What is the difference between an array and a slice in Go?

An array has a fixed length that is part of its type. A slice is a dynamically sized view onto an array and is what you'll use in most code.

Can I rely on the order of items in a map?

No. Go deliberately randomizes map iteration order. If you need a stable order, collect the keys into a slice and sort them.

Go Slices and Maps Explained

Introduction

Slices and maps are the workhorse collections in Go. A slice is a flexible, ordered view onto an array, and a map stores key-value pairs. This tutorial shows how to create, grow, loop over, and safely use both.

Prerequisites

Go installed
Basic Go syntax: variables and functions

Step-by-Step Guide

Step 1: Create and grow a slice

A slice literal has no size between the brackets. append returns the updated slice, so always assign the result back.

nums := []int{1, 2, 3}
nums = append(nums, 4, 5)

fmt.Println(nums, len(nums))   // [1 2 3 4 5] 5

Step 2: Slice a slice

The expression s[low:high] includes low and excludes high. The new slice shares the same underlying array, so changes to one can show up in the other.

part := nums[1:3]
fmt.Println(part)   // [2 3]

Step 3: Create and use a map

Maps are created with a literal or make. Reading a missing key returns the zero value, so use the two-value form to check whether it exists.

ages := map[string]int{"Ava": 30, "Ben": 25}
ages["Cy"] = 41

age, ok := ages["Dee"]
if !ok {
    fmt.Println("Dee not found")
}
fmt.Println(age)   // 0 (zero value)

Step 4: Delete entries and loop with range

delete removes a key. range gives the index and value for slices, and the key and value for maps. Map iteration order is random by design.

delete(ages, "Ben")

for i, n := range nums {
    fmt.Println(i, n)
}

for name, age := range ages {
    fmt.Println(name, age)
}

FAQs

What is the difference between an array and a slice in Go?: An array has a fixed length that is part of its type. A slice is a dynamically sized view onto an array and is what you'll use in most code.
Can I rely on the order of items in a map?: No. Go deliberately randomizes map iteration order. If you need a stable order, collect the keys into a slice and sort them.

Summary

Use slices for ordered data and maps for lookups by key. Remember to assign the result of append, and use the comma-ok form to tell a missing map key from a zero value.