create two dimensional string array in golang

Question

I need to create a 2 dimensional string array as shown below -

matrix = [['cat,'cat','cat'],['dog','dog']]

Code:-

package main

import (
    "fmt"
)

func main() {
    { // using append

    var matrix [][]string
    matrix[0] = append(matrix[0],'cat')
        fmt.Println(matrix)
    }
}

Error:-

panic: runtime error: index out of range

goroutine 1 [running]:
main.main()
    /tmp/sandbox863026592/main.go:11 +0x20

Possible duplicate of [What is a concise way to create a 2D slice in Go?](https://stackoverflow.com/questions/39804861/what-is-a-concise-way-to-create-a-2d-slice-in-go) — Keveloper, Aug 14 '18 at 20:06

score 4 · Accepted Answer · answered Aug 14 '18 at 20:05

4

You have a slice of slices, and the outer slice is nil until it's initialized:

matrix := make([][]string, 1)
matrix[0] = append(matrix[0],'cat')
fmt.Println(matrix)

Or:

var matrix [][]string
matrix = append(matrix, []string{"cat"})
fmt.Println(matrix)

Or:

var matrix [][]string
var row []string
row = append(row, "cat")
matrix = append(matrix, row)

answered Aug 14 '18 at 20:05

Adrian

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Thanks, How do I add the same element to 2 position(for the last option given). append(matrix,row) by default adds to the first element... – user1050619 Aug 14 '18 at 20:15
@user1050619 initialize your outer matrix first (as in the first example). – Adam Smith Aug 14 '18 at 20:18
1

`append(matrix,row)` does not add to the first element, it adds the row to the matrix. – Adrian Aug 14 '18 at 20:23

score 3 · Answer 2 · answered May 18 '19 at 01:04

The problem with doing two-dimensional arrays with Go is that you have to initialise each part individually, e.g., if you have a [][]bool, you gotta allocate []([]bool) first, and then allocate the individual []bool afterwards; this is the same logic regardless of whether you're using make() or append() to perform the allocations.

In your example, the matrix[0] doesn't exist yet after a mere var matrix [][]string, hence you're getting the index out of range error.

For example, the code below would create another slice based on the size of an existing slice of a different type:

func solve(board [][]rune, …) {

    x := len(board)
    y := len(board[0])
    visited := make([][]bool, x)
    for i := range visited {
        visited[i] = make([]bool, y)
    }
…

If you simply want to initialise the slice based on a static array you have, you can do it directly like this, without even having to use append() or make():

package main

import (
    "fmt"
)

func main() {
    matrix := [][]string{{"cat", "cat", "cat"}, {"dog", "dog"}}
    fmt.Println(matrix)
}

https://play.golang.org/p/iWgts-m7c4u

create two dimensional string array in golang

2 Answers2