What is the difference between `array` and `slice{array,array,...}` in Golang?

What is the difference between `array` and `slice{array,array,...}` in Golang? This is a question often asked by many beginners. PHP editor Zimo will answer for you: In Golang, `array` is a fixed-length sequence. Once defined, the length cannot be changed. `slice` is a sequence of dynamic length that can be expanded or reduced as needed. In addition, the length of `array` is determined at definition time, while the length of `slice` can be changed dynamically at runtime. Therefore, when using it, you need to choose an appropriate data structure based on actual needs.

Question content

I would like to know why, please give me a hint.

I want to append an array to res, res is a 2D slice. So I need to convert first. When I convert the array to slice, I get an error.

// i need a map to remove duplicates
mm := map[[3]int]bool{}
mm[[3]int{-1, -1, 2}] = true
mm[[3]int{-1, -1, 2}] = true
mm[[3]int{-1, 0, 1}] = true

var res [][]int
for k, _ := range mm {
    res = append(res, k[:])
}
fmt.printf("the res is %v\n", res)

the res is [[-1 0 1] [-1 0 1]]

But the result is not what I want.

Then I tentatively modified the for loop

for k, _ := range mm {
    //res = append(res, k[:])
    res = append(res, []int{k[0], k[1], k[2]})
}

the res is [[-1 0 1] [-1 -1 2]]

Now the result is correct, but why? What is the difference between k[:] and []int{k[0],k[1],k[2]}?

Solution

Change the loop to

for k, _ := range mm {
    j := k
    res = append(res, j[:])
}

Your original loop declares a variable k of type [3]int that has a specific location in memory. Each iteration of the loop, a different key from the map mm is copied to this variable. So far, so good.

When you convert it to a slice using k[:], it creates a slice header that points to the array k. Something went wrong here - the next iteration of the loop, the value of k was overwritten. All slices created in the loop point to the same backing array k at the same location in memory.

You avoid the problem by giving each slice its own backing array by first copying the value of k to a variable declared inside the loop.

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