Decisions: Conditional Logic

In the previous chapter we learned how programs store values.

But storing data is only part of the story.

Programs also need to make choices.

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Why Decisions Are Necessary

Real software is full of branching behavior.

Examples:

• if password is correct, allow login
• if stock is unavailable, show error
• if user is admin, show extra controls

Without conditional logic, programs would follow only one fixed path.

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Boolean Conditions

A condition evaluates to either:

• true
• false

Conditions are usually built from comparisons:

• == equal to
• != not equal to
• <, >, <=, >=

Example:

temperature = -2
if temperature < 0:
    wear_jacket = True

This is the exact idea: compare values, then choose behavior.

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If, Else, and Branching

The most common structure:

if condition:
    # run this block when condition is true
else:
    # run this block otherwise

You can also chain multiple branches:

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

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Visualizing Branches

graph TD
  A[Check Condition] --> B{True?}
  B -->|Yes| C[Run Branch A]
  B -->|No| D[Run Branch B]

At runtime, only one branch is executed for each decision point.

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Combining Conditions

Programs often combine multiple checks:

• and: both must be true
• or: at least one true
• not: flips true/false

Example:

if is_logged_in and has_permission:
    show_dashboard = True

This is how programs express more realistic rules.

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Common Beginner Mistakes

• confusing assignment (=) with comparison (==)
• forgetting edge cases
• writing deeply nested conditions that are hard to read

A practical habit is to test with several inputs, including boundary values.

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Key Ideas to Remember

• Conditional logic allows programs to choose behavior.
• Conditions evaluate to true or false.
• if/elif/else structures create program branches.
• Combined logical operators model real-world rules.
• Testing different inputs is essential for reliable decisions.

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What Comes Next

Conditions choose between paths.

Now we need a second core control structure:

repetition with loops, so programs can automate repeated tasks.