How Can I Guarantee Ordered Iteration Over Go Maps?

Maps in Go Preserve Order - But Not in the Way You Might Think

In Go, maps are unsorted data structures. This means that the order of the key-value pairs in a map is not guaranteed to be consistent. This can be a problem when you need to iterate over a map in a specific order.

Consider the following code:

package main

import (
    "fmt"
)

func main() {
    months := map[int]string{
        1:  "January",
        2:  "February",
        3:  "March",
        4:  "April",
        5:  "May",
        6:  "June",
        7:  "July",
        8:  "August",
        9:  "September",
        10: "October",
        11: "November",
        12: "December",
    }

    for index, month := range months {
        fmt.Println(index, "-", month)
    }
}

When you run this code, you might expect the output to be the months in order from January to December. However, this is not what happens. Instead, you will see that the months are printed in a seemingly random order.

Why does this happen?

The reason for this is that the keys in a Go map are stored in an unsorted array. When you iterate over a map, the keys are accessed in the order in which they are stored in the array.

Solution

There are a few different ways to get around this issue. One way is to use an array to store the keys in order. You can then iterate over the array to access the elements of the map in the desired order.

Another way to get around this issue is to use a sorted map. A sorted map is a map that guarantees that the keys are stored in sorted order. This can be useful when you need to iterate over a map in a specific order.

Example

The following code shows how to use an array to store the keys in order:

package main

import (
    "fmt"
    "sort"
)

func main() {
    months := map[int]string{
        1:  "January",
        2:  "February",
        3:  "March",
        4:  "April",
        5:  "May",
        6:  "June",
        7:  "July",
        8:  "August",
        9:  "September",
        10: "October",
        11: "November",
        12: "December",
    }

    keys := []int{}
    for key := range months {
        keys = append(keys, key)
    }

    sort.Ints(keys)

    for _, key := range keys {
        fmt.Println(key, "-", months[key])
    }
}

When you run this code, you will see that the output is the months in order from January to December.

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