What is a concise way to create a 2D slice in Go?


I am learning Go by going through A Tour of Go. One of the exercises there asks me to create a 2D slice of dy rows and dx columns containing uint8. My current approach, which works, is this:

a:= make([][]uint8, dy)       // initialize a slice of dy slices
for i:=0;i<dy;i++ {
    a[i] = make([]uint8, dx)  // initialize a slice of dx unit8 in each of dy slices

I think that iterating through each slice to initialize it is too verbose. And if the slice had more dimensions, the code would become unwieldy. Is there a concise way to initialize 2D (or n-dimensional) slices in Go?


There isn’t a more concise way, what you did is the “right” way; because slices are always one-dimensional but may be composed to construct higher-dimensional objects. See this question for more details: Go: How is two dimensional array's memory representation.

One thing you can simplify on it is to use the for range construct:

a := make([][]uint8, dy)
for i := range a {
    a[i] = make([]uint8, dx)

Also note that if you initialize your slice with a composite literal, you get this for “free”, for example:

a := [][]uint8{
    {0, 1, 2, 3},
    {4, 5, 6, 7},
fmt.Println(a) // Output is [[0 1 2 3] [4 5 6 7]]

Yes, this has its limits as seemingly you have to enumerate all the elements; but there are some tricks, namely you don’t have to enumerate all values, only the ones that are not the zero values of the element type of the slice. For more details about this, see Keyed items in golang array initialization.

For example if you want a slice where the first 10 elements are zeros, and then follows 1 and 2, it can be created like this:

b := []uint{10: 1, 2}
fmt.Println(b) // Prints [0 0 0 0 0 0 0 0 0 0 1 2]

Also note that if you’d use arrays instead of slices, it can be created very easily:

c := [5][5]uint8{}

Output is:

[[0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0]]

In case of arrays you don’t have to iterate over the “outer” array and initialize “inner” arrays, as arrays are not descriptors but values. See blog post Arrays, slices (and strings): The mechanics of ‘append’ for more details.

Try the examples on the Go Playground.

Answered By – icza

Answer Checked By – Willingham (GoLangFix Volunteer)

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