foo.c: In function 'main':
foo.c:9:2: warning: statement with no effect [-Wunused-value]
  for(num;num>0;num--)
  ^~~
foo.c:7:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a[i]);
   ^~~~~~~~~~~~~~~~~
foo.c:8:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&num);
  ^~~~~~~~~~~~~~~~