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Fix: Add init=0 to prevent empty collection crash during threshold tuning. - #432

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Fix: Add init=0 to prevent empty collection crash during threshold tuning.#432
vatsalyar wants to merge 4 commits into
rohitg00:mainfrom
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@vatsalyar vatsalyar commented Aug 24, 2026 •
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What this PR does

Fixes #434

Fixes an ArgumentError in main.jl by adding ; init=0 to the sum() generators in the build_metrics function to handle edge cases during threshold tuning

Kind of change

  • New lesson
  • Fix to an existing lesson
  • Translation
  • New output (prompt, skill, agent, MCP server)
  • Docs / website / tooling

Checklist

  • Code runs without errors with the listed dependencies
  • No comments in code files (docs explain, code is self-explanatory)
  • Built from scratch first, then shown with a framework (for new lessons)
  • Lesson folder matches LESSON_TEMPLATE.md structure
  • ROADMAP.md row for the lesson is a markdown link ([Name](phases/...)), not bare text
  • One lesson per commit (atomic per-lesson rule)
  • Tested locally / code output matches what docs/en.md claims

Phase / lesson

Phase 2 · 03-logistic-regression

Notes for reviewer

When running demo_threshold_tuning, extreme thresholds (like 0.7) can result in the model predicting exactly zero positive cases. When y_pred contains only 0s, the generator comprehensions inside build_metrics. (e.g., sum(1 for i in 1:length(y_true) if y_true[i] == 1 && y_pred[i] == 1)) becomes empty. This causes Julia to throw an ArgumentError: reducing over an empty collection is not allowed. Adding the ; init=0 keyword argument resolves this by safely returning 0 and allowing the script to complete successfully.)

@vatsalyar
Fix: Add init=0 to build_metrics to prevent empty collection crash du…
85eea7e 
…ring threshold tuning
@vatsalyar
Merge pull request #1 from vatsalyar/vatsalyar-fix-metrics-empty-coll…
f984f53 
…ection

Fix: Add init=0 to prevent empty collection crash during threshold tuning
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coderabbitai Bot commented Aug 24, 2026 •
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Review Change Stack

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info
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Run ID: e4bf7daa-e232-45d8-9845-a179082b4d58

📥 Commits

Reviewing files that changed from the base of the PR and between f97dc6b and 64c7d1b.

📒 Files selected for processing (1)
  • phases/02-ml-fundamentals/03-logistic-regression/code/main.jl
🚧 Files skipped from review as they are similar to previous changes (1)
  • phases/02-ml-fundamentals/03-logistic-regression/code/main.jl

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📝 Walkthrough

Walkthrough

build_metrics now uses indexed predicate counts for all four confusion-matrix metrics. Empty matches return zero without raising an ArgumentError.

Changes

Logistic regression metrics

Layer / File(s) Summary
Count confusion-matrix matches
phases/02-ml-fundamentals/03-logistic-regression/code/main.jl		 The true-positive, true-negative, false-positive, and false-negative calculations now use count over eachindex(y_true). Empty matches produce zero.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Merge Risk: ⚪ Minimal · up to 64c7d

This localized fix prevents threshold-tuning failures when no positive cases are predicted. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly identifies the fix for empty collections during threshold tuning. It accurately describes the intended behavior, although the implementation uses count over indexed predicates instea…
Description check ✅ Passed The description directly explains the build_metrics failure, the empty-generator edge case, and the intended fix for threshold tuning.
Linked Issues check ✅ Passed The changes satisfy issue #434 by updating build_metrics so confusion-matrix counts return zero when no predictions match. Threshold tuning can therefore complete without the empty-collection Argument…
Out of Scope Changes check ✅ Passed The changes are limited to build_metrics in the logistic-regression lesson and directly address the linked issue. No unrelated changes are identified.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Full details: Title check

Explanation

The title clearly identifies the fix for empty collections during threshold tuning. It accurately describes the intended behavior, although the implementation uses count over indexed predicates instead of adding init=0 directly.

Full details: Linked Issues check

Explanation

The changes satisfy issue #434 by updating build_metrics so confusion-matrix counts return zero when no predictions match. Threshold tuning can therefore complete without the empty-collection ArgumentError.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches 💡 1
🛠️ Fix failing CI checks 💡
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@vatsalyar
Refactor build_metrics to use count() for metrics
f97dc6b 
Refactored the build_metrics function to use count() for better performance and readability.
@vatsalyar
Minor fix
64c7d1b
@vatsalyar vatsalyar mentioned this pull request Aug 25, 2026
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[bug] ArgumentError in build_metrics during threshold tuning (03-logistic-regression)

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