Slice appending not working as intended when generating a binary sequence - go

Question

I have this piece of code that generate all binary sequences of length n.
so:
allBitsSeqs(2) gives a 1-by-n int slice [[1 1][0 0] [1 0] [0 1]]
Here's the code:
func allBitSeqs(n int) [][]int {
seq := [][]int{{1}, {0}}
for floor := 1; floor < n; floor++ {
remember := [][]int{}
for i := 0; i < len(seq); i++ {
one := append(seq[i], 1)
remember = append(remember, one)
zero := append(seq[i], 0)
remember = append(remember, zero)
}
seq = remember
}
return seq
}
playground link: https://play.golang.org/p/s40RS7qEKfL
problem is, when n = 4 or more, I get:
[[1 1 1 0] [1 1 1 0] [1 1 0 0] [1 1 0 0] [1 0 1 0] [1 0 1 0] [1 0 0 0] [1 0 0 0] [0 1 1 0] [0 1 1 0] [0 1 0 0] [0 1 0 0] [0 0 1 0] [0 0 1 0] [0 0 0 0] [0 0 0 0]]
Notice how they are repeating in pairs.
I have tried some debugging and I think it has something to do with how Go copy its slices when it needs to allocate more space.

Answer 1

A slice is a view of an underlying array. If you have multiple slices working with the same underlying array, you may get unexpected results if you don't know how slices work.
one := append(seq[i], 1)
Say, one is a slice pointing to an array that has the capacity to hold the added element. Something like the following:
[ 1 1 . . . ]
The array capacity is 5, with first two elements set to 1 (slice len=2). When you add another 1, it becomes:
[ 1 1 1 . .]
Then you do the following:
remember = append(remember, one)
With this, you added a slice pointing to the above array, with len=3 and capacity=5.
Then:
zero := append(seq[i], 0)
Remember that seq[i] is still pointing to the same array. Further, seq[i] has len=2. So the array becomes:
[ 1 1 0 . . ]
That is your zero. However, when you add that 0, you also changed the slice one. You end up with both zero and one pointing to [1 1 0 . .].
Long story short: when you assign a slice to a variable, you assign the view to an array. If you modify the contents of the underlying array, the contents of the view also change.
To fix: create a new slice, copy data, and store that copy.