Maps and structs

Maps

A map in Go is a built-in hash table written map[K]V. The key type K must be comparable (strings, numbers, booleans, structs of comparable fields), and V is any type. Create one with a literal — map[string]int{"a": 1} — or with make(map[string]int). Indexing a missing key returns the zero value for V, not an error, so reads never panic.

To tell "present but zero" from "absent", use the comma-ok form: v, ok := m[key]. ok is true only when the key exists.

Structs

A struct is a fixed set of named fields, declared with type Point struct { X, Y int }. You build one with a literal — Point{X: 1, Y: 2} — and read fields with the dot operator: p.X. Structs are value types: assigning or passing one copies every field.

Methods

You attach behaviour to a struct by declaring a method — a function with a receiver before its name. A value receiver (p Point) works on a copy; a pointer receiver (p *Point) can mutate the original. Methods are how Go models objects without classes.

Iterating in order

Ranging over a map visits keys in a deliberately randomised order, so output is non-deterministic. When you need a stable order, collect the keys into a slice, sort them, then range the slice and look each key up.