Why does appending to a copy of a Go slice also modify the original slice?

Understanding Slice Append and Its Effect on Original Slice

When working with slices in Go, the append function is often used to append new elements to an existing slice. However, many developers may be surprised to discover that this append operation can also modify the original slice.

The Code Under Examination

Consider the following code snippet:

func someFunc(A []int) int {
    for i := 0; i < len(A); i++ {
        tempA := A // copy the slice by value

        fmt.Println("A: ", A)
        fmt.Println("tempA: ", A)
        fmt.Println()

        newArr = remove(tempA, i)

        if isAesthetic(newArr) {
            ways++
        }
    }
}

func remove(slice []int, s int) []int {
    return append(slice[:s], slice[s+1:]...)
}

Here, the someFunc function takes a slice A as input and then creates a copy of A called tempA, before invoking the remove function to remove an element from tempA. Upon inspecting the function in action, you may notice the following console output:

A:  [3 4 5 3 7]
tempA:  [3 4 5 3 7]

A:  [4 5 3 7 7]
tempA:  [4 5 3 7 7]

A:  [4 3 7 7 7]
tempA:  [4 3 7 7 7]

A:  [4 3 7 7 7]
tempA:  [4 3 7 7 7]

The Surprising Side Effect

As the code runs, it prints the content of both A and tempA, revealing that the original slice A is also modified after append is called on tempA. This behavior might seem counterintuitive at first glance, as you would expect a by-value copy of A to be independent from any changes made to tempA.

However, this phenomenon is a direct consequence of the way slices are implemented in Go. Slices are essentially a lightweight data structure that consists of a header and a pointer to the underlying array. The header includes information about the slice's length and capacity, while the pointer points to the first element in the slice.

When you assign the value of A to tempA, you are essentially creating a new slice header that points to the same underlying array as A. Thus, any changes made to tempA will also be reflected in A, as both slices are referencing the same data.

Understanding Slice Headers and Arrays

To grasp this behavior further, it helps to understand how slice headers and arrays interact in Go. An array contains a contiguous block of elements of the same type. A slice, on the other hand, provides a dynamic view of a section of the array. It describes a consecutive set of elements within the array, but it does not own the underlying array data.

When you create a slice from an array using the syntax []T{e1, e2, ..., en}, you are essentially creating a new slice header that points to the first element in the array. The length of the slice is set to n, and the capacity is set to the remaining length of the array after the slice.

Similarly, when you create a slice header using the syntax []T(arr), you are creating a slice that points to the same underlying array as arr. The length of the slice is set to the length of arr, and the capacity is set to the capacity of arr.

Implications and Best Practices

Understanding the relationship between slices and arrays can help you avoid potential pitfalls and write more efficient Go code. When working with slices, keep in mind the following:

• Avoid assigning a slice to another slice if you wish to maintain independent references to the same underlying array.
• Use copy(dst, src) to create a new slice with a different header while referencing the same underlying array.
• Consider using the make function to create a slice with a specific length and capacity, avoiding the overhead of potentially allocating and copying a new underlying array.

Understanding the internals of Go slices allows you to leverage their flexibility and efficiency while ensuring that your code behaves as intended. By embracing the nuances of slice headers and arrays, you can master the art of slicing in Go and unlock the full potential of this versatile data structure.

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