foo.c: In function 'main':
foo.c:6:10: error: expected declaration specifiers or '...' before string constant
   printf("请输入数字正整数\n");
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:7:14: error: 'x' undeclared (first use in this function)
  scanf("%d",&x);
              ^
foo.c:7:14: note: each undeclared identifier is reported only once for each function it appears in
foo.c:10:3: error: 'a' undeclared (first use in this function)
   a[i]=x%10;
   ^
foo.c:10:5: error: 'i' undeclared (first use in this function)
   a[i]=x%10;
     ^
foo.c:11:3: error: 'sum' undeclared (first use in this function)
   sum=sum*10+a[i];
   ^~~

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