Convert map, slice

In Go a map is ideal for fast lookups—we can find, add and remove elements with great speed. But it is not good for iterating (looping) over elements.

Slice forms

We sometimes want a slice of keys, values, or pairs. And a "flat slice" one where all the keys and values are stored together one after another is also helpful.

Example program

Here we begin with a map of string keys and string values. We then show how to get just the keys, or just the values, in a separate string slice.

package main

import "fmt"

func main() {
    // Create example map.
    m := map[string]string{
        "java": "coffee",
        "go": "verb",
        "ruby": "gemstone",
    }

    // Convert map to slice of keys.
    keys := []string{}
    for key, _ := range m {
        keys = append(keys, key)
    }

    // Convert map to slice of values.
    values := []string{}
    for _, value := range m {
        values = append(values, value)
    }

    // Print the results.
    fmt.Println("MAP         ", m)
    fmt.Println("KEYS SLICE  ", keys)
    fmt.Println("VALUES SLICE", values)
}
MAP          map[go:verb java:coffee ruby:gemstone]
KEYS SLICE   [java go ruby]
VALUES SLICE [coffee verb gemstone]

Get pairs

Sometimes in Go programs we want a slice of 2-element string arrays from our map. We can create these in a slice of pairs.

package main

import "fmt"

func main() {
    m := map[string]string{
        "java": "coffee",
        "go": "verb",
        "ruby": "gemstone",
    }

    // Convert map to slice of key-value pairs.
    pairs := [][]string{}
    for key, value := range m {
        pairs = append(pairs, []string{key, value})
    }

    // Convert map to flattened slice of keys and values.
    flat := []string{}
    for key, value := range m {
        flat = append(flat, key)
        flat = append(flat, value)
    }

    // Results.
    fmt.Println("MAP         ", m)
    fmt.Println("PAIRS SLICE ", pairs)
    fmt.Println("FLAT SLICE  ", flat)
}
MAP          map[go:verb java:coffee ruby:gemstone]
PAIRS SLICE  [[java coffee] [go verb] [ruby gemstone]]
FLAT SLICE   [java coffee go verb ruby gemstone]

Some notes

We can extract data from a map by using for-range loops. When a map is iterated over in this kind of loop, we access each key and value.

A summary

Often constructs like slices that are "flattened" contain all keys and values together are useful in real programs. They may also be simpler to use.