cc-control-flow-quality

Skill: cc-control-flow-quality

STOP - Never Skip (even under time pressure)

• Check nesting depth before adding levels (max 3)
• Verify loop exit conditions are reachable
• Name loop indexes meaningfully in nested loops
• McCabe complexity >10 = redesign required

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Quick Reference

Threshold	Value	Source
Max nesting depth	3 levels	Chomsky, Weinberg via Yourdon 1986a
McCabe complexity	>10 decision points = redesign	McCabe 1976
Else clause consideration	50-80% of ifs need else	Elshoff 1976
Loop-with-exit comprehension	25% better than top/bottom test	Soloway 1983
Mental entity limit	5-9 items	Miller 1956

Key Principles:

• Use true/false not 0/1 for booleans
• Use for when iteration count is known, while when unknown, foreach for collections
• Test at beginning (may not execute) vs end (executes at least once)
• Put nominal case in if, error case in else (exception: guard clauses invert this intentionally—see below)
• Fully parenthesize complex boolean expressions

NEVER Skip (even under time pressure):

• Check nesting depth before adding levels
• Verify loop exit conditions are reachable
• Name loop indexes meaningfully in nested loops

Core Patterns

Deep Nesting → Guard Clauses

Reconciling with "nominal in if": Guard clauses are an exception pattern where error/invalid cases exit early at the top of a function. This contradicts the general "nominal in if" rule intentionally—the goal is to get preconditions out of the way so the nominal path flows without nesting. Use guard clauses at function entry; use "nominal in if" for conditionals within the function body.

// BEFORE: Arrow-shaped code (4 levels) [ANTI-PATTERN]
if (file.validName()) {
    if (file.open()) {
        if (encryptionKey.valid()) {
            if (file.decrypt(encryptionKey)) {
                // lots of code (nominal case buried)
            }
        }
    }
}

// AFTER: Guard clauses flatten structure
if (!file.validName()) return;
if (!file.open()) return;
if (!encryptionKey.valid()) return;
if (!file.decrypt(encryptionKey)) return;

// lots of code (nominal case at top level)

Complex Boolean → Intermediate Variables

// BEFORE: Monstrous test [ANTI-PATTERN]
if (inputStatus == SUCCESS && printerReady && !printerBusy &&
    paperAvailable && paperLevel > MIN_PAPER &&
    documentValid && !documentEmpty && documentSize < MAX_SIZE) {
    print();
}

// AFTER: Named intermediate booleans
bool printerCanPrint = printerReady && !printerBusy &&
                       paperAvailable && paperLevel > MIN_PAPER;
bool documentCanBePrinted = documentValid && !documentEmpty &&
                            documentSize < MAX_SIZE;

if (inputStatus == SUCCESS && printerCanPrint && documentCanBePrinted) {
    print();
}

Meaningless Loop Indexes → Descriptive Names

// BEFORE: Impossible to verify correct array access [ANTI-PATTERN]
for (int i = 0; i < numPayCodes; i++) {
    for (int j = 0; j < 12; j++) {
        for (int k = 0; k < numDivisions; k++) {
            sum = sum + transaction[j][i][k];  // Is this right? Who knows!
        }
    }
}

// AFTER: Self-documenting and verifiable
for (int payCodeIdx = 0; payCodeIdx < numPayCodes; payCodeIdx++) {
    for (int month = 0; month < 12; month++) {
        for (int divisionIdx = 0; divisionIdx < numDivisions; divisionIdx++) {
            sum = sum + transaction[month][payCodeIdx][divisionIdx];
        }
    }
}

Single-Use Test → Named Boolean Function

// BEFORE: Complex inline test obscures main flow
if (((('a' <= inputChar) && (inputChar <= 'z')) ||
     (('A' <= inputChar) && (inputChar <= 'Z'))) &&
    (inputChar != 'X') && (inputChar != 'x')) {
    processLetter(inputChar);
}

// AFTER: Named function (even for single use!)
// "Putting a test into a well-named function improves readability,
//  and that's a sufficient reason to do it." [KEY POINT p.433]
bool isProcessableLetter(char c) {
    bool isLetter = (('a' <= c) && (c <= 'z')) ||
                    (('A' <= c) && (c <= 'Z'));
    bool isExcluded = (c == 'X') || (c == 'x');
    return isLetter && !isExcluded;
}

if (isProcessableLetter(inputChar)) {
    processLetter(inputChar);
}

Number-Line Ordering for Comparisons

// Range checks: order smallest → variable → largest
if (MIN_VALUE <= count && count <= MAX_VALUE)  // count is between

// Out-of-range: variable on outside
if (count < MIN_VALUE || MAX_VALUE < count)    // count is outside

// This visual mapping aids comprehension [KEY POINT p.440]

Type-Appropriate Zero Comparisons

// Boolean: compare implicitly
while (!done)                    // not: while (done == false)

// Numeric: compare explicitly
while (balance != 0)             // not: while (balance)

// Character (C): compare to null terminator explicitly
while (*charPtr != '\0')         // not: while (*charPtr)

// Pointer: compare to NULL explicitly
while (bufferPtr != NULL)        // not: while (bufferPtr)

Loop-With-Exit Avoiding Duplication

// BEFORE: Duplicated code that will break under maintenance [ANTI-PATTERN]
GetNextRating(&ratingIncrement);
rating = rating + ratingIncrement;
while ((score < targetScore) && (ratingIncrement != 0)) {
    GetNextScore(&scoreIncrement);
    score = score + scoreIncrement;
    GetNextRating(&ratingIncrement);      // Duplicated!
    rating = rating + ratingIncrement;    // Duplicated!
}

// AFTER: Loop-with-exit eliminates duplication
while (true) {
    GetNextRating(&ratingIncrement);
    rating = rating + ratingIncrement;
    if (!((score < targetScore) && (ratingIncrement != 0))) {
        break;
    }
    GetNextScore(&scoreIncrement);
    score = score + scoreIncrement;
}
// Students scored 25% higher on comprehension with this pattern [Soloway 1983]

Decision Guidance

Loop Selection

1. Is iteration count known ahead of time?
   YES → Use `for` or `foreach`
   NO  → Use `while`

2. Must body execute at least once?
   YES → Use loop with test at END (`do-while`) or MIDDLE
   NO  → Use loop with test at BEGINNING (`while`)

3. Simple iteration through container?
   YES → Use `foreach` (eliminates housekeeping arithmetic errors)
   NO  → Use appropriate `for` or `while`

4. Need to exit from middle of loop?
   YES → Consider loop-with-exit (`while(true)` + `break`)
   NO  → Standard loop structure

Nesting Reduction Techniques (by invasiveness, not strict priority)

These techniques are ordered from least to most invasive changes—not a strict "try #1 before #2" sequence. Choose based on what fits the problem:

1. Guard clauses - Exit early on error conditions (least invasive; see Core Patterns)
2. Extract to routine - Move nested code into well-named function
3. Combine conditions - Merge nested ifs into single compound test
4. Convert to case/switch - When testing same variable repeatedly
5. Use polymorphism - Replace type-checking conditionals with dispatch
6. Table-driven methods - When logic maps data to outcomes (most architectural)

Note: #5 and #6 are context-dependent, not ranked. Tables can be simpler than polymorphism (see "The fact that a design uses inheritance and polymorphism doesn't make it a good design" [p.423]).

When to Use Table-Driven Methods

Consider tables instead of logic when:

• Writing 4th+ branch in if-else chain for same classification
• Creating subclass just to change a data value
• Format/rules change frequently without code changes desired
• Same lookup calculation duplicated in multiple places

"The fact that a design uses inheritance and polymorphism doesn't make it a good design." [p.423]

Table Access Method Selection

1. Can data key directly into table? (e.g., month 1-12, char code 0-255)
   YES → Direct Access
   NO  → Continue

2. Is keyspace large/sparse but entries few? (e.g., 100 items with 4-digit part numbers)
   YES → Indexed Access (index table + main table)
   NO  → Continue

3. Are entries valid for ranges, not distinct points? (e.g., grade thresholds)
   YES → Stair-Step Access (loop through range boundaries)
   NO  → Reconsider if table-driven is appropriate

Table-Driven Example: Message Processing

// BEFORE: 20 routines, one per message type [ANTI-PATTERN]
void processMessage(Message& msg) {
    if (msg.type == CYCLIC_REDUNDANCY_CHECK) {
        processCRC(msg);
    } else if (msg.type == BUOY_TEMPERATURE) {
        processBuoyTemp(msg);
    } else if (msg.type == SONOBUOY_PING) {
        processSonobuoyPing(msg);
    }
    // ... 17 more branches
}

// AFTER: One generic routine + table definition [p.418-420]
// Table describes message format; routine interprets any message
struct FieldDef { FieldType type; string name; };
struct MessageDef { string name; vector<FieldDef> fields; };

map<MessageType, MessageDef> messageTable = {
    {BUOY_TEMPERATURE, {"Buoy Temperature", {
        {FLOAT, "Average Temperature"},
        {FLOAT, "Temperature Range"},
        {INT, "Number of Samples"},
        {STRING, "Location"},
        {TIME, "Time of Measurement"}
    }}}
    // New message types = new table entries, zero code changes
};

void processMessage(Message& msg) {
    auto& def = messageTable[msg.type];
    for (auto& field : def.fields) {
        readAndPrint(field, msg);  // One routine handles all types
    }
}

McCabe Complexity Counting [p.458]

How to count decision points in a routine:

1. Start with 1 for the straight path through the routine
2. Add 1 for each: if, while, repeat, for, and, or
3. Add 1 for each case in a case/switch statement

Interpretation:
  0-5   → Routine is probably fine
  6-10  → Start thinking about simplification
  10-20 → Strong case needed to keep as-is; review for extraction
  20+   → Mandatory refactor (no exceptions)

Exception criteria for 10-20 range: A routine may legitimately exceed 10 ONLY when ALL of:

1. It's a flat dispatch (switch/case with no nesting within cases)
2. Each case is ≤3 lines (ideally just a function call)
3. Cases are exhaustive and unlikely to grow unboundedly
4. Cognitive complexity is low despite high McCabe count

NOT a valid exception: "It's just a big if-else chain" or "The complexity is inherent."

Emergency Minimum (Crisis Mode)

When production is down and you must fix immediately, you still MUST:

1. Check nesting depth - Don't add a 4th level to fix a 3-level problem
2. Verify loop exits - Ensure your fix doesn't create infinite loops
3. Name any new variables meaningfully - tempFix guarantees confusion later
4. Document WHY - A one-line comment explaining the crisis fix

After crisis resolved: Return within 24 hours to apply full control flow review. Crisis fixes that "work" often hide deeper issues.

Modern Patterns (Beyond Code Complete)

These patterns weren't covered in Code Complete (2004) but follow the same principles:

Async/Await (Promise Chains)

// BEFORE: Callback hell (deep nesting equivalent)
getData(url, (data) => {
    process(data, (result) => {
        save(result, (saved) => {
            notify(saved, () => { /* ... */ });
        });
    });
});

// AFTER: Flattened with async/await
const data = await getData(url);
const result = await process(data);
const saved = await save(result);
await notify(saved);

Same principle as guard clauses: flatten the structure.

Pattern Matching (Rust, C#, etc.)

// Pattern matching can exceed McCabe threshold but remain clear
// because it's flat dispatch with exhaustiveness checking
match status {
    Status::Pending => handle_pending(),
    Status::Active => handle_active(),
    Status::Suspended => handle_suspended(),
    Status::Closed => handle_closed(),
}

This is the "valid exception" for McCabe—flat dispatch with ≤3 lines per case.

Functional Pipelines

// Functional pipelines replace explicit loops
const result = items
    .filter(item => item.active)
    .map(item => transform(item))
    .reduce((acc, item) => acc + item.value, 0);

Prefer over explicit loops when: operations are independent, no early exit needed, and pipeline reads clearly.

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Chain

After	Next
Control flow verified	code-clarity-and-docs (CHECKER)