How to Create Nested Map in Go: Syntax and Example

In Go, you create a nested map by declaring a map whose values are also maps, like map[string]map[string]int. Initialize the outer map first, then initialize inner maps before assigning values to avoid nil map errors.

Syntax

A nested map in Go is a map where each value is itself another map. The general syntax looks like this:

• map[KeyType]map[InnerKeyType]ValueType declares a nested map.
• The outer map's keys are of type KeyType.
• The inner map's keys are of type InnerKeyType.
• The inner map's values are of type ValueType.

You must initialize the outer map first, then initialize each inner map before adding values.

go

var nestedMap map[string]map[string]int

nestedMap = make(map[string]map[string]int)

Example

This example shows how to create a nested map where the outer keys are strings, inner keys are strings, and values are integers. It initializes the maps and assigns values safely.

go

package main

import "fmt"

func main() {
    // Declare and initialize the outer map
    nestedMap := make(map[string]map[string]int)

    // Initialize inner map for key "fruits"
    nestedMap["fruits"] = make(map[string]int)
    nestedMap["fruits"]["apple"] = 5
    nestedMap["fruits"]["banana"] = 3

    // Initialize inner map for key "vegetables"
    nestedMap["vegetables"] = make(map[string]int)
    nestedMap["vegetables"]["carrot"] = 7
    nestedMap["vegetables"]["lettuce"] = 2

    // Print the nested map
    fmt.Println(nestedMap)
}

Output

map[fruits:map[apple:5 banana:3] vegetables:map[carrot:7 lettuce:2]]

Common Pitfalls

A common mistake is to try assigning values to an inner map before initializing it, which causes a runtime panic because the inner map is nil. Always initialize the inner map with make before adding keys and values.

Wrong way:

nestedMap := make(map[string]map[string]int)
nestedMap["fruits"]["apple"] = 5 // panic: assignment to entry in nil map

Right way:

nestedMap["fruits"] = make(map[string]int)
nestedMap["fruits"]["apple"] = 5

Quick Reference

• Declare nested map: map[KeyType]map[InnerKeyType]ValueType
• Initialize outer map with make
• Initialize each inner map with make before use
• Assign values using nestedMap[outerKey][innerKey] = value

Key Takeaways

Declare nested maps with map types where values are maps themselves.

Always initialize the outer map using make before use.

Initialize each inner map with make before assigning values to avoid nil map errors.

Assign values by first ensuring inner maps exist for the outer keys.

Print nested maps directly to see their structure in Go.