Python用while循环下标越界怎么解决？

报错内容：
Traceback (most recent call last):
File "C:/Users/kh/PycharmProjects/pythonProject1/main.py", line 24, in <module>
ddh1 = df1.iloc[a,7]
File "C:\Users\kh\.conda\envs\pythonProject1\lib\site-packages\pandas\core\indexing.py", line 889, in __getitem__
return self._getitem_tuple(key)
File "C:\Users\kh\.conda\envs\pythonProject1\lib\site-packages\pandas\core\indexing.py", line 1450, in _getitem_tuple
self._has_valid_tuple(tup)
File "C:\Users\kh\.conda\envs\pythonProject1\lib\site-packages\pandas\core\indexing.py", line 723, in _has_valid_tuple
self._validate_key(k, i)
File "C:\Users\kh\.conda\envs\pythonProject1\lib\site-packages\pandas\core\indexing.py", line 1358, in _validate_key
self._validate_integer(key, axis)
File "C:\Users\kh\.conda\envs\pythonProject1\lib\site-packages\pandas\core\indexing.py", line 1444, in _validate_integer
raise IndexError("single positional indexer is out-of-bounds")
IndexError: single positional indexer is out-of-bounds

12行到17行的代码 我想让他循环起来 就用了20行到27行的代码，但是到了24行就开始报错，该怎么解决，还有我发现执行if语句后变量a的值不会加1

第二个循环之前a和b要重新设置为0

你加点打印信息，可能if条件都不满足

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