ruby 35 lines · 9 steps

Building a pivot table in Ruby

Aggregate flat sales records into a region-by-quarter grid with row and column totals, then render it as tab-separated text.

Explained by highlit

Intermediate aggregation nested-hashes default-values data-transformation

1def build_pivot_table(sales)
pivot = sales.each_with_object(Hash.new { |h, k| h[k] = Hash.new(0.0) }) do |record, table|
  region = record.fetch(:region)
  quarter = record.fetch(:quarter)
  table[region][quarter] += record.fetch(:amount)
end
7 
quarters = pivot.values.flat_map(&:keys).uniq.sort
9 
rows = pivot.map do |region, by_quarter|
  cells = quarters.map { |q| by_quarter[q] }
  { region: region, cells: cells, total: cells.sum }
end
14 
column_totals = quarters.each_with_index.map do |_, i|
  rows.sum { |row| row[:cells][i] }
end
18 
{
  quarters: quarters,
  rows: rows.sort_by { |row| -row[:total] },
  column_totals: column_totals,
  grand_total: column_totals.sum
}
25end
26 
27def format_pivot(pivot)
header = ["Region", *pivot[:quarters], "Total"].join("\t")
body = pivot[:rows].map do |row|
  [row[:region], *row[:cells].map { |c| format('%.2f', c) }, format('%.2f', row[:total])].join("\t")
end
footer = ["Total", *pivot[:column_totals].map { |c| format('%.2f', c) }, format('%.2f', pivot[:grand_total])].join("\t")
33 
[header, *body, footer].join("\n")
35end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1A Hash with a block default lets you accumulate into nested structures without pre-initializing keys.
2Deriving the column axis from the data keeps the table robust when records skip some quarters.
3Separating aggregation from formatting makes each stage independently testable and reusable.

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