Night is a cut on the hour token

Lesson map

Lesson 07. Map, example, EX053.

flowchart TD
  T["Time token"] --> S["Split on spaces"]
  S --> H["First two characters of part two"]
  H --> I["Integer hour"]
  I --> Q{"Hour below 6, or hour at least 22?"}
  Q -->|Yes| N["Night flag"]
  Q -->|No| D["No night flag"]

The hour is parsed from characters. It is not a timezone.

Logical drawing

Fri 23:47 -> hour 23 -> night
Sat 00:02 -> hour 0 -> night
Fri 08:52 -> hour 8 -> not night
22:00 would be hour 22 -> night
06:00 would be hour 6 -> not night

Learning outcomes

After this chapter, you should be able to:

1. Show how the hour integer is parsed.
2. Apply the two cuts, including the boundaries.
3. Refuse a timezone reading the script does not perform.

7.1 The hour is two characters, then an integer cut.

when.split() breaks the time token on spaces. For Fri 23:47 the parts are Fri and 23:47. Index 1 selects the second part. [:2] keeps 23. int makes the hour 23. The day word is never parsed. The minutes after the colon are never parsed. No timezone name is read, and no clock is shifted.

The night test is hour < 6 or hour >= 22. Hour 23 meets the second cut. Hour 0, from Sat 00:02, meets the first cut. Hour 8, from Fri 08:52, meets neither cut, and hour 13 meets neither cut. Hour 22 is the first integer the late cut accepts, so a token written 22:00 would still count as night, and that token is not stored here. Morning equality is the excluded edge: parsing 06:00 leaves the flag down because six sits outside both cuts, and that edge is absent from the roster.

Section 2.2 says the lab flags a time between 22:00 and 06:00. The predicate is the precise form: from 22:00 inclusive, and before 06:00. The exercise answer already in the appendix says "Before 06:00 or from 22:00." That wording matches the two cuts. This chapter does not replace the earlier sentence. It shows the characters the script actually reads.

Lesson table; scroll horizontally to read all columns →

7.2 Worked example

Lesson table; scroll horizontally to read all columns →

Synthetic case. Boundary hours 6 and 22 are not rows in the file.

Errors and limits

Do not insert a 06:00 row or a 22:00 row into the published list.

Apply

• EX053 — Hour cuts. Using the same predicate, is 22:00 night, is 06:00 night, and does the script read a timezone?
  Show marking criteria

  Rubric: 3 = 22:00 is night because hour 22 meets >= 22; 06:00 is not night because hour 6 fails both cuts; the day word and the minutes are not a timezone;; 2 = 22:00 is night and 06:00 is not;; 1 = one boundary;; 0 = none.