foo.c: In function 'main':
foo.c:19:15: error: 'j' undeclared (first use in this function)
   19 |         for ( j = i + 1; j <= n; j++)
      |               ^
foo.c:19:15: note: each undeclared identifier is reported only once for each function it appears in
foo.c:14:13: warning: unused variable 'k' [-Wunused-variable]
   14 |     int n,i,k;
      |             ^

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