SliceTricks.md

---
title: SliceTricks
---

Since the introduction of the ` append ` built-in, most of the functionality of the ` container/vector ` package, which was removed in Go 1, can be replicated using ` append ` and ` copy `.

Since the introduction of generics, generic implementations of several of these functions are available in the [`golang.org/x/exp/slices`](https://pkg.go.dev/golang.org/x/exp/slices) package.

Here are the vector methods and their slice-manipulation analogues:

#### AppendVector
```go
a = append(a, b...)
```

#### Copy
```go
b := make([]T, len(a))
copy(b, a)

// These two are often a little slower than the above one,
// but they would be more efficient if there are more
// elements to be appended to b after copying.
b = append([]T(nil), a...)
b = append(a[:0:0], a...)

// This one-line implementation is equivalent to the above
// two-line make+copy implementation logically. But it is
// actually a bit slower (as of Go toolchain v1.16).
b = append(make([]T, 0, len(a)), a...)
```

#### Cut
```go
a = append(a[:i], a[j:]...)
```

#### Delete
```go
a = append(a[:i], a[i+1:]...)
// or
a = a[:i+copy(a[i:], a[i+1:])]
```

#### Delete without preserving order
```go
a[i] = a[len(a)-1] 
a = a[:len(a)-1]

```
**NOTE** If the type of the element is a _pointer_ or a struct with pointer fields, which need to be garbage collected, the above implementations of ` Cut ` and ` Delete ` have a potential _memory leak_ problem: some elements with values are still referenced by slice ` a `'s underlying array, just not "visible" in the slice. Because the "deleted" value is referenced in the underlying array, the deleted value is still "reachable" during GC, even though the value cannot be referenced by your code. If the underlying array is long-lived, this represents a leak. The following code can fix this problem:
> **Cut**
```go
copy(a[i:], a[j:])
for k, n := len(a)-j+i, len(a); k < n; k++ {
	a[k] = nil // or the zero value of T
}
a = a[:len(a)-j+i]
```

> **Delete**
```go
copy(a[i:], a[i+1:])
a[len(a)-1] = nil // or the zero value of T
a = a[:len(a)-1]
```

> **Delete without preserving order**
```go
a[i] = a[len(a)-1]
a[len(a)-1] = nil
a = a[:len(a)-1]
```

#### Expand
Insert `n` elements at position `i`:
```go
a = append(a[:i], append(make([]T, n), a[i:]...)...)
```

#### Extend
Append `n` elements:
```go
a = append(a, make([]T, n)...)
```

#### Extend Capacity
Make sure there is space to append `n` elements without re-allocating:
```go
if cap(a)-len(a) < n {
	a = append(make([]T, 0, len(a)+n), a...)
}
```

#### Filter (in place)

```go
n := 0
for _, x := range a {
	if keep(x) {
		a[n] = x
		n++
	}
}
a = a[:n]
```

#### Insert
```go
a = append(a[:i], append([]T{x}, a[i:]...)...)
```
**NOTE**: The second ` append ` creates a new slice with its own underlying storage and  copies elements in ` a[i:] ` to that slice, and these elements are then copied back to slice ` a ` (by the first ` append `). The creation of the new slice (and thus memory garbage) and the second copy can be avoided by using an alternative way:
> **Insert**
```go
s = append(s, 0 /* use the zero value of the element type */)
copy(s[i+1:], s[i:])
s[i] = x
```

#### InsertVector
```go
a = append(a[:i], append(b, a[i:]...)...)

// The above one-line way copies a[i:] twice and
// allocates at least once.
// The following verbose way only copies elements
// in a[i:] once and allocates at most once.
// But, as of Go toolchain 1.16, due to lacking of
// optimizations to avoid elements clearing in the
// "make" call, the verbose way is not always faster.
//
// Future compiler optimizations might implement
// both in the most efficient ways.
//
// Assume element type is int.
func Insert(s []int, k int, vs ...int) []int {
	if n := len(s) + len(vs); n <= cap(s) {
		s2 := s[:n]
		copy(s2[k+len(vs):], s[k:])
		copy(s2[k:], vs)
		return s2
	}
	s2 := make([]int, len(s) + len(vs))
	copy(s2, s[:k])
	copy(s2[k:], vs)
	copy(s2[k+len(vs):], s[k:])
	return s2
}

a = Insert(a, i, b...)
```

#### Push
```go
a = append(a, x)
```

#### Pop
```go
x, a = a[len(a)-1], a[:len(a)-1]
```

#### Push Front/Unshift
```go
a = append([]T{x}, a...)
```

#### Pop Front/Shift
```go
x, a = a[0], a[1:]
```

## Additional Tricks
### Filtering without allocating

This trick uses the fact that a slice shares the same backing array and capacity as the original, so the storage is reused for the filtered slice. Of course, the original contents are modified.

```go
b := a[:0]
for _, x := range a {
	if f(x) {
		b = append(b, x)
	}
}
```

For elements which must be garbage collected, the following code can be included afterwards:

```go
for i := len(b); i < len(a); i++ {
	a[i] = nil // or the zero value of T
}
```

### Reversing

To replace the contents of a slice with the same elements but in reverse order:
```go
for i := len(a)/2-1; i >= 0; i-- {
	opp := len(a)-1-i
	a[i], a[opp] = a[opp], a[i]
}
```
The same thing, except with two indices:
```go
for left, right := 0, len(a)-1; left < right; left, right = left+1, right-1 {
	a[left], a[right] = a[right], a[left]
}
```

### Shuffling

Fisher–Yates algorithm:

> Since go1.10, this is available at [math/rand.Shuffle](https://pkg.go.dev/math/rand#Shuffle)

```go
for i := len(a) - 1; i > 0; i-- {
    j := rand.Intn(i + 1)
    a[i], a[j] = a[j], a[i]
}
```

### Batching with minimal allocation

Useful if you want to do batch processing on large slices.

```go
actions := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
batchSize := 3
batches := make([][]int, 0, (len(actions) + batchSize - 1) / batchSize)

for batchSize < len(actions) {
    actions, batches = actions[batchSize:], append(batches, actions[0:batchSize:batchSize])
}
batches = append(batches, actions)
```

Yields the following:
```go
[[0 1 2] [3 4 5] [6 7 8] [9]]
```

### In-place deduplicate (comparable) 

```go
import "sort"

in := []int{3,2,1,4,3,2,1,4,1} // any item can be sorted
sort.Ints(in)
j := 0
for i := 1; i < len(in); i++ {
	if in[j] == in[i] {
		continue
	}
	j++
	// preserve the original data
	// in[i], in[j] = in[j], in[i]
	// only set what is required
	in[j] = in[i]
}
result := in[:j+1]
fmt.Println(result) // [1 2 3 4]
```

### Move to front, or prepend if not present, in place if possible.

```go
// moveToFront moves needle to the front of haystack, in place if possible.
func moveToFront(needle string, haystack []string) []string {
	if len(haystack) != 0 && haystack[0] == needle {
		return haystack
	}
	prev := needle
	for i, elem := range haystack {
		switch {
		case i == 0:
			haystack[0] = needle
			prev = elem
		case elem == needle:
			haystack[i] = prev
			return haystack
		default:
			haystack[i] = prev
			prev = elem
		}
	}
	return append(haystack, prev)
}

haystack := []string{"a", "b", "c", "d", "e"} // [a b c d e]
haystack = moveToFront("c", haystack)         // [c a b d e]
haystack = moveToFront("f", haystack)         // [f c a b d e]
```

### Sliding Window
```go
func slidingWindow(size int, input []int) [][]int {
	// returns the input slice as the first element
	if len(input) <= size {
		return [][]int{input}
	}

	// allocate slice at the precise size we need
	r := make([][]int, 0, len(input)-size+1)

	for i, j := 0, size; j <= len(input); i, j = i+1, j+1 {
		r = append(r, input[i:j])
	}

	return r
}
```