I have troubles translating this piece of code, which is effectively a left rotate on a slice, into a more generic version which accepts interface{} as an input parameter.
func rotate(a *[]int, i int) {
x, b := (*a)[:i], (*a)[i:]
*a = append(b, x...)
}
I have troubles with the final assignment:
func rotateSlice(a interface{}, i int) {
v := reflect.ValueOf(a)
x, b := v.Elem().Slice(0, i), v.Elem().Slice(i, v.Elem().Len())
*a = reflect.AppendSlice(b, x)
}
The error message is invalid indirect of a (type {}). The value of a is interface{}, hence *a = would be to assign the right-hand value to the space where the pointer is pointing to. My call to AppendSlice returns Value though. I am not sure where the type assertion needs to happen, I suppose on the left-hand side?
a is an interface{} not a pointer, so you can't dereference it. Even if you have a pointer to a slice, you can't assigned the result ofreflect.AppendSlice, because it returns the type reflect.Value. You need to set the value via Value.Set.
https://play.golang.org/p/JCF8jsRJ_O
func rotateSlice(a interface{}, i int) {
v := reflect.ValueOf(a).Elem()
x, b := v.Slice(0, i), v.Slice(i, v.Len())
v.Set(reflect.AppendSlice(b, x))
}
Eliminate the use of the reflect package in Go 1.18 and later by using generics:
func rotateSlice[T any](a []T, i int) []T {
x, b := a[:i], a[i:]
return append(b, x...)
}
Call it like this: x = rotateSlice(x, 2)
Here's now to implement rotate using the reflect package.
Use Value.Set to set the value in the slice.
func rotateSlice(a interface{}, i int) {
v := reflect.ValueOf(a).Elem()
x, b := v.Slice(0, i), v.Slice(i, v.Len())
v.Set(reflect.AppendSlice(b, x))
}
Call the function with a pointer to a slice:
a := []string{"a", "b", "c", "d"}
roateSlice(&a, 2)
Run all of the examples on the playground.
Related
I'm trying to convert key-value map to slice of pairs, for example given a map like:
m := make(map[int64]int64)
m[521] = 4
m[528] = 8
How do I convert that into a slice of its entries, like: [[521, 4], [528, 8]]
I'm thinking about ranging over all those key-values then create slice for that, but is there any simple code to do that?
package main
import "fmt"
func main() {
//create a map
m := map[int64]int64{512: 8, 513: 9, 234: 9, 392: 0}
//create a slice to hold required values
s := make([][]int64, 0)
//range over map `m` to append to slice `s`
for k, v := range m {
// append each element, with a new slice []int64{k, v}
s = append(s, []int64{k, v})
}
fmt.Println(s)
}
Go 1.18
It is now possible to write a generic function to extract all key-value pairs, i.e. the map entries, with any key and value types.
Notes:
the map iterations are still unordered — using generics doesn't change that.
the constraint for the map key must be comparable
type Pair[K, V any] struct {
First K
Second V
}
func Entries[M ~map[K]V, K comparable, V any](m M) []Pair[K, V] {
entries := make([]Pair[K, V], 0)
for k, v := range m {
entries = append(entries, Pair[K, V]{k, v})
}
return entries
}
The type Pair here is used to preserve type safety in the return value. If you really must return a slice of slices, then it can only be [][]any (or [][2]any) in order to hold different types.
If the map key and value have the same type, of course you can still use Pair but you can also use a type-safe variation of the above:
func Entries[T comparable](m map[T]T) [][2]T {
entries := make([][2]T, 0)
for k, v := range m {
entries = append(entries, [2]T{k, v})
}
return entries
}
Again, T must be comparable or stricter in order to work as a map key.
Playground: https://go.dev/play/p/RwCGmp7MHKW
Here is the code:
type field struct {
name string
}
func print(p *field) {
fmt.Println(p.name)
}
func fix1() {
data := []*field{{name: "one"}, {name: "two"}, {name: "three"}}
for _, v := range data {
go print(v)
}
time.Sleep(time.Millisecond * 200)
}
func wrong1() {
data := []*field{{name: "one"}, {name: "two"}, {name: "three"}}
for _, v := range data {
go func() {
print(v)
}()
}
time.Sleep(time.Millisecond * 200)
}
func main() {
wrong1()
}
As far as I understand, all goroutines in function wrong1 share the same local variable v. At the moment of a goroutine execution, the value of v may be equal to any value in data, therefore the function prints random data three times.
However, I am failing to understand why function fix1 behaves differently (it prints each value in data exactly once).
wrong1(): go func() { print(v) }()
Go: Frequently Asked Questions (FAQ)
What happens with closures running as goroutines?
Some confusion may arise when using closures with concurrency.
Consider the following program:
func main() {
done := make(chan bool)
values := []string{"a", "b", "c"}
for _, v := range values {
go func() {
fmt.Println(v)
done <- true
}()
}
// wait for all goroutines to complete before exiting
for _ = range values {
<-done
}
}
One might mistakenly expect to see a, b, c as the output. What you'll
probably see instead is c, c, c. This is because each iteration of the
loop uses the same instance of the variable v, so each closure shares
that single variable. When the closure runs, it prints the value of v
at the time fmt.Println is executed, but v may have been modified
since the goroutine was launched.
To bind the current value of v to each closure as it is launched, one
must modify the inner loop to create a new variable each iteration.
One way is to pass the variable as an argument to the closure:
for _, v := range values {
go func(u string) {
fmt.Println(u)
done <- true
}(v)
}
In this example, the value of v is passed as an argument to the
anonymous function. That value is then accessible inside the function
as the variable u.
Even easier is just to create a new variable, using a declaration
style that may seem odd but works fine in Go:
for _, v := range values {
v := v // create a new 'v'.
go func() {
fmt.Println(v)
done <- true
}()
}
Your wrong1 example,
for _, v := range data {
go func() {
print(v)
}()
}
Playground: https://play.golang.org/p/0w86nvVMt1g
Output:
three
three
three
Your wrong1 example, creating a new variable,
for _, v := range data {
v := v
go func() {
print(v)
}()
}
Playground: https://play.golang.org/p/z5RCI0ZZU8Z
Output:
one
two
three
Your wrong1 example, passing the variable as an argument,
for _, v := range data {
go func(v *field) {
print(v)
}(v)
}
Playground: https://play.golang.org/p/1JVI7XYSqvv
Output:
one
two
three
fix1(): go print(v)
The Go Programming Language Specification
Calls
Given an expression f of function type F,
f(a1, a2, … an)
calls f with arguments a1, a2, … an. Except for one special case,
arguments must be single-valued expressions assignable to the
parameter types of F and are evaluated before the function is called.
Go statements
The function value and parameters are evaluated as usual in the
calling goroutine.
Your fix1 example, evaluating the value of v before the function is called,
for _, v := range data {
go print(v)
}
Playground: https://play.golang.org/p/rN3UNaGi-ge
Output:
one
two
three
I have this code:
type countHolder struct {
count int
}
func main() {
a := &countHolder{1}
b := *a
a.count = 2
println(b.count)
}
I expected the output to be 2, but the output was 1.
My understanding was that:
a := &countHolder{1} // a is pointer to struct with data starting at address x
b := *a // b now equals address x
a.count = 2 // the struct stored at address x has its count value changed to 2
Where am I wrong? is b := *a creating a copy of the struct?
From the fine specification:
For an operand x of type T, the address operation &x generates a pointer of type *T to x. [...]
For an operand x of pointer type *T, the pointer indirection *x denotes the variable of type T pointed to by x. [...]
That means that the unary & operator gives you the address of something so a in:
a := &countHolder{1}
is a pointer. The unary * operator in:
b := *a
dereferences the pointer a and leaves you with a countHolder struct on the right side so b is a copy of the struct that a points to. Since b is a copy of the struct, modifying a.count:
a.count = 2
(which could also be written as (*a).count = 2) won't have any affect on b.
You could also have a look at (https://play.golang.org/p/Zubs8qYBA_K):
func main() {
a := &countHolder{1}
b := *a
fmt.Printf("%T\n%T\n", a, b)
}
to have a quick look at what types a and b are (*counterHolder and counterHolder, respectively, in this case).
Go has very neat multiple return values paradigm. But It looks like v, ok := map[key] and v, k := range m use different mechanism with same notation. Here is a simple example:
func f2() (k, v string) {
return "Hello", "World"
}
func main(){
k := f2() // Doesn't work : multiple-value f2() in single-value context
m := map[string]int{"One": 1}
// It works
v, ok := m["One"]
// How it all work?
v := m["One"]
for k := range m {}
}
In above example, k := f2() gives error as f2 returns two values, whereas v, ok := m["One"] and v := m["One"] - both expressions work without any error.
Why is that different behavior?
A fetch from the built in map, using range on a map, array or slice, and also type assertions allows for one or two variables. This is not the case for user defined functions and methods. If a function declares two return values, you must tell what to do with both of them, or ignore both:
k, _ := f2() // Specify what to do with each returned value
f2() // Ignoring both
Why? Because the specification says it is so:
Map (indexed expressions):
An index expression on a map a of type map[K]V may be used in an assignment or initialization of the special form
v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
where the result of the index expression is a pair of values with types (V, bool). In this form, the value of ok is true if the key x is present in the map, and false otherwise. The value of v is the value a[x] as in the single-result form.
Range (for statement):
For each iteration, iteration values are produced as follows:
Range expression: m map[K]V
1st value: key k K
2nd value (if 2nd variable is present): m[k] V
Type assertion:
For an expression x of interface type and a type T, the primary expression
x.(T)
asserts that x is not nil and that the value stored in x is of type T.
and
If a type assertion is used in an assignment or initialization of the form
v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)
the result of the assertion is a pair of values with types (T, bool)
If I have a map m is there a better way of getting a slice of the values v than this?
package main
import (
"fmt"
)
func main() {
m := make(map[int]string)
m[1] = "a"
m[2] = "b"
m[3] = "c"
m[4] = "d"
// Can this be done better?
v := make([]string, len(m), len(m))
idx := 0
for _, value := range m {
v[idx] = value
idx++
}
fmt.Println(v)
}
Is there a built-in feature of a map? Is there a function in a Go package, or is this the only way to do this?
As an addition to jimt's post:
You may also use append rather than explicitly assigning the values to their indices:
m := make(map[int]string)
m[1] = "a"
m[2] = "b"
m[3] = "c"
m[4] = "d"
v := make([]string, 0, len(m))
for _, value := range m {
v = append(v, value)
}
Note that the length is zero (no elements present yet) but the capacity (allocated space) is initialized with the number of elements of m. This is done so append does not need to allocate memory each time the capacity of the slice v runs out.
You could also make the slice without the capacity value and let append allocate the memory for itself.
Unfortunately, no. There is no builtin way to do this.
As a side note, you can omit the capacity argument in your slice creation:
v := make([]string, len(m))
The capacity is implied to be the same as the length here.
Go 1.18
You can use maps.Values from the golang.org/x/exp package.
Values returns the values of the map m. The values will be in an indeterminate order.
func main() {
m := map[int]string{1: "a", 2: "b", 3: "c", 4: "d"}
v := maps.Values(m)
fmt.Println(v)
}
The package exp includes experimental code. The signatures may or may not change in the future, and may or may not be promoted to the standard library.
If you don't want to depend on an experimental package, you can easily implement it yourself. In fact, this code is a copy-paste from the exp package:
func Values[M ~map[K]V, K comparable, V any](m M) []V {
r := make([]V, 0, len(m))
for _, v := range m {
r = append(r, v)
}
return r
}
Not necessarily better, but the cleaner way to do this is by defining both the Slice LENGTH and CAPACITY like txs := make([]Tx, 0, len(txMap))
// Defines the Slice capacity to match the Map elements count
txs := make([]Tx, 0, len(txMap))
for _, tx := range txMap {
txs = append(txs, tx)
}
Full example:
package main
import (
"github.com/davecgh/go-spew/spew"
)
type Tx struct {
from string
to string
value uint64
}
func main() {
// Extra touch pre-defining the Map length to avoid reallocation
txMap := make(map[string]Tx, 3)
txMap["tx1"] = Tx{"andrej", "babayaga", 10}
txMap["tx2"] = Tx{"andrej", "babayaga", 20}
txMap["tx3"] = Tx{"andrej", "babayaga", 30}
txSlice := getTXsAsSlice(txMap)
spew.Dump(txSlice)
}
func getTXsAsSlice(txMap map[string]Tx) []Tx {
// Defines the Slice capacity to match the Map elements count
txs := make([]Tx, 0, len(txMap))
for _, tx := range txMap {
txs = append(txs, tx)
}
return txs
}
Simple solution but a lot of gotchas. Read this blog post for more details: https://web3.coach/golang-how-to-convert-map-to-slice-three-gotchas
As far as I'm currently aware, go doesn't have a way method for concatenation of strings/bytes in to a resulting string without making at least /two/ copies.
You currently have to grow a []byte since all string values are const, THEN you have to use the string builtin to have the language create a 'blessed' string object, which it will copy the buffer for since something somewhere could have a reference to the address backing the []byte.
If a []byte is suitable then you can gain a very slight lead over the bytes.Join function by making one allocation and doing the copy calls your self.
package main
import (
"fmt"
)
func main() {
m := make(map[int]string)
m[1] = "a" ; m[2] = "b" ; m[3] = "c" ; m[4] = "d"
ip := 0
/* If the elements of m are not all of fixed length you must use a method like this;
* in that case also consider:
* bytes.Join() and/or
* strings.Join()
* They are likely preferable for maintainability over small performance change.
for _, v := range m {
ip += len(v)
}
*/
ip = len(m) * 1 // length of elements in m
r := make([]byte, ip, ip)
ip = 0
for _, v := range m {
ip += copy(r[ip:], v)
}
// r (return value) is currently a []byte, it mostly differs from 'string'
// in that it can be grown and has a different default fmt method.
fmt.Printf("%s\n", r)
}
As of 1.18, this is the best way:
https://stackoverflow.com/a/71635953/130427
Pre 1.18
You can use this maps package:
go get https://github.com/drgrib/maps
Then all you have to call is
values := maps.GetValuesIntString(m)
It's type-safe for that common map combination. You can generate other type-safe functions for any other type of map using the mapper tool in the same package.
Full disclosure: I am the creator of this package. I created it because I found myself rewriting these functions for map repeatedly.