Contracts: Making Tables and Displays

Students will be introduced to functions for working with tables, including: sort, count and first-n-rows. In the process they will review simple data types and be introduced to contracts for describing the Domain and Range of any function.

As you can imagine, there are many other functions that work with tables, each with a different set of inputs. For each of these functions, we need to keep track of three things:

The Name, Domain and Range are used to write a Contract.

The Domain in the Contract for sort tells us exactly what kind of inputs it needs, and in what order. The Range tells us exactly what we’ll get back.

Students are likely to think that sort changes the table, when instead it produces a new, sorted table. Encourage students to say out loud what they think they will get if they type animals-table after evaluating sort​(​animals-table, "name", true​). By testing their hypothesis, students who are surprised at the outcome are much more likely to remember the difference later on.

Students learn to work with more than one function at once, by way of Circles of Evaluation, a visual representation of the underlying structure.

What if we wanted to see the ten youngest animals?

One way to organize our thoughts is to diagram what we want to do, using the Circles of Evaluation.

The rules for Circles of Evaluation are simple:

(1) Every Circle of Evaluation must have a single function, written at the top.

(2) The arguments of the function are written left-to-right, in the middle of the Circle.

(3) Circles can contain other Circles!

Note: Values like Numbers, Strings, and Booleans are still written by themselves. It’s only when we want to use a function that we need to draw a Circle.

To convert a Circle of Evaluation into code:

This activity focuses on what we can learn about Pyret functions from the messages we get back in the Interactions Area. The error messages in this environment are specially-designed to be as student-friendly as possible. By explicitly drawing their attention to errors, you will be setting them up to be more independent in the future.

Let’s see how error messages in Pyret can help us to figure out the contract for a function we’ve never seen before.

Mistakes happen, especially if we’re just figuring things out! Diagnosing and fixing errors are skills that we will continue developing throughout this course.

Making images is a highly motivating context for reading error messages and writing contracts.

In order to make sure that all students both remain engaged and are prepared to engage in productive class discussion, when you become aware that the first student in your class has successfully used the text function, give your students directions about which functions to prioritize with the remaining time.

Students will be introduced to functions for making one-variable displays in Pyret, including: pie-chart, bar-chart, box-plot and histogram.

The goal here is for students to become familiar with using Contracts to write expressions that will produce displays. But knowing how to make a histogram doesn’t mean a student really understands histograms, and that’s OK!

The count function summarized the data for a single variable in a new table.

The same information could be communicated as a picture! This is called data visualization, and Pyret has functions that can make displays for us!

Just as we can use Circles of Evaluation to help us combine sort, count, and first-n-rows, we can put Circles of Evaluation to work to help us write code to build more specific displays. Consider this:

If your students would benefit from seeing a few more examples before drawing their own Circles of Evaluation, have them complete Composing Functions: Match Display Descriptions to Circles of Evaluation.

For more practice making tables and displays by composing functions, have students complete Circles of Evaluation: Composing Functions to Make Displays (2)