Arrays, Slices & Maps at a Glance

Q: What is the difference between len and cap?

len is how many elements you can read/write from index 0. cap is how many elements exist in the backing array starting at the slice's first element.

Q: Is a nil slice valid?

Yes - len and cap are 0, range works, and append allocates backing storage.

Q: How do I preallocate a slice?

make([]T, 0, expected) then append, or make([]T, expected) if you will index directly.

Q: What does s[low:high:max] do?

Sets len to high-low and cap to max-low, preventing append from reading past max.

Q: How are arrays passed to functions?

The entire array copies. For [1024]byte buffers, pass a slice buf[:] or pointer instead.

Q: Can I take the address of a map element?

No - &m[key] is illegal because map storage can move during growth. Store value in a variable first if you need a pointer.

Q: How do I merge maps?

Loop and assign: for k, v := range other { m[k] = v }. Watch for overwriting existing keys.

Q: What is the zero value of [3]int vs []int?

Array is [0 0 0] value type. Slice is nil header with no backing array.

Arrays are fixed-size values; slices and maps are reference-like collections built into the language.

Most Go code uses slices and maps daily, so len, cap, and sharing rules matter.

Summary

An array [N]T has size part of the type.

A slice []T is a header (pointer, len, cap) over an underlying array.

A map map[K]V is a hash table with undefined iteration order.

Slices and maps are passed by copying the header, so mutations are visible to callers.

Recipe

Quick-reference recipe card - copy-paste ready.

package main
 
import "fmt"
 
func main() {
    nums := make([]int, 0, 4)
    nums = append(nums, 1, 2, 3)
 
    ages := map[string]int{"ada": 36}
    ages["grace"] = 34
 
    fmt.Println(nums, len(nums), cap(nums), ages)
}

When to reach for this:

Use slices for ordered, growable sequences.
Use maps for keyed lookup when keys are comparable.
Use arrays only when size is a fixed protocol constraint (crypto, embedded).
Preallocate with make([]T, 0, n) when you know approximate size.

Working Example

package main
 
import "fmt"
 
func addSuffix(names []string, suffix string) {
    for i := range names {
        names[i] += suffix
    }
}
 
func main() {
    original := []string{"go", "rust"}
    view := original[0:2:2] // full slice expression caps capacity
    addSuffix(view, "!")
 
    counts := map[string]int{}
    for _, name := range original {
        counts[name]++
    }
 
    fmt.Println(original, counts)
 
    keys := make([]string, 0, len(counts))
    for k := range counts {
        keys = append(keys, k)
    }
    fmt.Println("keys (unordered):", keys)
}

What this demonstrates:

Slice headers copy but share backing arrays - addSuffix mutates original.
Full slice expression s[low:high:max] limits append growth capacity.
Maps need make or literals before writes.
Collecting map keys requires a separate slice; order is not stable.

Deep Dive

How It Works

Slice append grows capacity by allocating a new array when len == cap (typically doubling).
Slicing s[low:high] sets len to high-low and cap to cap(s)-low.
Map buckets are internal; iteration order is randomized each pass.
Arrays assign by value (full copy); slices and maps assign headers (shared storage).

Arrays vs Slices vs Maps

	Array `[n]T`	Slice `[]T`	Map `map[K]V`
Size	Fixed in type	Dynamic	Dynamic
Pass to func	Copy whole array	Copy header	Copy header
Zero value	All elements zero	`nil`	`nil`
Comparable	Yes (if `T` comparable)	No	No
Index access	`a[i]`	`s[i]`	`m[k]` with ok form

len and cap

Expression	Meaning
`len(s)`	Number of elements visible in slice
`cap(s)`	Elements from first index through end of backing array
`make([]T, len, cap)`	Allocates array, sets len and cap
`append(s, x)`	Grows len, may reallocate if `len == cap`

Go Notes

// Delete map entry
delete(m, key)
 
// Clear map in Go 1.21+
clear(m)
 
// Copy slices
dst := make([]int, len(src))
copy(dst, src)

Gotchas

Append alias surprise - a := []int{1,2,3}; b := a[:2]; append(b, 99) may overwrite a[2] if cap allows. Fix: use full slice a[:2:2] or append to a copy.
Nil map write - Assigning to uninitialized map panics. Fix: make first.
Expecting sorted map iteration - Order changes between loops. Fix: sort keys slice before display.
Using array as slice stand-in - [5]int is not []int without slicing arr[:]. Fix: slice with [:].
Large array copies - Passing big arrays copies all elements. Fix: pass pointer *[N]T or use slice.
Concurrent map writes - Maps are not safe for concurrent write without sync. Fix: sync.Map or mutex.

Alternatives

Alternative	Use When	Don't Use When
Slice	Default ordered collection	Need O(1) lookup by key
Map	Keyed lookup	Need stable order without extra sort
Array	Fixed wire format size	General-purpose lists
`container/list`	Frequent middle inserts (rare)	Most cases - slices win

FAQs

What is the difference between len and cap?

len is how many elements you can read/write from index 0.

cap is how many elements exist in the backing array starting at the slice's first element.

Is a nil slice valid?

Yes - len and cap are 0, range works, and append allocates backing storage.

How do I check if a map has a key?

v, ok := m[key]
if !ok { /* missing */ }

Can map keys be slices?

No - keys must be comparable; slices, maps, and functions are not comparable.

How do I preallocate a slice?

make([]T, 0, expected) then append, or make([]T, expected) if you will index directly.

What does s[low:high:max] do?

Sets len to high-low and cap to max-low, preventing append from reading past max.

How are arrays passed to functions?

The entire array copies.

For [1024]byte buffers, pass a slice buf[:] or pointer instead.

Can I take the address of a map element?

No - &m[key] is illegal because map storage can move during growth.

Store value in a variable first if you need a pointer.

How do I merge maps?

Loop and assign: for k, v := range other { m[k] = v }.

Watch for overwriting existing keys.

Does append return the same slice header?

It may return a new header if reallocation happens.

Always assign: s = append(s, x).

What is the zero value of [3]int vs []int?

Array is [0 0 0] value type.

Slice is nil header with no backing array.

How do I shallow-copy a slice?

append([]T(nil), s...) or s2 := make([]T, len(s)); copy(s2, s).

Zero Values and Initialization - nil maps and slices
Pointers & the Address-of Operator - Sharing via slice headers
Numeric Types, Strings, Runes & Byte Slices - []byte as byte slices
Structs, Embedding & Field Promotion - Structs containing slices and maps

A Tour of Go - Official interactive introduction to Go.
Go by Example - Annotated practical code examples.
Effective Go - Official guide to writing clear, idiomatic Go.
The Go Programming Language (ebook) - Donovan & Kernighan.
Learning Go (2nd Edition) - Jon Bodner.

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

Most Go code uses slices and maps daily, so len, cap, and sharing rules matter.

Summary

An array [N]T has size part of the type.

A slice []T is a header (pointer, len, cap) over an underlying array.

A map map[K]V is a hash table with undefined iteration order.

Slices and maps are passed by copying the header, so mutations are visible to callers.

Recipe

Quick-reference recipe card - copy-paste ready.

package main
 
import "fmt"
 
func main() {
    nums := make([]int, 0, 4)
    nums = append(nums, 1, 2, 3)
 
    ages := map[string]int{"ada": 36}
    ages["grace"] = 34
 
    fmt.Println(nums, len(nums), cap(nums), ages)
}

When to reach for this:

Use slices for ordered, growable sequences.
Use maps for keyed lookup when keys are comparable.
Use arrays only when size is a fixed protocol constraint (crypto, embedded).
Preallocate with make([]T, 0, n) when you know approximate size.

Working Example

package main
 
import "fmt"
 
func addSuffix(names []string, suffix string) {
    for i := range names {
        names[i] += suffix
    }
}
 
func main() {
    original := []string{"go", "rust"}
    view := original[0:2:2] // full slice expression caps capacity
    addSuffix(view, "!")
 
    counts := map[string]int{}
    for _, name := range original {
        counts[name]++
    }
 
    fmt.Println(original, counts)
 
    keys := make([]string, 0, len(counts))
    for k := range counts {
        keys = append(keys, k)
    }
    fmt.Println("keys (unordered):", keys)
}

What this demonstrates:

Slice headers copy but share backing arrays - addSuffix mutates original.
Full slice expression s[low:high:max] limits append growth capacity.
Maps need make or literals before writes.
Collecting map keys requires a separate slice; order is not stable.

Deep Dive

How It Works

Slice append grows capacity by allocating a new array when len == cap (typically doubling).
Slicing s[low:high] sets len to high-low and cap to cap(s)-low.
Map buckets are internal; iteration order is randomized each pass.
Arrays assign by value (full copy); slices and maps assign headers (shared storage).

Arrays vs Slices vs Maps

	Array `[n]T`	Slice `[]T`	Map `map[K]V`
Size	Fixed in type	Dynamic	Dynamic
Pass to func	Copy whole array	Copy header	Copy header
Zero value	All elements zero	`nil`	`nil`
Comparable	Yes (if `T` comparable)	No	No
Index access	`a[i]`	`s[i]`	`m[k]` with ok form

len and cap

Expression	Meaning
`len(s)`	Number of elements visible in slice
`cap(s)`	Elements from first index through end of backing array
`make([]T, len, cap)`	Allocates array, sets len and cap
`append(s, x)`	Grows len, may reallocate if `len == cap`

Go Notes

// Delete map entry
delete(m, key)
 
// Clear map in Go 1.21+
clear(m)
 
// Copy slices
dst := make([]int, len(src))
copy(dst, src)

Gotchas

Append alias surprise - a := []int{1,2,3}; b := a[:2]; append(b, 99) may overwrite a[2] if cap allows. Fix: use full slice a[:2:2] or append to a copy.
Nil map write - Assigning to uninitialized map panics. Fix: make first.
Expecting sorted map iteration - Order changes between loops. Fix: sort keys slice before display.
Using array as slice stand-in - [5]int is not []int without slicing arr[:]. Fix: slice with [:].
Large array copies - Passing big arrays copies all elements. Fix: pass pointer *[N]T or use slice.
Concurrent map writes - Maps are not safe for concurrent write without sync. Fix: sync.Map or mutex.

Alternatives

Alternative	Use When	Don't Use When
Slice	Default ordered collection	Need O(1) lookup by key
Map	Keyed lookup	Need stable order without extra sort
Array	Fixed wire format size	General-purpose lists
`container/list`	Frequent middle inserts (rare)	Most cases - slices win

FAQs

What is the difference between len and cap?

len is how many elements you can read/write from index 0.

cap is how many elements exist in the backing array starting at the slice's first element.

Is a nil slice valid?

Yes - len and cap are 0, range works, and append allocates backing storage.

How do I check if a map has a key?

v, ok := m[key]
if !ok { /* missing */ }

Can map keys be slices?

No - keys must be comparable; slices, maps, and functions are not comparable.

How do I preallocate a slice?

make([]T, 0, expected) then append, or make([]T, expected) if you will index directly.

What does s[low:high:max] do?

Sets len to high-low and cap to max-low, preventing append from reading past max.

How are arrays passed to functions?

The entire array copies.

For [1024]byte buffers, pass a slice buf[:] or pointer instead.

Can I take the address of a map element?

No - &m[key] is illegal because map storage can move during growth.

Store value in a variable first if you need a pointer.

How do I merge maps?

Loop and assign: for k, v := range other { m[k] = v }.

Watch for overwriting existing keys.

Does append return the same slice header?

It may return a new header if reallocation happens.

Always assign: s = append(s, x).

What is the zero value of [3]int vs []int?

Array is [0 0 0] value type.

Slice is nil header with no backing array.

How do I shallow-copy a slice?

append([]T(nil), s...) or s2 := make([]T, len(s)); copy(s2, s).

Zero Values and Initialization - nil maps and slices
Pointers & the Address-of Operator - Sharing via slice headers
Numeric Types, Strings, Runes & Byte Slices - []byte as byte slices
Structs, Embedding & Field Promotion - Structs containing slices and maps

A Tour of Go - Official interactive introduction to Go.
Go by Example - Annotated practical code examples.
Effective Go - Official guide to writing clear, idiomatic Go.
The Go Programming Language (ebook) - Donovan & Kernighan.
Learning Go (2nd Edition) - Jon Bodner.

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

Arrays, Slices & Maps at a Glance

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Arrays, Slices & Maps at a Glance