Topic 15

Topic 15

Ruby source files are converted directly from the English source tree.

Overview

Why this topic matters 💡

Topic 14 taught transformation pipelines. Topic 15 teaches the next step: problems where you need to build one result from many inputs. This is where reduce and each_with_object become practical tools rather than abstract ideas.

Learning outcomes 🎯

By the end of this topic, students should be able to:

Assessment focus ✅

Students should be able to defend the accumulator they chose and explain why it is clearer than a manual loop with external mutable state.

Short Note

Not every collection problem is a map . Some problems ask for:

That is where accumulator thinking matters.

Imperative style often looks like this:

total = 0 rows . each do |row| total += row [: amount ] end shortnote.md ruby Ruby accumulator style can look like this:

rows . reduce ( 0 ) { |sum , row| sum + row [: amount ] } shortnote.md ruby For hashes or grouped structures, each_with_object often reads even better:

rows . each_with_object ({}) do |row , grouped| grouped [ row [: currency ]] || = 0 grouped [ row [: currency ]] += row [: amount ] end shortnote.md ruby Ruby beauty in this topic:

Ruby caution in this topic:

Reflection prompt:

Worked Examples

Example 1: Invoice total 💡

Old style:

total = 0 invoices . each do |invoice| total += invoice [: amount ] end worked_examples.md ruby Accumulator style:

invoices . reduce ( 0 ) { |sum , invoice| sum + invoice [: amount ] } worked_examples.md ruby Why the second version helps: