How do maps work in Go? What are the key operations and pitfalls?

Maps are Go's built-in hash table. Keys must be comparable types (those that support == and !=). Maps are reference types — like slices, they are cheap to pass because only a header is copied.

// Creating maps
m := map[string]int{}             // empty map literal (ready to use)
m2 := make(map[string]int)        // same — make preferred when initial size is known
m3 := map[string]int{             // initialised map
    "alice": 30, "bob": 25,
}

// Insert / update
m3["carol"] = 28
m3["alice"] = 31                 // update — no error if key exists

// Read — returns zero value for missing keys, NOT an error
age := m3["alice"]               // 31
missing := m3["dave"]            // 0 — zero value for int

// Comma-ok idiom — check if key actually exists
age, ok := m3["alice"]
if ok {
    fmt.Printf("alice is %d\n", age)
}

// Delete
delete(m3, "bob")

// Iteration — ORDER IS NOT GUARANTEED
for name, age := range m3 {
    fmt.Printf("%s: %d\n", name, age)
}

// PITFALL: reading nil map is OK (returns zero value)
var bad map[string]int
_  = bad["key"]       // safe — returns 0
// bad["key"] = 1     // PANIC: assignment to entry in nil map

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