How do maps work in Go?

Question

Accepted Answer

A **map** is Go's built-in hash table — an unordered collection of **key-value pairs** with O(1) average lookup. It's the standard way to associate keys with values.

## Creating and using maps

```go
// create with make
ages := make(map[string]int)    // map from string keys to int values
ages["Ann"] = 30                 // set
age := ages["Ann"]               // get → 30

// or with a literal
scores := map[string]int{
    "math":    90,
    "science": 85,
}

delete(scores, "math")           // remove a key
len(scores)                       // number of entries
```

## The comma-ok idiom — distinguishing "missing" from "zero"

```go
// ⚠️ a missing key returns the ZERO VALUE, not an error
value := scores["missing"]       // 0 — but was it stored as 0, or absent? Ambiguous!

// ✅ the comma-ok form tells you if the key EXISTS
value, ok := scores["missing"]
if ok {
    fmt.Println("found:", value)
} else {
    fmt.Println("not present")    // key doesn't exist
}
```

This is essential: accessing a missing key returns the value type's zero value (0, "", nil), so you **can't tell** "key absent" from "key present with zero value" by the value alone — the `, ok` boolean disambiguates.

## Iterating (order is RANDOM)

```go
for key, value := range scores {
    fmt.Println(key, value)
}
// ⚠️ iteration order is RANDOMIZED — never rely on map order!
// To iterate in order: collect keys into a slice, sort it, then iterate.
```

Go deliberately randomizes map iteration order (to prevent code from depending on it) — if you need ordered iteration, sort the keys yourself.

## The nil map trap

```go
var m map[string]int        // nil map (declared but not initialized)
v := m["x"]                 // ✅ reading is OK → 0
m["x"] = 1                  // 💥 PANIC: assignment to entry in nil map
// must initialize first: m = make(map[string]int)
```

A nil map can be read but **panics on write** — you must `make()` it (or use a literal) before storing.

## Keys must be comparable

```text
Valid keys: comparable types (string, int, bool, structs of comparable fields, pointers)
Invalid keys: slices, maps, functions (not comparable) → compile error
```

## Why it matters

Maps are a fundamental, constantly-used Go data structure for key-value associations and fast lookups (caches, indexes, counting, grouping).

Several behaviors are essential to know and are common bug sources: the **comma-ok idiom** (the only reliable way to check key existence, since a missing key returns the zero value), the **randomized iteration order** (never depend on it — sort keys for ordered output), and the **nil-map write panic** (always initialize before writing).

Understanding these — plus that keys must be comparable types — is necessary for correct everyday Go and reflects the kind of practical detail that distinguishes solid Go developers and frequently appears in interviews.