如题！

#include<stdio.h>
main()
{float score,average=0;
  int n=0;
  scanf("%f",&score);
  while(score>=0)
  {average+=score;
   n++;
   scanf("%f",&score);
  }
  if(n!=0) average=average/n;
  printf("%6.2f",average);
}
大虾这个while和if是怎么执行的？