SliceTricks.md

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

**Copy**
```go
b = make([]T, len(a))
copy(b, a)
// or, if a is not the empty slice,
b = append([]T(nil), 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 ` and thus can not be collected. 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**
```go
a = append(a[:i], append(make([]T, j), a[i:]...)...)
```

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

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

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

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

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

**Shift**
```go
x, a := a[0], a[1:]
```

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

## 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)
	}
}
```

### 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]
}
```