# Slice tricks

suggest change

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

### Append a whole slice

``a = append(a, b...)``

### Copy

``````b = make([]T, len(a))
copy(b, a)
// or
b = append([]T(nil), a...)
// or
b = append(a[:0:0], a...)  // See <https://github.com/go101/go101/wiki>
``````

### Cut

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

### Delete

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

### Delete without preserving order

``````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` and thus can not be collected. The following code can fix this problem:

Cut
``````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
``````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
``````a[i] = a[len(a)-1]
a[len(a)-1] = nil
a = a[:len(a)-1]``````

### Expand

``a = append(a[:i], append(make([]T, j), a[i:]...)...)``

### Extend

``a = append(a, make([]T, j)...)``

### Insert

``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
``````s = append(s, 0 /* use the zero value of the element type */)
copy(s[i+1:], s[i:])
s[i] = x``````

### Insert slice at index i:

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

### Push

``a = append(a, x)``

### Pop

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

### Push Front/Unshift

``a = append([]T{x}, a...)``

### Pop Front/Shift

``x, a = a[0], a[1:]``

### 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.

``````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:

``````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:

``````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:

``````for left, right := 0, len(a)-1; left < right; left, right = left+1, right-1 {
a[left], a[right] = a[right], a[left]
}``````

### Shuffling

Fisherâ€“Yates algorithm:

Since go1.10, this is available at math/rand.Shuffle
``````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.

``````actions := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
batchSize := 3
var batches [][]int

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

Yields the following:

``[[0 1 2] [3 4 5] [6 7 8] [9]]``

### In-place deduplicate (comparable)

``````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]
``````