Chapter-2 code

Hi Ageron,

I'm new to ML and recently started reading your book. I have below question from Chapter 2.

```python
def split_train_test_by_id(data, test_ratio, id_column, hash=hashlib.md5):
ids = data[id_column]
in_test_set = ids.apply(lambda id_: test_set_check(id_, test_ratio, hash))
return data.loc[~in_test_set], data.loc[in_test_set]
```

1- In the above code, why we use same variable "id_" twice once with lambda and second with test_set_check function?
2- Exactly what value of "id_" is passing to "test_set_check" function? We didn't initialize any value to it anywhere.

Please respond in detail.

Thanks
Shweta