What is the difference between map and struct?

Question

Accepted Answer

**Map** — dynamic key-value store, O(1) lookups, reference type. **Struct** — fixed named fields, compile-time checked, value type.

Maps are ideal for dynamic data. They grow automatically and support `delete`. Not safe for concurrent writes.

```go
counters := make(map[string]int)
for _, word := range words {
    counters[word]++
}
```

Structs define a compile-time schema. The compiler catches errors. They compose via embedding.

```go
type Stats struct {
    WordCount map[string]int
    Total     int
}
```

**When to use:**
- **Map** — dynamic keys, caching.
- **Struct** — known-shape data.

**Common mistakes:**
- `map[string]interface{}` instead of structs.
- Reading without `ok` check.
- Concurrent writes without mutex.
- Nil maps readable but not writable.

Go: What is the difference between map and struct?