Procedures & Lists

📝 Procedures & Lists

Part 1 of 7 — List Operations, Traversals, and Procedure Design

Lists in AP CSP

A list (also called an array) stores an ordered collection of values.

// AP CSP pseudocode uses 1-based indexing!
myList ← ["apple", "banana", "cherry"]
DISPLAY(myList[1])    // "apple" (index 1 is the first element)
DISPLAY(myList[3])    // "cherry"

⚠️ AP CSP lists are 1-indexed (first element at index 1). Most programming languages are 0-indexed.

List Operations

Example

scores ← [90, 85, 78]
APPEND(scores, 92)           // [90, 85, 78, 92]
INSERT(scores, 2, 88)        // [90, 88, 85, 78, 92]
REMOVE(scores, 1)            // [88, 85, 78, 92]
DISPLAY(LENGTH(scores))      // 4

Concept Check 🎯

List Traversal

FOR EACH Loop

total ← 0
FOR EACH score IN scores
{
    total ← total + score
}
average ← total / LENGTH(scores)

Index-Based Loop

i ← 1
REPEAT UNTIL (i > LENGTH(scores))
{
    DISPLAY(scores[i])
    i ← i + 1
}

Common List Algorithms

Find the Maximum

PROCEDURE findMax(list)
{
    max ← list[1]
    FOR EACH item IN list
    {
        IF (item > max)
        {
            max ← item
        }
    }
    RETURN max
}

Filter a List

PROCEDURE getPositives(list)
{
    result ← []
    FOR EACH item IN list
    {
        IF (item > 0)
        {
            APPEND(result, item)
        }
    }
    RETURN result
}

Swap Two Elements

temp ← list[i]
list[i] ← list[j]
list[j] ← temp

Applied Recall ✍️

1. APPEND(list, value) adds the value to the _______ of the list.

2. REMOVE(list, i) removes the element at index i and shifts remaining elements _______.

3. To swap two list elements without losing data, you need a _______ variable.

List Operations 🔍

AP Exam Strategy: Procedures & Lists

• Lists are 1-indexed on the AP exam — first element is list[1], NOT list[0]
• Know all list operations: access, assign, INSERT, APPEND, REMOVE, LENGTH
• INSERT shifts elements RIGHT (list grows). REMOVE shifts elements LEFT (list shrinks)
• The swap algorithm (using temp) appears frequently — memorize it
• Common patterns: sum/average, find max/min, filter, count matches
• FOR EACH is read-only for the loop variable — changes to the variable do NOT change the list

AP-Style Application 🎯

📋 Procedures & Lists

Part 2 of 7 — Key Processes

Procedures Bundle Reusable Logic

A procedure (a.k.a. function or method) is a named, reusable block of instructions. Define once, call many times.

    PROCEDURE greet(name)       DISPLAY("Hello, " + name)

Calling greet("Alex") runs the body with name = "Alex".

Concept Check 🎯

Why Procedures Matter

📋 Procedures & Lists

Part 3 of 7 — Patterns & Examples

Procedure Patterns

List Patterns

Concept Check 🎯

Map / Filter / Reduce In Pseudocode

📋 Procedures & Lists

Part 4 of 7 — Connections & Interactions

Procedures & Lists Connect Across CSP

Concept Check 🎯

Lists Are Often Procedure Inputs / Outputs

Most non-trivial procedures take or return lists:

    PROCEDURE topThree(scores)       sorted ← SORT(scores, descending)       RETURN [sorted[1], sorted[2], sorted[3]]

Mutability Caution

When a procedure receives a list, modifying it may modify the caller's list (depending on language). To stay safe, document whether your procedure mutates inputs or returns a fresh list.

Pure Functions Are Easier To Test

A pure function (no side effects, output depends only on inputs) can be tested with simple input → expected-output pairs:

📋 Procedures & Lists

Part 5 of 7 — Change Over Time

How Procedures & Lists Have Evolved

Concept Check 🎯

Lists Got A Cousin: Other Collections

AP CSP focuses on lists, but real software uses all of these.

📋 Procedures & Lists

Part 6 of 7 — Problem-Solving Workshop

Procedures & Lists Workshop

Concept Check 🎯

Worked: Build A "Top K" Procedure

    PROCEDURE topK(scores, k)       sorted ← SORT(scores, descending)       result ← []       FOR i FROM 1 TO k:         APPEND(result, sorted[i])       RETURN result

Now any caller can ask for the top 3, top 10, etc.

Worked: Compose Map + Filter + Reduce

Sum the squares of even numbers in nums:

    total ← 0     FOR EACH n IN nums:       IF n MOD 2 = 0 THEN         total ← total + n * n

Conceptually: filter (even) → map (square) → reduce (sum).

Worked: 2D Sum

    PROCEDURE gridSum(grid)       total ← 0       FOR EACH row IN grid:         FOR EACH cell IN row:           total ← total + cell       RETURN total

Applied Recall ✍️

1. A "top K" procedure usually first _______ the list, then takes the first k elements.

2. Filter then map then reduce is a common functional _______.

3. Touching every cell in a 2D list requires _______ loops.

Targeted Practice 🔍

AP Exam Strategy: Workshop

• Recognize map / filter / reduce shape from the loop body.
• Top-K = sort + take.
• 2D traversal = nested loops.

AP-Style Application 🎯

📋 Procedures & Lists

Part 7 of 7 — AP Review

AP Exam Recap — Procedures & Lists

Concept Check 🎯

Final Vocab

Common Pitfalls

• Confusing parameter and argument.
• Off-by-one with 1-indexed lists.
• Forgetting to RETURN a value.
• Mutating a caller's list without documenting it.
• Writing one big function instead of decomposing.

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