foo.c: In function 'main':
foo.c:10:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &arr[i]);       //输入三个整数
     ^~~~~~~~~~~~~~~~~~~~
foo.c:26:2: warning: iteration 2 invokes undefined behavior [-Waggressive-loop-optimizations]
  printf("%-4d", arr[i]);
  ^~~~~~~~~~~~~~~~~~~~~~
foo.c:24:2: note: within this loop
  for(i=0; i<3; i++)          //循环输出
  ^~~
foo.c:16:17: warning: iteration 1 invokes undefined behavior [-Waggressive-loop-optimizations]
    if(arr[j]<arr[j+1])      //交换变量
              ~~~^~~~~
foo.c:14:6: note: within this loop
      for(j=0; j< 2-i; j++)      //两两比较
      ^~~
foo.c:16:17: warning: array subscript is above array bounds [-Warray-bounds]
    if(arr[j]<arr[j+1])      //交换变量
              ~~~^~~~~
foo.c:20:7: warning: array subscript is above array bounds [-Warray-bounds]
    arr[j+1]=t;
    ~~~^~~~~

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