Selection (if/else) and logic - Computer Science A AP Study Notes

Selection statements allow programs to make decisions and execute different code paths based on conditions. This is fundamental to creating intelligent, responsive software.

Key Terms:

Conditional statements (if/else) evaluate Boolean expressions to determine program flow. An if statement executes code only when a condition is true. An else clause provides an alternative path when the condition is false. else if (or elif) allows testing multiple conditions sequentially.

Boolean expressions return true or false values. They use relational operators: == (equal to), != (not equal), > (greater than), < (less than), >= (greater than or equal), <= (less than or equal).

Logical operators combine multiple conditions:

• AND (&& or and): Both conditions must be true
• OR (|| or or): At least one condition must be true
• NOT (! or not): Inverts the Boolean value

Truth Tables (essential for Cambridge exams):

| A | B | A AND B | A OR B | |---|---|---------|--------| | T | T | T | T | | T | F | F | T | | F | T | F | T | | F | F | F | F |

Nested selection places if statements inside others, enabling complex decision-making. The general syntax follows:

if (condition1)
    // code block
else if (condition2)
    // code block
else
    // default code

Order of evaluation matters: logical operators follow precedence (NOT > AND > OR). Use parentheses to control evaluation order explicitly.

Cambridge Note: Examiners expect precise use of terminology. "Selection" is preferred over "branching" or "decision-making" in formal answers.

Selection statements mirror everyday decision-making. Think of a traffic light system: IF light is red THEN stop, ELSE IF light is amber THEN prepare to stop, ELSE (green) proceed. This nested logic ensures only one action occurs.

Real-World Application 1: Authentication Systems

When you log into a website, the system uses selection:

if (username correct AND password correct)
    grant access
else if (username correct AND password incorrect)
    display "wrong password"
else
    display "account not found"

The AND operator ensures both credentials validate. Using OR would create a security vulnerability—imagine granting access with just a correct username!

Real-World Application 2: E-commerce Discount Logic

Online stores calculate prices using nested selection:

if (customer is VIP)
    discount = 20%
else if (order total > £100)
    discount = 15%
else if (first purchase)
    discount = 10%
else
    discount = 0%

Notice the order matters: VIP status checks first, preventing customers getting lesser discounts.

Analogy: A Restaurant Menu Decision Tree

Selecting a meal follows logical steps: IF vegetarian THEN show veggie options, ELSE IF allergic to nuts THEN filter nut dishes, ELSE show full menu. Each condition narrows choices systematically.

Short-Circuit Evaluation optimizes performance: In A AND B, if A is false, B isn't checked (result must be false). In A OR B, if A is true, B isn't evaluated. This prevents errors: if (divisor != 0 AND number/divisor > 5) safely checks division by placing the guard condition first.

Memory Aid (MNEMONIC): "AND needs ALL, OR needs ONE" helps remember logical operator requirements.