Feature #20811
closed`warning: in a**b, b may be too big` is really helpful?
Description
I tried to calculate the largest prime number recently discovered. However, it did not work. I was a bit disappointed.
$ ruby -e 'p 2**136279841-1'
-e:1: warning: in a**b, b may be too big
Infinity
I know this is not a realistic case, but I checked my past chat logs and found a record of the same thing six years ago and the same disappointment.
$ ruby -e 'p 2**77232917-1'
-e:1: warning: in a**b, b may be too big
Infinity
So I would like to ask, has anyone experienced that this behavior has actually been helpful?
I think that this limit is to prevent a program from unintentionally spending too much time and memory trying to compute a power that is too large. However, is it helpful to return Infinity in such a situation? I think raising an exception would be safer, if this limit is really needed.
Also, is this limit appropriate? On my machine, 2 ** 32537661
is calculated normally and 2 ** 32537662
returns Infinity. This calculation took about 100 ms and 400 kB. I think this is short enough for a single computation time. Incidentally, 2 ** 32537661
in decimal notation is 9,794,814 digits, which may be too loose a limit considering the display.
Note that this limit only applies when calculating the power directly. (2 ** 32537661) * 2
returns an Integer, not Infinity. Even 1 << 32537662
gives an Integer.
From the above, I suspect that this limit is not particularly useful and only spoils the fun of mathematics. What do you think?
Updated by byroot (Jean Boussier) 20 days ago
Agreed. Either Ruby can't do it and should raise some sort of error, or just do it even if it uses gigabytes of memory.
That warning isn't helping anyone, and returning Infinity
is a huge gotcha and totally unexpected.
Updated by Eregon (Benoit Daloze) 20 days ago
Agreed it should just compute it.
It should still be interruptible by Ctrl+C though, but it probably already is.
Updated by mame (Yusuke Endoh) 6 days ago
Matz said "let's calculate". I will create a PR
Updated by mame (Yusuke Endoh) 5 days ago
I have created https://github.com/ruby/ruby/pull/12033
This patch changes the behavior of Integer#**
and Rational#**
. If the generated bignum is not likely to exceed an estimated 16 GB, it will be calculated straightforwardly. If it is likely to exceed 16 GB, an ArgumentError is raised.
Complex#**
also prints a warning "in a**b, b may be too big", but I found a bit more subtle issue, so I will create a separate ticket.
Updated by mame (Yusuke Endoh) 5 days ago
- Status changed from Open to Closed
Applied in changeset git|45cd4a8296814f3b082dfb906cdef29974726731.
Do not round a**b
to infinity
... instead, just calculate the value unless it is too big.
Also, this change raises an ArgumentError if it is expected to exceed
16 GB in a 64-bit environment.
(It is possible to calculate it straightforward, but it would likely be
out-of-memory, so I didn't think it would make sense.)
[Feature #20811]