char_at_reverse(0) returns the first char of str

[dovahcrow]

could this be a bug? misunderstood the char_range_at_reverse methods?

[lifthrasiir]

It is correct. s.char_range_at_reverse(i) returns the last character from s.slice_to(i). Documentation. (It might be surprising that s.char_range_at_reverse(0) is defined, though.)

[dovahcrow]

"abcd".char_at(0) returns 'a'， but " abcd".char_at_reverse(0) not intuitively returns 'd'. It returns 'a' too.

[oli-obk]

taking the example from the docs: "中华" is 6 bytes long, taking char_at(1) will panic, char_at(2) will panic, char_at(3) will return '华'
reverse is the non-panicking version. char_at_reverse(1) will return '中'
since you do not know where the last character of your string ends, you need to use the char_range_at_reverse and use the next field to find the next character (in reverse iteration)

the only thing that is odd here is the inconsistency:
char_range_at_reverse: Given a byte position and a str, return the previous char and its position.
char_at_reverse: Plucks the character ending at the ith byte of a string.

char_range_at: Pluck a character out of a string and return the index of the next character.
char_at: Plucks the character starting at the ith byte of a string.

char_range_at_reverse should probably return the current char not the previous char. but this might be a documentation error

[oli-obk]

ok, not an inconsistency. just badly documented.

but there's a source for bugs: s.char_at(s.len()) panics, s.char_at_reverse(0) doesn't
so reverse iteration fails silently while forward iteration panics when going out of bounds.

[liigo]

very weird behavior, surprise

2014-12-11 17:35 GMT+08:00 oli-obk notifications@github.com:

taking the example from the docs: "中华" is 6 bytes long, taking char_at(1)
will panic, char_at(2) will panic, char_at(3) will return '华'
reverse is the non-panicking version. char_at_reverse(1) will return '中'
since you do not know where the last character of your string ends, you
need to use the char_range_at_reverse and use the next field to find the
next character (in reverse iteration)

the only thing that is odd here is the inconsistency:
char_range_at_reverse: Given a byte position and a str, return the
previous char and its position.
char_at_reverse: Plucks the character ending at the ith byte of a string.

char_range_at: Pluck a character out of a string and return the index of
the next character.
char_at: Plucks the character starting at the ith byte of a string.

char_range_at_reverse should probably return the current char not the
previous char. but this might be a documentation error

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[alexcrichton]

As @lifthrasiir mentioned, this is currently intended behavior in terms of indexing. I think what you're pointing out with char_range_at_reverse(0) may be a bug in the implementation where we may want to return an Option instead.

@oli-obk the functions for char_range and char with at and at_reverse variants are intended to have the same semantics, and I think any departure from this is definitely a bug in the implementation.