赋值运算符 += 与先前声明的变量的用法与声明的变量？

问：

我是 C 的新手，有点困惑。我有代码，在声明时使用带有变量的运算符，但它给了我这个运算符的错误，但在反向使用时工作正常，即.请解释一下为什么？+==+

代码如下：

  int i = 0;
  int g = 99;
  do
  {
    int  f += i;   //Here += is throwing error and =+ is working fine why?
    printf("%-6s = %d ","This is",f);
    i++;
  } while(i<10);

Error as 
ff.c:16:11: error: invalid '+=' at end of declaration; did you mean '='?
    int f += i;
          ^~
          =
1 error generated.

它是这样工作的：

  int i = 0;
  int g = 99;
  do
  {
    int  f =+ i;   //Here += is `your text throwing error and =+ is working fine why?
    printf("%-6s = %d ","This is",f);
    i++;
  } while(i<10);