Branching Programs

Mr. Whitfield runs the library on his own at lunch, and about a dozen students take turns helping at the desk. His problem was never the arithmetic — it was that the same overdue book got four different reactions depending on who was standing there. This program is his policy, written down once, in a form that cannot be forgotten at the end of a long week.

The program

# Return-desk helper for Mr. Whitfield, who wants the same book
# treated the same way every time, whoever is at the desk.
 
title = input("Title: ")
answer = input("Days past the due date (0 if on time): ")
 
try:
    days_late = int(answer)
except ValueError:
    print(f"I could not read '{answer}' as a number of days.")
    days_late = 0
 
print()
 
if days_late <= 0:
    print(f"{title}: on time. Shelve it and say thanks.")
elif days_late <= 7:
    print(f"{title}: less than a week late. Shelve it, no message.")
elif days_late <= 30:
    print(f"{title}: {days_late} days late. Send the standard reminder.")
else:
    print(f"{title}: {days_late} days late — over a month.")
    print("Ask in person, kindly: it is usually lost, not stolen.")

Title: Fifteen Dogs
Days past the due date (0 if on time): 12

Fifteen Dogs: 12 days late. Send the standard reminder.

How it works

Exactly one of the four branches runs, and which one depends entirely on days_late:

days_late	Branch that runs	Because
0 or less	on time	days_late <= 0 is the first True
1 to 7	no message	the first test failed, the second passed
8 to 30	standard reminder	the first two failed
31 or more	speak in person	nothing matched, so else catches it

The order is doing real work. Each elif is only ever reached when every condition above it was False, which is why days_late <= 30 does not need to also say “and more than 7” — that is already known. Swap two branches around and the program still runs, still prints something, and is silently wrong for half its inputs; that is the logic error described in Making Decisions.

The try block came from testing, not from good intentions. The test plan in Testing and Debugging includes the row “user types soon”, and the first version of this program failed it.

Two things this program deliberately does not do: it does not record who borrowed the book, and it does not mention fines. Both were decisions made with Mr. Whitfield, and both are the kind of thing Who Is This For exists to make you ask about out loud.

Change it

1. One line. Mr. Whitfield decides that “recently late” means two weeks, not one. Change days_late <= 7 to days_late <= 14 and nothing else — the reminder branch now starts at 15, automatically, because it never mentioned 7 in the first place.
2. A few lines. Put the day count into the second message, and handle the awkward case with a nested if: 1 day late, but 4 days late. It is a small thing that makes a program sound like it was written by somebody who cared.
3. A real change. Ask a third question — Is this a course text? (yes/no) — and make a course text under a week late get the standard reminder anyway, because somebody else’s class is waiting for it. You will need a condition with and in it, which is the whole subject of Boolean Logic. Then re-run the test plan: three of your five rows just changed their expected result.

Practise the branching itself in Decisions Practice.

Curriculum connection

A1.4

demonstrate the ability to use Boolean operators (e.g., AND, OR, NOT), comparison operators (i.e., equal to, not equal to, greater than, less than, greater than or equal to, less than or equal to), arithmetic operators (e.g., addition, subtraction, multiplication, division, exponentiation, parentheses), and order of operations correctly in computer programs;

Link to original

A2.2

use sequence, selection, and repetition control structures to create programming solutions;

Link to original

B3.1

design simple algorithms (e.g., add data to a sorted array, delete a datum from the middle of an array) according to specifications;

Link to original