Making Decisions

Everything you had written until now ran the same way every time. Then somebody described the library return desk: a book comes back, and what happens next depends on how late it is. Nobody could write that as a straight list of steps. On the whiteboard it came out as a diagram with branches — and a diagram with branches is a program with if in it.

One question, three answers

graph TD
    A["Book returned"] --> B{"Days late?"}
    B -->|"none"| C["Shelve it, say thanks"]
    B -->|"under a week"| D["Shelve it, no message"]
    B -->|"more than a week"| E["Send the reminder"]

Draw the diagram first, every time, until you stop needing to. It is also how you check a decision with the person who asked for the program: Mr. Whitfield can look at that picture and say “no — under a week I still want to know”, which is a thirty-second conversation instead of a rewrite. A flow chart is one of the design tools the course expects you to use, alongside the pseudocode and structure charts in Decomposition and Design.

The shape in Python

if days_late <= 0:
    print("On time. Shelve it and say thanks.")
elif days_late <= 7:
    print("Less than a week late. Shelve it, no message.")
else:
    print("Send the standard reminder.")

Four things are doing work here:

• The condition — days_late <= 0 — is an expression that comes out True or False, built from the comparison operators: equal to ==, not equal to !=, and <, >, <=, >=.
• The colon ends the question.
• The indentation says what belongs to that branch. Python is not being fussy; the indenting is the grouping, where other languages use braces.
• Exactly one branch runs. elif means “otherwise, if”, and else catches everything left over.

else is optional, but leaving it out is a decision too: it means “when none of these match, do nothing”. Say that out loud and check it is what you want.

Order matters more than you expect

This looks reasonable and is wrong:

mark = 91
 
if mark >= 50:
    print("Pass")
elif mark >= 80:
    print("Excellent")

It prints Pass. A 91 satisfies the first condition, so Python never looks at the second — the Excellent branch is unreachable for every mark that could ever reach it. In an elif chain, the most demanding condition goes first:

if mark >= 80:
    print("Excellent")
elif mark >= 50:
    print("Pass")
else:
    print("Not yet")

Nothing about the broken version is a syntax error, and Python will never warn you. This is a logic error, and the only thing that catches it is trying a value from each branch on purpose — the habit built in Testing and Debugging and drilled in Trace It.

Nesting, and when to stop

An if inside an if is legal and sometimes exactly right:

if days_late > 0:
    if days_late > 30:
        print("Ask in person.")
    else:
        print("Send the reminder.")

But three levels of nesting is usually a sign that the conditions want to be combined instead — and, or, and not are next, in Boolean Logic. Practise the branching itself in Decisions Practice, then read a complete decision-driven program in Branching Programs.

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;

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A2.2

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

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B2.4

represent the structure and components of a program using industry-standard programming tools (e.g., structure chart, flow chart, UML [Unified Modeling Language], data flow diagram, pseudocode);

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