Go语言：XXX类型不是表达式

Question

I have written a function:

func Pic(dx, dy int) [][]uint8 {
    type matrix [][]uint8 

    for i := 0; i < dx; i++ { // fills up the matrix with z's in their right places.
        for j := 0; j < dy; j++ {
            matrix[i][j] = Z(i,j)
        }
    }

    return matrix
}

that is supposed to fill up a matrix with z values for each x and y value and return it. As I want to have different dimensions for the matrix depending of parameters to the Pic function, I create a slice i line 2. Then in my for loops i fill the matrix up.

I get an error upon running this code: type matrix is not an expression for the matrix[i][j] = Z(i,j) line. What am I doing wrong? Should matrix[i][j] evaluate to an expression? Why should it, when I want to put something there (it's empty/non-existent now!) ?

Answer 1

While you can use var matrix [][]uint8, I'd recommend using the built-in make function since you know the desired lengths—dx for the outer slice and dy for the inner slice. The Two-dimensional slices section of Effective Go gives an example of using make to allocate a two-dimensional slice. Below is an example for your Pic function.

func Pic(dx, dy int) [][]uint8 {
    matrix := make([][]uint8, dx)

    for i := 0; i < dx; i++ {
        matrix[i] = make([]uint8, dy)
        for j := 0; j < dy; j++ {
            matrix[i][j] = Z(i, j)
        }
    }
    return matrix
}