Clojure Expressions in OpenRefine
Excel (or similar spreadsheet tools) is a simple and quick tool to get a high-level idea about a data-set and its characteristics, especially used by folks from a non-programming background. It has the ability to filter, splice and pull up graphs (and a whole host of features that I know nothing about).
While I’ve seen that Excel is a great tool for this task, I feel slightly hamstrung by the lack of programming support . I know there are macros and other geegaws available, but for someone coming from the Java/Clojure side of the world, investing the time to learn a new set of tools seems like an awfully large side project.
Enter OpenRefine. At first glance, it looks like a web based version of an Excel-like tool, but some of its features hit the sweet spot
- It can be run on a server and accessed remotely. If the size of the data is big, it can be run on a server machine and accessed from a lightweight laptop
- It keeps a record of all edits and maintains a comprehensive history, which makes it easy to roll back to a particular version and see the list of operations/transformations done at each step.
- It has elementary support for faceting and graphing.
- Did I mention it is open source and free? There are also commercial API connectors for things like reconciliation and geo-coding.
- What I found most interesting was that among the 3 mini-languages supported for expressions, Clojure is one of them.
- It has excellent documentation, including, but not limited to, several how-to guides, screencasts, a wiki and a book as well.
I didn’t find any documentation on using Clojure for doing transformations, so the rest of this post is on that topic.
Variables available in expression REPL
The Variables lists the set of variables that can be accessed in the expressions window. However, the list of Clojure-accessible variables are just these:
- value: the value of the cell in the base column of the current row. This is an instance of the String class
- row : the current row, an instance of WrappedRow
- cells : the cells of the current row, with fields that correspond to the column names, an instance of CellTuple
- cell : the cell in the base column of the current row, an instance of WrappedCell
These are Java instances,not pure Clojure, and the API backing these objects supports only a few methods.
Here are a few basic transformations:
- Getting the value of a cell
(.-value (.-cell cell))
the easier alternative is just
- Get a different cell in the row (indexed by number, which is 1 in this case)
(.getCellValue (.-row row) 1)
- Get a different cell in the row by column name
(.-value (.-cell (.getField cells "ColumnName" (new java.util.Properties))))
The above API requires a java.util.Properties argument, which seems to be unused in this class.
- Concatenate 2 column values separated by a colon character, the current cell along with another cell in the same row
(str value ":" (.-value (.-cell (.getField cells "Column2" (new java.util.Properties)))))
- a Test that returns true if current cell and Column2 cell have non-empty values or not.
(every? true? (map empty? [value (.-value (.-cell (.getField cells "Column2" (new java.util.Properties))))]))
As we can see, the API is not very user-friendly or Clojuresque in nature. However for quick transformations such as creating facets, it is quite capable.