foo.c: In function 'main':
foo.c:20:14: error: 'j' undeclared (first use in this function)
   20 |         for (j = 2; j < (n - i); j++) { // 判断(n - i)是否素数
      |              ^
foo.c:20:14: note: each undeclared identifier is reported only once for each function it appears in

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