Table Methods

Students review row definitions, and are introduced to Function Definitions in Pyret, and do some open-ended reading and interpreting of code.

Load the Table Methods Starter File, go to the File menu, and click "Save a Copy".

This program has several things you’ve never seen before! This activity is about reading carefully, and trying to make sense of new code.

Student work in groups or pairs.

Take a look at the three examples for is-dog. Each one shows us a different way of thinking about examples, in this case using a row that should return false:

Students learn to sort rows of a table in ascending or descending order, according to one column.

Have students find the contract for .order-by in their contracts pages. The .order-by method consumes a String (the name of the column by which we want to order) and a Boolean (true for ascending, false for descending). But what does it produce?

Students learn how to filter tables by removing Rows.

Explain to students that you have "Function Cards", which describe the purpose statement of a function that consumes a Row from a table of students, and produces a Boolean (e.g. - "this student is wearing glasses"). Select a volunteer to be the "filter method", and have them randomly choose a function card from from the Function Cards set, and make sure they read it without showing it to anyone else.

Have 6-8 students line up in front of the classroom, and have the filter method go to each student and say "stay" or "sit" depending on whether their function would return true or false for that student. If they say "sit", the student sits down. If they say "stay", the student stays standing.

Ask the class: based on who sat and who stayed, what function was on the card?

Suppose we want to get a table of only animals that have been fixed? Have students find the contract for .filter in their contracts pages. The .filter method is taking in a function. What is the contract for that function? Where have we seen functions-taking-functions before?

The .filter method walks through the table, applying whatever function it was given to each row, and producing a new table containing all the rows for which the function returned true. Notice that the Domain for .filter says that test must be a function (that’s the arrow), which consumes a Row and produces a Boolean. If it consumes anything besides a single Row, or if it produces anything else besides a Boolean, we’ll get an error.

Students often think that filtering a table changes the table. In Pyret, all table methods produce a brand new table. If we want to save that table, we need to define it. For example: cats = animals-table.filter(is-cat).

Debrief with students. Some guiding questions on filtering:

Students learn how to build columns, using the .build-column table method.

Suppose we want to transform our table, converting pounds to kilograms or weeks to days. Have students find the contract for .build-column in their contracts pages. The .build-column method is taking in a function and a string. What is the contract for that function?

The .build-column method walks through the table, applying whatever function it was given to each row. Whatever the function produces for that row becomes the value of our new column, which is named based on the string it was given. In the first example, we gave it the is-old function, so the new table had an extra Boolean column for every animal, indicating whether or not it was young. Notice that the Domain for .build-column says that the builder must be a function which consumes a Row and produces some other value. If it consumes anything besides a single Row, we’ll get an error.

Debrief with students. Ask them if they can think of a situation where they would want to use this. Some ideas:

Being able to define functions is a huge upgrade in our ability to analyze data! But as a wise person once said, "with great power comes great responsibility"! Dropping all the dogs from our dataset might be a cute exercise in this class, but suppose we want to drop certain populations from a national census? Even a small programming error could erase millions of people, impact funding for things like roads and schools, etc.

Functions are a powerful tool, and the next two lessons are all about thinking in terms of functions and how to build them. In the next lesson, we’ll learn how to view functions in three different ways. By making sure each representation matches the other two, it gives us a chance to check our work - twice! The lesson after that turns our attention back to Data Analysis, building functions specifically for analyzing our dataset.