this issue is to use the `order()` function in combination with the `-` operator. The correct syntax is `DT[order(x,-v)]`. This will first sort by column `x` and then by column `v` in ascending order, effectively achieving a decreasing order for `x`.
  2. Step 2: However, this method may not be as intuitive or user-friendly as using the `setorder()` function. Nevertheless, it can still provide a reliable solution for users who need to sort their data in a specific way.

💡 Conclusion

In conclusion, the error 'invalid argument to unary operator' in data.table 1.9.4 or earlier can be resolved by using either the `setorder()` function or a workaround with the `order()` function. By following these steps and understanding the root cause of this issue, users can easily sort their data in decreasing order without relying on deprecated syntax.

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