Github user viirya commented on the issue:
https://github.com/apache/spark/pull/20464
> One followup question is though, would it be difficult to match the
behaviour with substr in R when the index is 0 or minus? If i understood #20464
(comment) correctly, it sounds better to match it to substr's behaviour in R.
Took a quick look/test and seems we can just set start to 1 for both cases.
If we both consider the indices at starting and ending, setting them to 1
seems not enough. E.g.,
```R
> substr("abcdef", -2, -3)
[1] ""
> substr("abcdef", 1, 1)
[1] "a"
```
For the cases when only ending is zero/negative, no matter what starting
is, the result is empty string.
For the cases when only starting is zero/negative, we can set it to 1.
For the cases they are both zero/negative, the result is empty string.
We can address this in another PR.
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