How to assign string to bytes array - go

Question

I want to assign string to bytes array:
var arr [20]byte
str := "abc"
for k, v := range []byte(str) {
arr[k] = byte(v)
}
Have another method?

Answer 1

Safe and simple:
[]byte("Here is a string....")

Answer 2

For converting from a string to a byte slice, string -> []byte:
[]byte(str)
For converting an array to a slice, [20]byte -> []byte:
arr[:]
For copying a string to an array, string -> [20]byte:
copy(arr[:], str)
Same as above, but explicitly converting the string to a slice first:
copy(arr[:], []byte(str))
The built-in copy function only copies to a slice, from a slice.
Arrays are "the underlying data", while slices are "a viewport into underlying data".
Using [:] makes an array qualify as a slice.
A string does not qualify as a slice that can be copied to, but it qualifies as a slice that can be copied from (strings are immutable).
If the string is too long, copy will only copy the part of the string that fits (and multi-byte runes may then be copied only partly, which will corrupt the last rune of the resulting string).
This code:
var arr [20]byte
copy(arr[:], "abc")
fmt.Printf("array: %v (%T)\n", arr, arr)
...gives the following output:
array: [97 98 99 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] ([20]uint8)
I also made it available at the Go Playground

Answer 3

For example,
package main
import "fmt"
func main() {
s := "abc"
var a [20]byte
copy(a[:], s)
fmt.Println("s:", []byte(s), "a:", a)
}
Output:
s: [97 98 99] a: [97 98 99 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]

Answer 4

Piece of cake:
arr := []byte("That's all folks!!")

Answer 5

I think it's better..
package main
import "fmt"
func main() {
str := "abc"
mySlice := []byte(str)
fmt.Printf("%v -> '%s'",mySlice,mySlice )
}
Check here: http://play.golang.org/p/vpnAWHZZk7

Answer 6

Go, convert a string to a bytes slice
You need a fast way to convert a []string to []byte type. To use in situations such as storing text data into a random access file or other type of data manipulation that requires the input data to be in []byte type.
package main
func main() {
var s string
//...
b := []byte(s)
//...
}
which is useful when using ioutil.WriteFile, which accepts a bytes slice as its data parameter:
WriteFile func(filename string, data []byte, perm os.FileMode) error
Another example
package main
import (
"fmt"
"strings"
)
func main() {
stringSlice := []string{"hello", "world"}
stringByte := strings.Join(stringSlice, " ")
// Byte array value
fmt.Println([]byte(stringByte))
// Corresponding string value
fmt.Println(string([]byte(stringByte)))
}
Output:
[104 101 108 108 111 32 119 111 114 108 100] hello world
Please check the link playground

Answer 7

Besides the methods mentioned above, you can also do a trick as
s := "hello"
b := *(*[]byte)(unsafe.Pointer((*reflect.SliceHeader)(unsafe.Pointer(&s))))
Go Play: http://play.golang.org/p/xASsiSpQmC
You should never use this :-)

Answer 8

Ended up creating array specific methods to do this. Much like the encoding/binary package with specific methods for each int type. For example binary.BigEndian.PutUint16([]byte, uint16).
func byte16PutString(s string) [16]byte {
var a [16]byte
if len(s) > 16 {
copy(a[:], s)
} else {
copy(a[16-len(s):], s)
}
return a
}
var b [16]byte
b = byte16PutString("abc")
fmt.Printf("%v\n", b)
Output:
[0 0 0 0 0 0 0 0 0 0 0 0 0 97 98 99]
Notice how I wanted padding on the left, not the right.
http://play.golang.org/p/7tNumnJaiN

Answer 9

Arrays are values... slices are more like pointers. That is [n]type is not compatible with []type as they are fundamentally two different things. You can get a slice that points to an array by using arr[:] which returns a slice that has arr as it's backing storage.
One way to convert a slice of for example []byte to [20]byte is to actually allocate a [20]byte which you can do by using var [20]byte (as it's a value... no make needed) and then copy data into it:
buf := make([]byte, 10)
var arr [10]byte
copy(arr[:], buf)
Essentially what a lot of other answers get wrong is that []type is NOT an array.
[n]T and []T are completely different things!
When using reflect []T is not of kind Array but of kind Slice and [n]T is of kind Array.
You also can't use map[[]byte]T but you can use map[[n]byte]T.
This can sometimes be cumbersome because a lot of functions operate for example on []byte whereas some functions return [n]byte (most notably the hash functions in crypto/*).
A sha256 hash for example is [32]byte and not []byte so when beginners try to write it to a file for example:
sum := sha256.Sum256(data)
w.Write(sum)
they will get an error. The correct way of is to use
w.Write(sum[:])
However, what is it that you want? Just accessing the string bytewise? You can easily convert a string to []byte using:
bytes := []byte(str)
but this isn't an array, it's a slice. Also, byte != rune. In case you want to operate on "characters" you need to use rune... not byte.

Answer 10

If someone is looking for a quick consider use unsafe conversion between slices, you can refer to the following comparison.
package demo_test
import (
"testing"
"unsafe"
)
var testStr = "hello world"
var testBytes = []byte("hello world")
// Avoid copying the data.
func UnsafeStrToBytes(s string) []byte {
return *(*[]byte)(unsafe.Pointer(&s))
}
// Avoid copying the data.
func UnsafeBytesToStr(b []byte) string {
return *(*string)(unsafe.Pointer(&b))
}
func Benchmark_UnsafeStrToBytes(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = UnsafeStrToBytes(testStr)
}
}
func Benchmark_SafeStrToBytes(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = []byte(testStr)
}
}
func Benchmark_UnSafeBytesToStr(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = UnsafeBytesToStr(testBytes)
}
}
func Benchmark_SafeBytesToStr(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = string(testBytes)
}
}
go test -v -bench="^Benchmark" -run=none
output
cpu: Intel(R) Core(TM) i7-8565U CPU # 1.80GHz
Benchmark_UnsafeStrToBytes
Benchmark_UnsafeStrToBytes-8 1000000000 0.2465 ns/op
Benchmark_SafeStrToBytes
Benchmark_SafeStrToBytes-8 289119562 4.181 ns/op
Benchmark_UnSafeBytesToStr
Benchmark_UnSafeBytesToStr-8 1000000000 0.2530 ns/op
Benchmark_SafeBytesToStr
Benchmark_SafeBytesToStr-8 342842938 3.623 ns/op
PASS