foo.c: In function 'main':
foo.c:11:25: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
   11 |                 int n = sqrt(y);
      |                         ^~~~
foo.c:2:1: note: include '<math.h>' or provide a declaration of 'sqrt'
    1 | #include <stdio.h>
  +++ |+#include <math.h>
    2 | 
foo.c:11:25: warning: incompatible implicit declaration of built-in function 'sqrt' [-Wbuiltin-declaration-mismatch]
   11 |                 int n = sqrt(y);
      |                         ^~~~
foo.c:11:25: note: include '<math.h>' or provide a declaration of 'sqrt'
foo.c:12:17: warning: statement with no effect [-Wunused-value]
   12 |                 for (a;a <= n;a++)
      |                 ^~~

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