Go: What is the difference between map and struct?

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

Maps are ideal when keys aren't known at compile time. They use hash tables internally and grow dynamically. Maps are reference types — passing one to a function shares the same data. They are not safe for concurrent writes without synchronization.

1wordCount := make(map[string]int)
2for _, w := range words {
3    wordCount[w]++
4}
5if count, ok := wordCount["hello"]; ok {
6    fmt.Println("found", count, "times")
7}

Structs define a fixed schema at compile time. The compiler catches type errors, IDEs provide autocompletion, and the memory layout is deterministic. Structs compose via embedding for reuse.

1type WordStats struct {
2    WordCount map[string]int
3    Total     int
4}

When to use:

• Map — dynamic keys, caching, counting, grouping, configuration from unstructured data.
• Struct — domain models, API payloads, 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 the ok value — zero values may mask missing data.
• Concurrent map access without mutex or sync.Map — runtime panic.
• Nil maps are readable (return zero value) but not writable (writing panics).