Go/Arrays

Go blog post: arrays, slices, and strings: https://blog.golang.org/slices

Arrays in Go

"Arrays are not often seen in Go programs because the size of an array is part of its type, which limits its expressive power."

The most common use of arrays are to store slices, which we will see in a moment.

Array size is a part of the type

An important characteristic of arrays is that their size is a part of their type.

The two variables defined here are of two distinct types:

var buffer [256]byte
var buffer2 [512]byte

This is because the size of the array is allocated at initialization time. You can use the square bracket syntax to access elements of an array, buffer[0] through buffer[255]. The program crashes if you access an index outside of its range.

Array slices

The bracket notation with colons can be used to refer to a slice of an array. For example:

var buffer [256]byte
...
var slice []byte = buffer[100:150]

alternatively,

slice := buffer[100:150]

Think of a slice variable as a data structure with two elements: a length, and a pointer to an element of the array.

We can also take a slice of the slice:

slice2 := slice[5:10]

Here's a shortcut to drop the first and last elements:

slice3 := slice[1:len(slice)-1]

References vs Copies

It is important to distinguish between the slice data structure, which is a small bundle that contains a reference to the underlying array data and a length, and the underlying array data itself.

if we use a slice to modify the underlying array data, those changes will be persistent across functions and scopes. But if we modify the slice variable directly, we are modifying a copy of the slice data structure, and we will discard that copy when we leave that scope.

Example modifying underlying array data

The following example modifies the underlying array data:

func AddOneToEachElement(slice []byte) {
    for i := range slice {
        slice[i]++
    }
}

func main() {
    slice := buffer[10:20]
    for i := 0; i < len(slice); i++ {
        slice[i] = byte(i)
    }
    fmt.Println("before", slice)
    AddOneToEachElement(slice)
    fmt.Println("after", slice)
}

Again, this requires us to think about the array slice as a data structure, which contains a pointer to an array and a length. The bundle, the data structure, is not a pointer itself!

Even though the slice (also called the "slice header") is passed by value, the header includes a pointer to elements of an array. Thus, both the original and the copy passed to the function refer to the same underlying array (slots in memory).

Example modifying copy of slice

The argument to the SubtractOneFromLength function is a slice, which in this case is a copy of the original slice data structure, containing a reference to the same underlying data.

When we modify the slice variable directly, those changes only affect the copy of the data slice, so the original slice is not affected.

func SubtractOneFromLength(slice []byte) []byte {
    slice = slice[0 : len(slice)-1]
    return slice
}

func main() {
    fmt.Println("Before: len(slice) =", len(slice))
    newSlice := SubtractOneFromLength(slice)
    fmt.Println("After:  len(slice) =", len(slice))
    fmt.Println("After:  len(newSlice) =", len(newSlice))
}

Array Slice Capacity

The capacity of an array is fixed; to make a new array with a new capacity, use the make() function.

The make function allocates space in memory for an array slice with a specified capacity.

The make function takes three arguments: the type of the slice, its initial length, and its capacity, which is the length of the array that make allocates to hold the slice data.

// Make an array slice with a length of 100 and room for 5 more
slice := make([]int, 100, 15)

To double the array slice capacity:

    // Original slice:
    slice := make([]int, 10, 15)
    fmt.Printf("len: %d, cap: %d\n", len(slice), cap(slice))

    // New slice (double the capacity):
    newSlice := make([]int, len(slice), 2*cap(slice))
    for i := range slice {
        newSlice[i] = slice[i]
    }

    // Replace old with new
    slice = newSlice
    fmt.Printf("len: %d, cap: %d\n", len(slice), cap(slice))

The above code is functional but awkward; Go provides a copy function to copy an old array into a new array, to accomplish the above in fewer lines.

newSlice := make([]int, len(slice), 2*cap(slice))
copy(newSlice, slice)

Note that copy is smart - it will copy as much as it can, but will respect the lengths of both array slices rfg.

Array Insertion

The general algorithm used for array insertion, therefore, looks something like this:

• 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

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

package main

import "fmt"

// 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
}

func main() {
    orig := []int{0,1,2,3,4,5,6,7,8,9,10}

    fmt.Println("-----------------------------")
    fmt.Println("Inserting Into Slice:")

    //// If we define slice this way,
    //// it has no capacity, and Insert fails.
    //slice := orig[0:4]

    // If we define slice this way,
    // we have extra capacity for 
    // new elements.
    slice := make([]int, 4, 8)
    for i := range slice {
        slice[i] = i
    }

    // Before:
    fmt.Println("Before (slice/orig):")
    fmt.Println(slice)
    fmt.Println(orig)

    // Insert int 99 into position 2
    slice = Insert(slice, 2, 99)

    // After
    fmt.Println("After (slice/orig):")
    fmt.Println(slice)
    fmt.Println(orig)
}

Output:

-----------------------------
Inserting Into Slice:
Before (slice/orig):
[0 1 2 3]
[0 1 2 3 4 5 6 7 8 9 10]
After (slice/orig):
[0 1 99 2 3]
[0 1 2 3 4 5 6 7 8 9 10]

QUESTION: the array slice is just a pointer to underlying array data - it doesn't contain the array data itself.

So the question I have is, when we say slice[index] = value in the Insert function, and we insert the value of 99 into the slice, why aren't we modifying the underlying array at that index?

In other words, why doesn't "After" look like this?

After (slice/orig):
[0 1 99 2 3]
[0 1 99 3 4 5 6 7 8 9 10]
     ^^
     this is value (99) because we set
     slice[index] equal to value (99)

I understand that we're dealing with the variable slice, which is a copy of the slice header.

But the slice header points to an underlying array data, which does not change. And when we say slice[index] = value, we're changing the value of the underlying array of data at index index, are we not?