How do you insert values into a Go slice at a given index?

Insert Values into a Slice at a Given Index

In Go, inserting values into a slice at a specific index requires careful consideration of the slice's length, capacity, and whether the index falls within its bounds.

Inserting at an Available Index

If the index you want to insert at is within the range of the slice's existing elements, you can use the following approach:

slice1 := []int{1, 3, 4, 5}
slice1 = append(slice1[:index+1], slice1[index:]...)
slice1[index] = value

This approach:

1. Creates a new slice composed of the elements before and after the insertion point (excluding the element at the insertion point).
2. Inserts the new value at the desired index.

Inserting at a New Index

If the index you want to insert at is greater than the current length of the slice, you need to expand the slice's capacity to accommodate the new element.

index := 7
if index > cap(slice1) {
    newCap := cap(slice1) * 2 // Double the capacity
    slice1 = append(make([]int, len(slice1), newCap), slice1...)
}
slice1 = append(slice1[:index+1], slice1[index:]...)
slice1[index] = value

This approach:

1. Checks if the index is beyond the slice's capacity and, if necessary, creates a new slice with double the capacity.
2. Inserts the new value at the desired index.

Inserting at the End of the Slice

To insert at the end of the slice, you can simply append the new value:

slice1 := []int{1, 3, 4, 5}
slice1 = append(slice1, value)

Using the 'slices' Package (for Go 1.21 )

For Go version 1.21 and above, you can use the slices.Insert() function from the github.com/golang/exp/slices package:

import "github.com/golang/exp/slices"

slice1 := []int{1, 3, 4, 5}
slices.Insert(slice1, index, value)

Example:

array1 := []int{1, 3, 4, 5}
array2 := []int{2, 4, 6, 8}

index := 1 // Index to insert at
value := array2[2] // Value to insert

// Insert value into array1 at index
slice1 := array1[:index+1]
slice2 := array1[index:]
slice1 = append(slice1, value)
slice1 = append(slice1, slice2...)
array1 = slice1

// Result: [1, 6, 3, 4, 5]

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