Flattening a Slice

In this lesson you will learn how to flatten a nested slice in Go, combining append with variadic expansion. This is for anyone searching for "Go slice flatten implementation".

Expanding a slice that contains other slices into one flat slice is called flattening. You implement it by combining append with ... (variadic expansion).

The sample code has flatten() take a nested [][]int and append each inner slice, expanded, into one result with result = append(result, inner...). Adding ... is what spreads the slice's contents as individual values.

A common stumbling block for beginners is forgetting the ... and trying to append the inner slice itself as a single element. You need to recognise when expansion is required and when it is not.

In real-world development this idea applies whenever you want to treat nested data as one list, including collapsing multi-level tree data into a single list.

📖 Reference code

package main

import "fmt"

func flatten(nested [][]int) []int {
    result := []int{}
    for _, inner := range nested {
        result = append(result, inner...)
    }
    return result
}

func main() {
    nested := [][]int{{1, 2}, {3, 4}, {5}}
    fmt.Println(flatten(nested))
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Go directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).