2nd/10_Wiki/Topic_Programming/Topic_C/C_Random_Numbers.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
c-random-numbers	C Random Numbers	Programming_Language	draft	conceptual		rand() srand() seeding random dice roll example C 난수		B	0.85	2026-07-04	2026-07-04			c programming-language w3schools random stdlib	https://www.w3schools.com/c/c_random_numbers.php

C Random Numbers

🎯 한 줄 통찰 (One-line insight)

Without calling srand(), rand() produces the EXACT SAME sequence of numbers every single time the program runs — it's not "random" by default at all, just deterministic-looking — and the standard fix (seeding with time(NULL)) works only because the current time is itself always different, meaning C's "randomness" is really pseudo-randomness bootstrapped from an ever-changing external value. [S1]

🧠 핵심 개념 (Core concepts)

• rand() (from <stdlib.h>) — returns a random integer, but deterministic/repeatable unless seeded. [S1]
• srand(seed) — sets the starting point for rand()'s sequence; without it, the sequence repeats identically on every run. [S1]
• Time as a seed — srand(time(NULL)) (requiring <time.h>) uses the current time as the seed since it's always changing, producing different sequences run to run. [S1]
• Call srand() exactly once — specifically at the start of main(), NOT repeatedly inside a loop (re-seeding inside a loop would reset the sequence and undermine randomness). [S1]
• Range restriction via modulo — rand() % N maps the result into the range 0 to N-1; adding an offset shifts the range further (e.g. (rand() % 6) + 1 for 1-6). [S1]

📖 세부 내용 (Details)

• Unseeded rand() (repeats every run): int r = rand(); printf("%d\n", r);. [S1]
• Properly seeded rand(): srand(time(NULL)); printf("%d\n", rand()); — different results each run. [S1]
• Restricting to a 0-9 range: int x = rand() % 10; // 0..9. [S1]
• Dice-roll simulation combining modulo and offset: int dice1 = (rand() % 6) + 1; int dice2 = (rand() % 6) + 1; printf("You rolled %d and %d (total = %d)\n", dice1, dice2, dice1 + dice2);. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

• 시드 없이는 진짜 랜덤이 아님: srand()를 호출하지 않으면 프로그램을 몇 번을 실행해도 항상 같은 숫자가 나온다는 점이 명시적으로 경고됨 — 진짜 다른 결과를 원하면 반드시 시드를 설정해야 함. [S1]
• srand()는 반복문 안에서 재호출 금지: main 시작 시 딱 한 번만 호출해야 하며, 루프 안에서 반복 호출하면 안 된다는 점이 팁으로 강조됨. [S1]

🛠️ 적용 사례 (Applied in summary)

주사위 두 개를 굴려 1-6 범위의 숫자를 뽑고 합계를 계산하는 예제가 원문에서 직접 실전 활용 사례로 제시됨. [S1]

💻 코드 패턴 (Code patterns)

Simulating a dice roll using rand(), modulo, and an offset (C):

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main() {
  srand(time(NULL)); // seed once, with the current time
  int dice1 = (rand() % 6) + 1;
  int dice2 = (rand() % 6) + 1;
  printf("You rolled %d and %d (total = %d)\n", dice1, dice2, dice1 + dice2);
  return 0;
}

✅ 검증 상태 및 신뢰도

• 상태: draft
• 검증 단계: conceptual
• 출처 신뢰도: B (W3Schools — widely used educational reference, not a primary standards body)
• 신뢰 점수: 0.85
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: C Tutorial
• 관련 개념: C Math, C Date Time, C Booleans
• 참조 맥락: 수학 및 난수 섹션 마지막 — 불리언(Booleans) 섹션으로 이어짐.

📚 출처 (Sources)

• [S1] W3Schools — C Random Numbers — https://www.w3schools.com/c/c_random_numbers.php

📝 변경 이력 (Change history)

• 2026-07-04: Initial draft synthesized from the W3Schools "C Random Numbers" page (Astra wiki-curation, P-Reinforce v3.1 format).