Go: What is the difference between map and struct?

Map — dynamic key-value container, keys must be comparable. Struct — fixed set of named, typed fields defined at compile time.

Maps are reference types with O(1) average lookups. They're perfect for grouping, counting, or indexing data where the keys aren't known in advance. Maps grow dynamically but cannot be compared with == (except nil check).

1wordCount := make(map[string]int)
2for _, w := range strings.Fields(text) {
3    wordCount[w]++
4}
5// safe lookup with comma-ok idiom
6if count, ok := wordCount["hello"]; ok {
7    fmt.Println("found", count, "times")
8}

Structs give you a schema. Every field is known at compile time — IDEs provide autocompletion, the compiler catches type errors, and the memory layout is deterministic. Structs compose via embedding.

1type User struct {
2    ID    int    `json:"id" db:"user_id"`
3    Email string `json:"email"`
4}
5
6type Admin struct {
7    User           // embed User
8    Role string
9}

When to use:

• Map — configuration, caching, counting, dynamic grouping, JSON parsing of unstructured data.
• Struct — domain models, request/response types, any data with a known shape.

Common mistakes:

• Using map[string]interface{} for complex structures — lose type safety and discoverability.
• Reading map values without checking ok — zero values may mask missing data.
• Concurrent map access without mutex or sync.Map — runtime panic.
• Forgetting that nil maps are readable but not writable (reading returns zero value, writing panics).