Simple question, is it possible to do a for loop on a range of negative values? Specifically starting at -1 and iterating down to some negative value. for i in -1..x apparently isn't kosher.
Other than the ugly solution of iterating on the absolute values and then applying the sign within the loop.
The problem isn't in the negative numbers, the problem is in the reverse range. If we assume that x < -1 then you have a range (a,b) with a > b.
The default step of a rust range in a for loop is +1, so the loop never executes. If you write it out, C++ style, you'd be saying
for (i=-1; i < x; i++) { ... }
So what you want to do is reverse the range:
for i in (x..=-1).rev()
EDIT: Because (x..-1) does not include -1, we need to change it to (x..=-1). In that case the range will include both x and 1. If we don't want that, we have to fiddle with the x bound as well, e.g. ((x+1)..=-1).rev
step_by parameter supposed to be unsigned?
for i in (x..-1).rev() iterates from -2 down to x, since the range x..-1 does not include the endpoint.
(x..-1).rev() starts at -2. It should be (x..=-1).rev().The standard Range type (what you get with a..b) is a half-open increasing range. If you have a range where a >= b, then the range is empty.
If you want to iterate a range in reverse (decreasing) order, then you need to use Iterator::rev to reverse the range, which is enabled by range being a DoubleEndedIterator and providing the reverse iteration next_back.
So in this case, you'd iterate over ((x + 1)..0).rev().
Just as a fun side note, you can also get basically any iterator you want out of iter::successors, by just providing a fn next(&T) -> Option<T>; in this case, |x| x - 1, if that's clearer in a specific use case than reversing an increasing range.
(x..-1).rev() will start at -2. You need to use (x..=-1).rev() instead.