How to Efficiently Delete an Element from a Go Slice Using the Append and Spread Operator?

Delete Element in a Slice

This snippet of Go code illustrates how to remove an element from a slice:

func main() {
    a := []string{"Hello1", "Hello2", "Hello3"}
    fmt.Println(a)
    // [Hello1 Hello2 Hello3]
    a = append(a[:0], a[1:]...)
    fmt.Println(a)
    // [Hello2 Hello3]
}

This code demonstrates a trick to delete an element using the append function. How does this work and what does the ... (dot dot dot) operator do?

The syntax for deleting an element at index i from a slice a is:

a = append(a[:i], a[i+1:]...)

Explanation:

• [:i] creates an empty slice from the beginning of the slice up to (but not including) index i.
• [i 1:] creates a slice that starts after the element at index i and goes to the end of the slice.
• ... is the spread operator, which unpacks the elements of a slice or array into variadic arguments.

When append is called, the first argument must be a slice of the same type as the slice being modified (a in this case), the second argument is a variadic parameter that accepts any number of elements of the same type, and the return value is the new slice with the elements added.

In the given code, the spread operator unpacks the elements from a[1:] into the variadic argument of append. This is equivalent to manually passing in the elements a[1], a[2], .... The first argument is an empty slice to ensure that the old elements are overwritten.

Conclusion:

This trick allows us to delete an element from a slice by reconstructing the slice with the elements before and after the one we wish to remove. The spread operator plays a crucial role in unpacking the elements to be added back into the slice.

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