Go: What is the difference between map and struct?

Map — dynamic, runtime-checked key-value store with O(1) average lookups. Struct — fixed, compile-time-checked collection of named, typed fields.

Maps are reference types — sharing a map between goroutines requires synchronization. They grow dynamically but have per-entry overhead. Keys must be comparable.

1// Dynamic grouping — great for runtime data
2byDept := make(map[string][]Employee)
3for _, emp := range employees {
4    byDept[emp.Department] = append(byDept[emp.Department], emp)
5}

Structs define a compile-time schema. The compiler catches field name typos and type mismatches. Structs are value types — no nil-pointer concerns for the struct itself.

1type Employee struct {
2    Name       string `json:"name"`
3    Department string `json:"department"`
4    Salary     float64 `json:"salary"`
5}

When to use:

• Map — dynamic keys, caching, counting, configuration.
• Struct — domain models, API payloads, any known-shape data.

Common mistakes:

• map[string]interface{} for complex data — use structs.
• Reading without ok check — zero value masks missing data.
• Concurrent writes without mutex — panic.
• Nil maps are readable but not writable — writing panics.