(Also available in CODAP)
Students learn how to chain Methods together, and define more sophisticated subsets.
Lesson Goals |
Students will be able to…
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Student-facing Lesson Goals |
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Materials |
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Preparation |
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🔗Design Recipe Practice 25 minutes
Overview
Students practice more of what they learned in the previous lesson, applying the Design Recipe to make table functions that operate on rows of the Animals Dataset. These become the basis of the chaining activity that follows.
Launch
The Design Recipe is a powerful tool for solving problems by writing functions . It’s important for this to be like second nature, so let’s get some more practice using it!
Investigate
Define the Compute functions on The Design Recipe: is-dog / is-female and The Design Recipe: is-old / name-has-s.
Optional: Combining Booleans Suppose we want to build a table of Animals that are fixed and old, or a table of animals that are cats or dogs? By using the For many of the situations where you might use |
Synthesize
Did students find themselves getting faster at using the Design Recipe? Can students share any patterns they noticed, or shortcuts they used?
🔗Chaining 25 minutes
Overview
Students learn how to compose multiple table operations (sorting, filtering, building) on the same table - a technique called "chaining".
Launch
Now that we are doing more sophisticated analyses, we might find ourselves writing the following code:
# get a table with the nametags of all the fixed animals, ordered by species
with-labels = animals-table.build-column("labels", nametag)
fixed-with-labels = with-nametags.filter(is-fixed)
result = fixed-with-labels.order-by("species", true)That’s a lot of code, and it also requires us to come up with names for each intermediate step! Pyret allows table methods to be chained together, so that we can build, filter and order a Table in one shot. For example:
# get a table with the nametags of all the fixed animals, ordered by species
result = animals-table.build-column("labels", nametag).filter(is-fixed).order-by("species", true)This code takes the animals-table, and builds a new column. According to our Contracts Page, .build-column produces a new Table, and that’s the Table whose .filter method we use. That method produces yet another Table, and we call that Table’s order-by method. The Table that comes back from that is our final result.
Teaching Tip Use different color markers to draw nested boxes around each part of the expression, showing where each Table came from. |
It can be difficult to read code that has lots of method calls chained together, so we can add a line-break before each “.” to make it more readable. Here’s the exact same code, written with each method on its own line:
# get a table with the nametags of all the fixed animals, order by species
animals-table
.build-column("label", nametag)
.filter(is-fixed)
.order-by("species", true)Order matters: Build, Filter, Sort.
Suppose we want to build a column and then use it to filter our table. If we use the methods in the wrong order (trying to filter by a column that doesn’t exist yet), we might wind up crashing the program. Even worse, the program might work, but produce results that are incorrect!
Investigate
When chaining methods , it’s important to build first, then filter, and then order.
How well do you know your table methods? Complete Chaining Methods and Chaining Methods 2: Order Matters in your Student Workbook to find out.
Synthesize
As our analysis gets more complex, chaining is a great way to re-use work we’ve already done. And less duplicate work means a smaller chance of bugs. Chaining is a powerful way to work, so it’s critical to think carefully when we use it!
🔗Additional Exercises
These materials were developed partly through support of the National Science Foundation,
(awards 1042210, 1535276, 1648684, and 1738598). 
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