Adding Function Transformers to CODAP

Posted on 22 August 2021.

CODAP is a wonderful tool for data transformation. However, it also has important limitations, especially from the perspective of our curricula. So we’ve set about addressing them so that we can incorporate CODAP into our teaching.


We at Brown PLT and Bootstrap are big fans of CODAP, a data-analysis tool from the Concord Consortium. CODAP has very pleasant support for working with tables and generating plots, and we often turn to it to perform a quick analysis and generate a graph.

One of the nice things about CODAP, that sets it apart from traditional spreadsheets, is that the basic unit of space is not a table or spreadsheet but a desktop that can contain several objects on it. A workspace can therefore contain many objects side-by-side: a table, a graph, some text, etc.:

View of CODAP desktop with several objects.

Also, a lot of things in CODAP are done through direct manipulation. This is helpful for younger students, who may struggle with formal programming but can use a GUI to manipulate objects.

There are many other nice features in CODAP, such as the ability to track a data case cross representations, and so on. We urge you to go try it out! When you launch a new CODAP instance, CODAP will offer you a folder of examples, which can help you get acquainted with it and appreciate its features.

What’s Not to Love?

Unfortunately, we don’t love everything about CODAP. We’ll illustrate with an example. To be clear, this is not a bug in CODAP, but rather an important difference of opinion in ease-of-use.

Let’s say we want to find all the people who are below 50 years of age. In CODAP, there are a few ways to do it, all of which have their issues.

If you don’t mind being imprecise (which may be okay for a quick data exploration, but isn’t if you want to, say, compute a statistic over the result):

  • Create a new graph.
  • Drag the Age column to the graph.
  • Select all the items that are under 50 using visual inspection. (Depending on how much data you have and their spread, you’ll quite possibly under- and/or over-shoot.)
  • Then do the last few steps below.

If you care to get an accurate selection, instead begin with:

  • First, add a new column to the original table.
  • Enter a formula for that column (in this case, Age < 50).
  • Obtain a selection of the desired items, which can be done in several different ways, also all with trade-offs:
  1. Sort by that column. Unfortunately, this won’t work if there’s grouping in the table. You’d have to select manually. (Try it. This may be a bit harder than it seems.)

  2. Create a graph as above, but of the new column. This will give you a clean separation into two values. Manually select all the values in the true column. At least now it will be visually clear if you didn’t select all the right values (if the dataset is not too large).

  3. Remove the formula for the new column. Now drag it to the leftmost end of the table. (If you don’t remove the formula, you’ll get an error!) Now you have all the elements grouped by true and false (and operations performed to one can also be performed to the other).

You’re not done! You still have more steps to go:

  • If you aren’t already in the table (e.g., if you made a graph), select the table.
  • Click on the “Eye” icon.
  • Choose the “Set Aside Unselected Cases” entry.

Note that, in most or all of these cases:

  • You’ve added a completely superfluous column to your dataset.
  • You may have changed the order of items in your dataset.
  • You’ve lost the ability to see the original data alongside the filtered data.
  • You had to take numerous steps.
  • You had to remember to use the Eye icon for filtering, as opposed to other GUI operations for other tasks.
  • You had to remember where the Eye icon even is: it’s hidden when a table isn’t selected.

But most of all, in every single case:

  • You had to perform all these operations manually.

Why does this matter? We need data science to be reproducible: we should be able to give others our datasets and scripts so they can re-run them to check that they get the same answer, tweak them so they can check the robustness of our answers, and so on. But when all the operations are done manually, there’s no “script”, only output. That focuses on answers rather than processes, and is anti-reproducibility.

In contrast, we think of filtering as a program operation that we apply to a table to produce a new table, leaving the original intact: e.g., the way it works in Pyret. This addresses almost all of the issues above.

Other Pedagogic Consequences

CODAP had to make certain design choices. They made good choices for some settings: for younger children, in particular, the direct manipulation interface works very nicely. It’s a low floor. However, we feel it’s also a lower-than-we’d-like ceiling. There are many things that the CODAP view of data transformation inhibits:

  • Making operations explicit, as we noted above.
  • Introducing the idea of functions or transformations of data as objects in their own right, not only as manual operations.
  • Having explicit data-transformation functions also connects to other related school curricula, such as algebra.
  • Saving and naming repeated operations, to learn a bottom-up process of developing abstractions.
  • Examining old and new tables side-by-side.

This last point is especially important. A critical task in data science is performing “what if” analyses. What-if fundamentally means we should be able to perform some operation (the “if” part) and compare the output (the “what” part). We might even want to look at multiple different scenarios, representing different possible outcomes. But traditional what-if analysis, whether in CODAP or on spreadsheets, often requires you, the human, to remember what has changed, rather than letting the computer do it for you. (Microsoft Excel has limited support to get around this, but its very presence indicates that spreadsheets, traditionally, did not support what-if analysis—even though that’s how they have often been marketed.)

Finally, there’s also a subtle consequence to CODAP’s design: derived tables must look substantially similar to their parents. In computing terms, the schema should be largely the same. That works fine when an operation has little impact on the schema: filtering doesn’t change the schema at all (in principle, though in CODAP you have to add an extra column…), and adding a new column is a conservative extension. But what if you want to perform an operation that results in a radically different schema? For instance, consider the “pivot wider” and “pivot longer” operations when we create tidy data. The results of those operations have substantially different schemas!

Introducing CODAP Transformers

In response to this critique, we’ve added a new plugin to CODAP called Transformers:

CODAP Plugins dropdown showing Transformers entry.

(This work was done by undergrad trio Paul Biberstein, Thomas Castleman, and Jason Chen.)

This introduces a new pane that lists several transformation operations, grouped by functionality:

Transformers pane.

For instance, with no more textual programming than before (the formula is the same), we can perform our same example as before, i.e., finding all the people younger than 50:

Filtering the People table for Age under 50.

The result is a new table, which co-exists with the original:

Original and filtered tables.

The resulting table is just as much a table as the original. For instance, we can graph the ages in the two tables and see exactly the difference we’d expect:

Graphs of ages in original and filtered tables.

(Over time, of course, you may build up many tables. The Transformers plugin chooses names based on the operations, to make them easy to tell apart. CODAP also lets you resize, minimize, and delete tables. In practice, we don’t expect users to have more than 3–4 tables up at a time.)

Saving Transformers

We might want to perform the same operation on multiple tables. This is valuable in several contexts:

  • We create a hand-curated table, with known answers, as a test case to make sure our operations perform what we expect. After confirming this, we want to be sure that we applied the exact same operation to the real dataset.

  • We want to perform the same operation to several related datasets: e.g., a table per year.

We might also simply want to give a meaningful name to the operation.

In such cases, we can use the “Save This Transformer” option at the bottom of the Transformers pane:

Saving a transformer.

Following the programming processes we follow and teach, we naturally want you to think about the Design Recipe steps when saving it because, in programming terms, you’re creating a new function.

This now creates a new named transformer:

Saved transformer.

Every part of this is frozen other than the choice of dataset; it can be applied as many times as you want, to as many datasets as you want. The above use-cases are suggestions, but you can use it however you wish.

A Note on Errors

Suppose you try to apply an operation improperly. Say, for instance, you have a table of people that does not have an Age column, and you try to filter people with Age < 50. There are at least two choices that Transformers can take:

  • Allow you to try to perform the operation, and report an error.

  • Prevent you from even trying by simply not showing tables that are invalid in the drop-down list of tables that the operation can be applied to.

We know exactly what the programming languages reader reading this is thinking: “You’re going to choose the latter, right? Right?!? PLEASE TELL ME YOU ARE!!!”

Gentle Reader: we’re not.

Here’s why we chose not to.

  • There’s the messy implementation detail of figuring out exactly when a table should or shouldn’t be shown in the drop-down. And we’d have to maintain that across changes to the CODAP language. There are no such problems in the dynamic version.

But hey, we’re language implementors, we can figure these things out. Rather, our real reason comes from human factors:

  • Imagine you’re a teacher with a classroom full of students. A student tries to apply an operation to the wrong table. They probably don’t even realize that the operation can’t be applied. All they know is that the table doesn’t appear in the list. Their table doesn’t appear in the list! Their reaction is (perhaps rightly) going to be to raise their hand and say to their teacher, “This tool is broken! It won’t even show me my table!!!” And the teacher, dealing with a whole bunch of students, all in different states, may not immediately realize why the table doesn’t show. Everyone’s frustrated; the student feels stuck, and the teacher may be left feeling inadequate.

In contrast, if we just let the operation happen, here’s what the student sees:

Error when `Age` is not present.

Now the student has a pretty good chance of figuring out for themselves what went wrong: not pulling away the teacher from helping someone else, not blaming the tool, and instead giving themselves a chance of fixing their own problem.

There’s potentially a broader lesson here about making invalid states unrepresentable. Potentially.

Many, Many Transformers!

We’ve focused on just one transformation here. There are many more. We even have the pivoting operations for tidy data! (It would have been wrong to tease you with that up top, otherwise.)

We even take the what-if part seriously: the Compare Transformer lets you compare numeric and categorical data. Believe it or not, the categorical comparison operator was actually inspired by prior work we’ve done for many years on comparing access control policies, router configurations, and SDN programs (see also our two brief position papers). It’s pretty thrilling to see the flow of ideas from security and networking research to data science education in a small but very non-obvious way: the grouping in the categorical output is directly inspired by the multi-terminal decision diagrams of our original Margrave system.


For your benefit, we’ve set up a bunch of pre-built CODAP examples that show you the operations in action:

Make Your Own!

As you might have guessed from the examples above, transformers are now part of the official CODAP tool. You can go play with them right now on the CODAP site. Have fun! Tell us what you learned.


Special thanks to our friends at the Concord Consortium, especially William Finzer, Jonathan Sandoe, and Chad Dorsey, for their support.