foo.c: In function 'main':
foo.c:9:7: warning: implicit declaration of function 'isprime' [-Wimplicit-function-declaration]
   if( isprime(i)&&isprime(num - i))
       ^~~~~~~
foo.c: In function 'isprime':
foo.c:19:6: warning: unused variable 'a' [-Wunused-variable]
  int a;
      ^
foo.c: In function 'main':
foo.c:5:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&num);
  ^~~~~~~~~~~~~~~~

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