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10_Wiki/Dev/Topic_C/C_Functions_Recursion.md

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+---
+id: c-functions-recursion
+title: "C Recursion"
+category: "Programming_Language"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["recursive function", "base case", "factorial recursion", "C 재귀"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.86
+created_at: 2026-07-04
+updated_at: 2026-07-04
+review_reason: ""
+merge_history: []
+tags: ["c", "programming-language", "w3schools", "functions", "recursion"]
+raw_sources: ["https://www.w3schools.com/c/c_functions_recursion.php"]
+applied_in: []
+github_commit: ""
+---
+
+# [[C Recursion]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Every recursive function in the source's examples relies on a single structural guarantee: the function ONLY calls itself when a shrinking condition still holds (`k > 0`, `n > 0`, `n > 1`) and returns a plain value OTHERWISE — the source's own warning that recursion "can be quite easy to slip into writing a function which never terminates" is really pointing at what happens if that shrinking condition is missing or wrong, not at recursion itself being inherently risky. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Recursion** — a function calling ITSELF, breaking a complex problem into smaller instances of the same problem. [S1]
+- **Base case** — the condition under which the function STOPS calling itself and returns directly (e.g. `k > 0` becoming false), essential for termination. [S1]
+- **Unwinding accumulation** — each recursive call's `return` combines its own contribution with the result of the smaller sub-call (`return k + sum(k - 1);`), building up the final answer as calls return in reverse order. [S1]
+- **Risk of non-termination or excess resource use** — without a solid base case, recursion can run forever or exhaust memory/processor time. [S1]
+
+## 📖 세부 내용 (Details)
+- Summing 1 to 10 via recursion: `int sum(int k) { if (k > 0) { return k + sum(k - 1); } else { return 0; } }` — unfolds as `10 + (9 + (8 + ... + sum(0)))`. [S1]
+- Countdown via recursion: `void countdown(int n) { if (n > 0) { printf("%d ", n); countdown(n - 1); } }` — stops calling itself once `n` reaches 0. [S1]
+- Factorial via recursion: `int factorial(int n) { if (n > 1) { return n * factorial(n - 1); } else { return 1; } }` — factorial(5) = 5×4×3×2×1 = 120; by definition 0! is also 1. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+- **재귀의 종료 조건이 핵심 위험 요소**: 종료되지 않는 함수나 과도한 메모리/CPU를 쓰는 함수를 실수로 작성하기 쉽다는 경고가 있지만, 이는 결국 축소 조건(base case)이 없거나 잘못됐을 때 발생하는 문제임이 예제들에서 공통적으로 드러남. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+5의 팩토리얼(5×4×3×2×1=120)을 재귀로 계산하는 예제가 원문에서 직접 실전 활용 사례로 제시됨. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+A recursive function with a clear base case that guarantees termination (C):
+```c
+int factorial(int n) {
+  if (n > 1) {
+    return n * factorial(n - 1);
+  } else {
+    return 1; // base case
+  }
+}
+```
+
+## ✅ 검증 상태 및 신뢰도
+- **상태:** draft
+- **검증 단계:** conceptual
+- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
+- **신뢰 점수:** 0.86
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[C Tutorial]]
+- **관련 개념:** [[C Functions Parameters]], [[C Memory Allocate]], [[C Functions Pointers]]
+- **참조 맥락:** 재귀 — 함수 포인터(Functions Pointers) 챕터로 이어짐, 스택 메모리(Memory Allocate 챕터의 스택 오버플로우 개념)와 연결.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — C Recursion — https://www.w3schools.com/c/c_functions_recursion.php
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "C Recursion" page (Astra wiki-curation, P-Reinforce v3.1 format).