How to Convert a Slice to a Fixed Size Array in Go?

Converting Slices to Fixed Size Arrays in Go

A common scenario in Go is the need to convert a slice into a fixed size array. While this may seem straightforward, it can result in a compilation error. To understand this error and its solution, let's analyze the following code snippet:

func gen(bricks []Brick) {
    if len(bricks) == 16 {
        if check(Sculpture{bricks}) {
            var b [16]Brick = bricks[0:16]; // This line generates the error
        }
     }
}

The error message indicates that the slice object cannot be directly assigned to a fixed size array. This is because Go treats slices and arrays as distinct types.

Solution: Using the 'copy' Function

To convert a slice to a fixed size array, you can use the built-in 'copy' function. This function takes two slices as input and copies a specified number of elements from the source slice to the destination slice. By providing the correct inputs, you can effectively transfer the elements of the slice into the fixed size array.

Here's an example demonstrating the use of 'copy':

slice := []byte("abcdefgh")

var arr [4]byte

copy(arr[:], slice[:4])

fmt.Println(arr) // Output: [97 98 99 100]

In this case, 'copy' transfers the first four elements of the slice into the array 'arr'.

Alternative Syntax

Another concise way to achieve the same result using 'copy' is to omit the slice indices:

copy(arr[:], slice)

The 'copy' function will automatically copy the minimum number of elements that both slices can accommodate. Therefore, you can skip specifying the indices when the sizes match or when you want to fill the entire fixed size array.

Go 1.17 and Above

It's worth noting that Go 1.17 introduced support for slice-to-array conversions. This feature allows you to directly assign a slice to an array pointer of the same base type:

s := make([]byte, 2, 4)
s0 := (*[0]byte)(s)      // s0 != nil
s1 := (*[1]byte)(s[1:])  // &s1[0] == &s[1]
s2 := (*[2]byte)(s)      // &s2[0] == &s[0]

However, this functionality is still considered experimental and may not be available in all environments.

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