## Chapter 13 — An empty MISSING loop still ran ### Lesson map Lesson 13. Map, example, EX059. ```mermaid flowchart TD B["baseline minus today"] --> E["Empty set"] E --> L["The MISSING loop runs"] L --> Z["Zero lines"] Z --> Q{"Were OneDrive, AudioService, PrinterHelper removed?"} Q -->|No| K["They are still in today"] ``` No MISSING line is a result. It is not a loop that was skipped. ### Logical drawing ```text still in both sets OneDrive AudioService PrinterHelper baseline minus today empty printed under MISSING nothing ``` ### Learning outcomes After this chapter, you should be able to: 1. Name the three baseline programs that remain. 2. Explain an empty difference. 3. Refuse an invented MISSING line. ### 13.1 The second loop prints once per leftover name. There are none. After the NEW lines, the script walks `sorted(baseline - today)` and prints `MISSING:` plus the name. `baseline - today` holds names that were in the saved set and are absent from `today`. OneDrive, AudioService, and PrinterHelper are members of both sets, so that difference is empty. `sorted` of an empty set is an empty sequence. The loop body never runs, and the page shows no MISSING line. An empty result is still information. Some harmful programs disable or replace a normal startup name. Priya's note records that none of the three baseline names was removed. She gets that from the absence, not from a sentence the script failed to emit. Adding a fake MISSING line "for completeness" would claim a removal that the sets contradict. The length line agrees. Baseline length 3 and today length 5, with two names added and zero names removed, is 3 + 2 - 0 = 5. That check uses only the sets. It does not require a clock, and it does not require opening either new program. | Set fact | Print | |---|---| | Name in both sets | No MISSING line | | Name only in `today` | A NEW line | | Name only in `baseline` | A MISSING line, and this run has none | | An invented removal | Not supported | ### 13.2 Worked example | Offered reading | What the sets support | Decision | |---|---|---| | Three baseline names remain | They are members of `today` | Keep | | The MISSING loop printed nothing | The difference is empty | Keep | | "PrinterHelper was replaced." | It is still in `today` | Refuse | | "Add MISSING: none so the page looks finished." | That word is not a member | Refuse | > **Synthetic case.** The baseline three are still present. ### Errors and limits Do not fill an empty difference with a comforting extra line. ### Apply - **EX059 — Empty difference.** Why is there no MISSING line, and which baseline names remain? *Rubric:* 3 = OneDrive, AudioService, and PrinterHelper are still in `today`; the difference is empty; the loop ran and printed nothing;; 2 = the three names remain and nothing was missing;; 1 = one name;; 0 = none. ### Continue studying — lesson 13 (chapter 13 of 20) [← Chapter 12](#ch-12) · [Course chapter index](../SUBJECTS.md) · [Chapter 14 →](#ch-14) Continue to the next authored lesson. ## Worked answers **EX059.** OneDrive, AudioService, and PrinterHelper are still in `today`. The difference is empty, so the MISSING loop prints nothing.