Go/Arrays/Array Slice Insertion

Main article: Go/Arrays

The Go blog (this post: https://blog.golang.org/slices) gets lost on a (rather stupid and uninformative) topic, which is, array slices.

To insert an element into a slice of an array, we can do the following (NOTE: This is NOT the same as inserting an element into an array):

• Extend the slice window by 1
• Shift the upper part of the slice's underlying array data up by 1, using the copy function
  • Copying FROM the variable slice, starting at index and running to the end
  • Copying TO the variable slice, starting at index + 1 and running to the end
• Return the new slice

This operation WILL NOT affect the underlying data.

Here is the Insert function, plus a main method that calls it:

// Insert inserts the value into the slice at the specified index,
// which must be in range.
// The slice must have room for the new element.
func Insert(slice []int, index, value int) []int {

    // Grow the slice by one element.
    slice = slice[0 : len(slice)+1]

    // Use copy to move the upper part of the slice out of the way and open a hole.
    copy(slice[index+1:], slice[index:])

    // Store the new value.
    slice[index] = value

    // Return the result.
    return slice
}

Aside from the stupidity of the example (not making clear why you would want to insert something into a slice of an array but not the array itself; not making clear the underlying data or the memory/reference model in Go).

Basically this whole blog post is a hot mess.

The confusion here is, the text seems to contradict the behavior of this function.

• The text describing this example insists we are passing a copy of the slice, but that copy should refer to the same data underneath.
• However, when we expand the length of the slice, and shift everything over, and insert a value, this doesn't affect the data underneath.

The answer: we aren't creating a slice by saying slice = orig[5:10], we are creating a slice by saying slice = make([]int, 4, 8) and then copy(slice,orig).

When we call make, we're basically allocating an underlying array for a slice.

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