In Go, there are three built-in functions to manipulate slices:
make
function is used to create a slice with specified length and capacity. All the elements are set to zero values.copy
function is used to copy some elements from one slice to another if the two slices have identical element types.append
function is used to append some elements to a base slice and result in a new slice.
The base slice might share all of its elements with the new slice or not,
depending on whether or not the capacity of the base slice is large enough to hold all the appended elements.When the capacity of the base slice if an append
function call is not larger enough.
the call can be viewed as a way to merge two slices.
As the way is built-in, it is performant.
However, there is not a performant way to merge 3+ slices. The following implementation might be the most performant way to do the task:
func merge[S ~[]E, E any](ss ...S) S {
var n, allNils, k = 0, true, -1
for i := range ss {
if m := len(ss[i]); n != 0 {
n += m
if n < 0 {
panic("sum of lengths is too large")
}
} else if m > 0 {
n = m
allNils = false
k = i
} else {
allNils = allNils && ss[i] == nil
}
}
if allNils {
return nil
}
if n == 0 {
return S{}
}
// Make use of this optimization:
// https://github.com/golang/go/commit/6ed4661807b219781d1aa452b7f210e21ad1974b
s := ss[k]
r := make(S, n)
copy(r, s)
r = r[:len(s)]
ss = ss[k+1:]
for _, s := range ss {
r = append(r, s...)
}
return r
}
Generally, a make
call will zero the elements allocated during its execution, which is unnecessarily for this particular use case.
The implementation does its best to zero as few as possible elements within the make
call,
by using the mentioned optimization, it doesn't zero the elements in r[:len(ss[0])]
,
but it still fails to do so for the remaining elements (the ones in r[len(ss[0]):]
).
If the merge
function is built-in, then it is able to avoid all the unnecessary element zeroing operations.
As mentioned above, an append
function call will reuse the backing array
of the first slice argument (call it base
here) for the result slice if the capacity of base
is large enough to hold all the appended elements.
For some scenarios, we want to prevent an append
call from modifying the elements in base[len(base):]
and we achieve this by clipping the capacity of base
to its length:
var x, y, z T
... = append(base[:len(base):len(base)], x, y, z)
Nothing to complain about, except that it is some verbose,
in particular when the base
expression is verbose, such as
... = append(aValue.Field.Slice[:len(aValue.Field.Slice):len(aValue.Field.Slice)], x, y, z)
If the base
expression is a function call, then we must store the result of a call in a temporary intermediate variable:
base := aFunctionCall()
... = append(base[:len(base):len(base)], x, y, z)
We can use the Clip
function in the golang.org/x/exp/slices
package, but this way is still not clean enough.
import "golang.org/x/exp/slices"
... = append(slices.Clip((base), x, y, z)
... = append(slices.Clip(aValue.Field.Slice), x, y, z)
... = append(slices.Clip(aFunctionCall()), x, y, z)
I do perfer this proposal, but it has been rejected:
aSlice[ : n : ] // <=> aSlice[ : n : n]
aSlice[m : n : ] // <=> aSlice[m : n : n]
sSlice[ : : ] // <=> aSlice[ : len(s) : len(s)]
aSlice[m : : ] // <=> aSlice[m : len(s) : len(s)]
If the proposal is adopted, then we may just write the code as
... = append(base[::], x, y, z)
... = append(aValue.Field.Slice[::], x, y, z)
... = append(aFunctionCall()[::], x, y, z)
which is much cleaner.
This fact prevents custom Go packages to make perfect implementations for some slice manipulations.
Some standard packages are using (ever used) unsafe ways to do the job. For example:
These unsafe usages actually don't follow any valid unsafe use patterns, so they are bad and dangerous implementations.
The Go 101 project is hosted on Github. Welcome to improve Go 101 articles by submitting corrections for all kinds of mistakes, such as typos, grammar errors, wording inaccuracies, description flaws, code bugs and broken links.
If you would like to learn some Go details and facts every serveral days, please follow Go 101's official Twitter account @go100and1.