Category with highest average expense
Requirement
Given a month of expenses — each with a date, category, and amount — find the category with the highest average amount per transaction, and print it with the average formatted to two decimals. The data is in the code below; both versions must print the line shown under Expected output.
Expected output
Highest average spend: Travel ($111.00 per transaction)
Side by side
Native Dart
FxDart
Why they differ
Core Dart has neither groupBy nor maxBy;
package:collection supplies both — but as top-level
functions, not chain steps. The native version therefore reads
inside-out: maxBy(… wrapping a map over
entries of a groupBy(… — three idioms (function call,
method chain, function call) for one three-step thought. FxDart keeps
the reading order equal to the data flow: groupBy the
transactions, map each group to
(category, average), maxBy the average. Same
algorithm, but the sentence runs left to right.
Benchmark
N = 100
Time Tie
Peak memory Tie
N = 10,000
Time Tie
Peak memory Tie
N = 1,000,000
Time Native wins
Peak memory Tie
Bars are medians of repeated timed iterations in fresh processes per side (small N is batched for timer resolution). Sides within 5% of each other — or within 0.6 ms, a difference no person can perceive — count as a tie; close relative races are re-measured up to 5 runs. In an app, anything under a few milliseconds is invisible to the user regardless of which bar is shorter. Memory is peak process RSS. The Dart VM and the dataset are identical on both sides, so the difference between the two bars is what the pipeline itself holds onto.