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Tags: golang go slice
Clipped on: 2016-11-10

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# SliceTricks

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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 `.

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

AppendVector

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

Copy

```b = make([]T, len(a))
copy(b, a)
// or, if a is not the empty slice,
b = append([]T(nil), a...)```

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)
copy(s[i+1:], s[i:])
s[i] = x```

InsertVector

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

Pop

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

Push

`a = append(a, x)`

Shift

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

Unshift

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

### 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)
}
}```

### 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]
}```