minimally changed code that works: https://play.golang.org/p/Tcfw_EC8jMp
On Thursday, May 27, 2021 at 4:11:40 AM UTC+3 johnfor...@gmail.com wrote: > Here’s the code. <https://play.golang.org/p/JYXqCWsXeIj> It might be > possible to come up with a shorter reproducer, but I’m not sure where to > start. The recursion takes place in line 21. The code works for 7 or > smaller, and fails for 8 and larger. > > J > > > On May 26, 2021, at 20:34, Martin Schnabel <m...@mb0.org> wrote: > > Could you send a https://play.golang.org/ link showing a short example? > > Without much information it sounds like a you update the same slice at two > different indices. It would be interesting to know how you populate the > temp slice. > > On 27.05.21 01:52, John Olson wrote: > > I have a recursive function that seems to be reusing memory it shouldn't. > I'm developing in Goland 2021.1.1 with go version 1.16.2. > I've written a function to generate the partitions of an integer. The > partitions of n are all the sets of integers that add up to n, so the > partitions of 3 are {3}, {2,1} and {1,1,1}. As n grows, the number of > partitions grows exponentially (to be precise, as e^(n^½)). I wrote a > recursive function, which works great up to n=7. At n=8, one of the > partitions is {2,2,2,1}, which is obviously wrong; and since the function > is recursive, every subsequent n is wrong, too. > I just spent quite a long time stepping through the code, and I found that > what seems to be happening is that a slice declared in my recursive > function is being reused in two different recursions. Specifically, my > function declares > var temp [][]int > to build up the list of partitions. When I compute Partitions(8), I have > to compute Partitions(4), 5, 6 and 7. I memoize each set of partitions so I > only have to compute them once. > It all goes wrong when I'm working on Partitions(7). At this point, > Partitions(8) has already added 6 cases, the last of which is {2,2,2,2}. > This is stored in temp[5], the temp corresponding to n=8, of course. Then I > compute Partitions(7), which should create a new temp [][]int; I'll call > this temp//7. temp[6]//7 gets {2,2,2,1}, and at that point temp[5]//8 > changes from {2,2,2,2} to {2,2,2,1}. The addresses of temp[6]//7 and > temp[5]//8 are different, but the addresses of the /elements/ of these > slices are the same. > This has to be wrong. > I suspect a bug in go, but I suppose it's possible there's a bug in > goland. I'm running on macOS 11.3.1, just in case that's relevant. > I'm happy to share the source code if anyone is interested. > Thanks, > J > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts...@googlegroups.com < > mailto:golang-nuts...@googlegroups.com>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/fb9031ba-bfa0-4c92-9cb7-6ad3a8781184n%40googlegroups.com > < > https://groups.google.com/d/msgid/golang-nuts/fb9031ba-bfa0-4c92-9cb7-6ad3a8781184n%40googlegroups.com?utm_medium=email&utm_source=footer > >. > > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/de91d8a7-63eb-40aa-bf20-b1a18897fa84n%40googlegroups.com.
