Go Data Structures

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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. For defensive copying at API boundaries, see go-defensive. For pre-sizing capacity for performance, see go-performance.

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Slices

The append Function

Always assign the result — the underlying array may change:

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):

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

Single allocation (more efficient for fixed sizes):

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

Read 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:

// 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.

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Maps

Implementing a Set

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

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

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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.

// 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()
}

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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 when copying slices or maps at API boundaries to prevent mutation
• Capacity hints: See go-performance when pre-sizing slices or maps for known workloads
• Iteration patterns: See go-control-flow when using range loops over slices, maps, or channels
• Declaration style: See go-declarations when choosing between new, make, var, and composite literals