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randの既定シード値は1

C

C標準ライブラリ提供の擬似乱数生成関数randでは、既定のシード値は 1 と定義される。

#include <stdlib.h>

int main()
{
  // 暗黙にsrand(1)相当でシード値を設定
  int x = rand();
}

C11 7.22.2.2/p2より引用（下線部は強調）。

The srand function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand. If srand is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. If rand is called before any calls to srand have been made, the same sequence shall be generated as when srand is first called with a seed value of 1.

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