Go Data Structures

---
name: go-data-structures
description: Use when working with Go slices, maps, or arrays — choosing between new and make, using append, declaring empty slices (nil vs literal for JSON), implementing sets with maps, and copying data at boundaries. Also use when building or manipulating collections, even if the user doesn't ask about allocation idioms. Does not cover concurrent data structure safety (see go-concurrency).
license: Apache-2.0
metadata:
  sources: "Effective Go, Google Style Guide, Uber Style Guide, Go Wiki CodeReviewComments"
---

# Go Data Structures

---

## Choosing a Data Structure

```
What do you need?
├─ Ordered collection of items
│  ├─ Fixed size known at compile time → Array [N]T
│  └─ Dynamic size → Slice []T
│     ├─ Know approximate size? → make([]T, 0, capacity)
│     └─ Unknown size or nil-safe for JSON? → var s []T (nil)
├─ Key-value lookup
│  └─ Map map[K]V
│     ├─ Know approximate size? → make(map[K]V, capacity)
│     └─ Need a set? → map[T]struct{} (zero-size values)
└─ Need to pass to a function?
   └─ Copy at the boundary if the caller might mutate it
```

> **When this skill does NOT apply**: For concurrent access to data structures (mutexes, atomic operations), see [go-concurrency](../go-concurrency/SKILL.md). For defensive copying at API boundaries, see [go-defensive](../go-defensive/SKILL.md). For pre-sizing capacity for performance, see [go-performance](../go-performance/SKILL.md).

---

## Slices

### The append Function

**Always assign the result** — the underlying array may change:

```go
x := []int{1, 2, 3}
x = append(x, 4, 5, 6)

// Append a slice to a slice
x = append(x, y...)  // Note the ...
```

### Two-Dimensional Slices

**Independent inner slices** (can grow/shrink independently):

```go
picture := make([][]uint8, YSize)
for i := range picture {
    picture[i] = make([]uint8, XSize)
}
```

**Single allocation** (more efficient for fixed sizes):

```go
picture := make([][]uint8, YSize)
pixels := make([]uint8, XSize*YSize)
for i := range picture {
    picture[i], pixels = pixels[:XSize], pixels[XSize:]
}
```

> Read [references/SLICES.md](references/SLICES.md) when debugging unexpected slice behavior, sharing slices across goroutines, or working with slice headers.

### Declaring Empty Slices

Prefer nil slices over empty literals:

```go
// Good: nil slice
var t []string

// Avoid: non-nil but zero-length
t := []string{}
```

Both have `len` and `cap` of zero, but the nil slice is the preferred style.

**Exception for JSON**: A nil slice encodes to `null`, while `[]string{}`
encodes to `[]`. Use non-nil when you need a JSON array.

When designing interfaces, avoid distinguishing between nil and non-nil
zero-length slices.

---

## Maps

### Implementing a Set

Use `map[T]bool` — idiomatic and reads naturally:

```go
attended := map[string]bool{"Ann": true, "Joe": true}
if attended[person] {  // false if not in map
    fmt.Println(person, "was at the meeting")
}
```

---

## Copying

Be careful when copying a struct from another package. If the type has methods
on its pointer type (`*T`), copying the value can cause aliasing bugs.

**General rule:** Do not copy a value of type `T` if its methods are associated
with the pointer type `*T`. This applies to `bytes.Buffer`, `sync.Mutex`,
`sync.WaitGroup`, and types containing them.

```go
// Bad: copying a mutex
var mu sync.Mutex
mu2 := mu  // almost always a bug

// Good: pass by pointer
func increment(sc *SafeCounter) {
    sc.mu.Lock()
    sc.count++
    sc.mu.Unlock()
}
```

---

## Quick Reference

| Topic | Key Point |
|-------|-----------|
| Slices | Always assign `append` result; `nil` slice preferred over `[]T{}` |
| Sets | `map[T]bool` is idiomatic |
| Copying | Don't copy `T` if methods are on `*T`; beware aliasing |

## Related Skills

- **Defensive copying**: See [go-defensive](../go-defensive/SKILL.md) when copying slices or maps at API boundaries to prevent mutation
- **Capacity hints**: See [go-performance](../go-performance/SKILL.md) when pre-sizing slices or maps for known workloads
- **Iteration patterns**: See [go-control-flow](../go-control-flow/SKILL.md) when using range loops over slices, maps, or channels
- **Declaration style**: See [go-declarations](../go-declarations/SKILL.md) when choosing between `new`, `make`, `var`, and composite literals