Like many other programming languages, string is also one important kind of types in Go. This article will list all the facts of strings.
type _string struct {
elements *byte // underlying bytes
len int // number of bytes
}
From the declaration, we know that a string is actually a byte sequence wrapper.
In fact, we can really view a string as an (element-immutable) byte slice.
Note, in Go, byte is a built-in alias of type uint8.
"" or `` in literal.
+ and += operators.
== and != operators).
And like integer and floating-point values, two values of the same string type can also be compared with
>, <, >= and <= operators.
When comparing two strings, their underlying bytes will be compared, one byte by one byte.
If one string is a prefix of the other one and the other one is longer,
then the other one will be viewed as the larger one.
package main
import "fmt"
func main() {
const World = "world"
var hello = "hello"
// Concatenate strings.
var helloWorld = hello + " " + World
helloWorld += "!"
fmt.Println(helloWorld) // hello world!
// Compare strings.
fmt.Println(hello == "hello") // true
fmt.Println(hello > helloWorld) // false
}
More facts about string types and values in Go.
string type has no methods (just like most other built-in types in Go), but we can
strings standard package
to do all kinds of string manipulations.
len function to get the length of a string (number of bytes stored in the string).
aString[i] introduced in
container element accesses
to get the ith byte value stored in aString.
The expression aString[i] is not addressable.
In other words, value aString[i] can't be modified.
aString[start:end]
to get a substring of aString.
Here, start and end are both indexes of bytes stored in aString.
aString[start:end]
also shares the same underlying byte sequence with the base string
aString in memory.
package main
import (
"fmt"
"strings"
)
func main() {
var helloWorld = "hello world!"
var hello = helloWorld[:5] // substring
// 104 is the ASCII code (and Unicode) of char 'h'.
fmt.Println(hello[0]) // 104
fmt.Printf("%T \n", hello[0]) // uint8
// hello[0] is unaddressable and immutable,
// so the following two lines fail to compile.
/*
hello[0] = 'H' // error
fmt.Println(&hello[0]) // error
*/
// The next statement prints: 5 12 true
fmt.Println(len(hello), len(helloWorld),
strings.HasPrefix(helloWorld, hello))
}
Note, if
aString and the indexes in expressions aString[i] and aString[start:end] are all constants,
then out-of-range constant indexes will make compilations fail.
And please note that the evaluation results of such expressions are always non-constants
(this might or might not change since a later Go version).
For example, the following program will print 4 0.
package main
import "fmt"
const s = "Go101.org" // len(s) == 9
// len(s) is a constant expression,
// whereas len(s[:]) is not.
var a byte = 1 << len(s) / 128
var b byte = 1 << len(s[:]) / 128
func main() {
fmt.Println(a, b) // 4 0
}
About why variables a and b are evaluated to different values,
please read the special type deduction rule in bitwise shift operator operation and which function calls are evaluated at compile time。
Unicode standard specifies a unique value for each character in all kinds of human languages. But the basic unit in Unicode is not character, it is code point instead. For most code points, each of them corresponds to a character, but for a few characters, each of them consists of several code points.
Code points are represented as rune values in Go.
In Go, rune is a built-in alias of type int32.
In applications, there are several encoding methods to represent code points, such as UTF-8 encoding and UTF-16 encoding. Nowadays, the most popularly used encoding method is UTF-8 encoding. In Go, all string constants are viewed as UTF-8 encoded. At compile time, illegal UTF-8 encoded string constants will make compilation fail. However, at run time, Go runtime can't prevent some strings from being illegally UTF-8 encoded.
For UTF-8 encoding, each code point value may be stored as one or more bytes (up to four bytes). For example, each English code point (which corresponds to one English character) is stored as one byte, however each Chinese code point (which corresponds to one Chinese character) is stored as three bytes.
In the article constants and variables, we have learned that integers can be explicitly converted to strings (but not vice versa).
Here introduces two more string related conversions rules in Go:[]byte.
[]rune.
In a conversion from a rune slice to string,
each slice element (a rune value) will be UTF-8 encoded as from one to four bytes
and stored in the result string.
If a slice rune element value is outside the range of valid Unicode code points,
then it will be viewed as 0xFFFD, the code point for the Unicode replacement character.
0xFFFD will be UTF-8 encoded as three bytes (0xef 0xbf 0xbd).
When a string is converted to a rune slice, the bytes stored in the string
will be viewed as successive UTF-8 encoding byte sequence representations
of many Unicode code points. Bad UTF-8 encoding representations will be
converted to a rune value 0xFFFD.
When a string is converted to a byte slice, the result byte slice is just a deep copy of the underlying byte sequence of the string. When a byte slice is converted to a string, the underlying byte sequence of the result string is also just a deep copy of the byte slice. A memory allocation is needed to store the deep copy in each of such conversions. The reason why a deep copy is essential is slice elements are mutable but the bytes stored in strings are immutable, so a byte slice and a string can't share byte elements.
Please note, for conversions between strings and byte slices,Runes function in the bytes standard package
to convert a []byte value to a []rune value.
There is not a function in this package to convert a rune slice to byte slice.
package main
import (
"bytes"
"unicode/utf8"
)
func Runes2Bytes(rs []rune) []byte {
n := 0
for _, r := range rs {
n += utf8.RuneLen(r)
}
n, bs := 0, make([]byte, n)
for _, r := range rs {
n += utf8.EncodeRune(bs[n:], r)
}
return bs
}
func main() {
s := "Color Infection is a fun game."
bs := []byte(s) // string -> []byte
s = string(bs) // []byte -> string
rs := []rune(s) // string -> []rune
s = string(rs) // []rune -> string
rs = bytes.Runes(bs) // []byte -> []rune
bs = Runes2Bytes(rs) // []rune -> []byte
}
range keyword in a for-range loop.
package main
import "fmt"
func main() {
var str = "world"
// Here, the []byte(str) conversion will
// not copy the underlying bytes of str.
for i, b := range []byte(str) {
fmt.Println(i, ":", b)
}
key := []byte{'k', 'e', 'y'}
m := map[string]string{}
// The string(key) conversion copys the bytes in key.
m[string(key)] = "value"
// Here, this string(key) conversion doesn't copy
// the bytes in key. The optimization will be still
// made, even if key is a package-level variable.
fmt.Println(m[string(key)]) // value (very possible)
}
Note, the last line might not output value if
there are data races in evaluating string(key).
However, such data races will never cause panics.
package main
import "fmt"
import "testing"
var s string
var x = []byte{1023: 'x'}
var y = []byte{1023: 'y'}
func fc() {
// None of the below 4 conversions will
// copy the underlying bytes of x and y.
// Surely, the underlying bytes of x and y will
// be still copied in the string concatenation.
if string(x) != string(y) {
s = (" " + string(x) + string(y))[1:]
}
}
func fd() {
// Only the two conversions in the comparison
// will not copy the underlying bytes of x and y.
if string(x) != string(y) {
// Please note the difference between the
// following concatenation and the one in fc.
s = string(x) + string(y)
}
}
func main() {
fmt.Println(testing.AllocsPerRun(1, fc)) // 1
fmt.Println(testing.AllocsPerRun(1, fd)) // 3
}
for-range on Strings
The for-range loop control flow applies to strings.
But please note, for-range will iterate the Unicode code points
(as rune values), instead of bytes, in a string.
Bad UTF-8 encoding representations in the string will be
interpreted as rune value 0xFFFD.
package main
import "fmt"
func main() {
s := "éक्षिaπ囧"
for i, rn := range s {
fmt.Printf("%2v: 0x%x %v \n", i, rn, string(rn))
}
fmt.Println(len(s))
}
0: 0x65 e
1: 0x301 ́
3: 0x915 क
6: 0x94d ्
9: 0x937 ष
12: 0x93f ि
15: 0x61 a
16: 0x3c0 π
18: 0x56e7 囧
21
é, is composed of two runes (3 bytes total)क्षि, is composed of four runes (12 bytes total).a, is composed of one rune (1 byte).π, is composed of one rune (2 bytes).囧, is composed of one rune (3 bytes).package main
import "fmt"
func main() {
s := "éक्षिaπ囧"
for i := 0; i < len(s); i++ {
fmt.Printf("The byte at index %v: 0x%x \n", i, s[i])
}
}
Surely, we can also make use of the compiler optimization mentioned above to iterate bytes in a string. For the standard Go compiler, this way is a little more efficient than the above one.
package main
import "fmt"
func main() {
s := "éक्षिaπ囧"
// Here, the underlying bytes of s are not copied.
for i, b := range []byte(s) {
fmt.Printf("The byte at index %v: 0x%x \n", i, b)
}
}
From the above several examples, we know that
len(s) will return the number of bytes in string s.
The time complexity of len(s) is O(1).
How to get the number of runes in a string?
Using a for-range loop to iterate and count all runes is a way, and using the
RuneCountInString
function in the unicode/utf8 standard package is another way.
The efficiencies of the two ways are almost the same.
The third way is to use len([]rune(s)) to get the count of runes in string s.
Since Go Toolchain 1.11, the standard Go compiler makes an optimization for the third way
to avoid an unnecessary deep copy so that it is as efficient as the former two ways.
Please note that the time complexities of these ways are all O(n).
+ operator to concatenate strings, we can also use following ways to concatenate strings.
Sprintf/Sprint/Sprintln functions
in the fmt standard package
can be used to concatenate values of any types, including string types.
Join function in the strings standard package.
Buffer type in the bytes standard package
(or the built-in copy function) can be used to build byte slices,
which afterwards can be converted to string values.
Builder type in the strings standard package
can be used to build strings. Comparing with bytes.Buffer way,
this way avoids making an unnecessary duplicated copy of underlying bytes for the result string.
The standard Go compiler makes optimizations for string concatenations by using the + operator.
So generally, using + operator to concatenate strings is convenient and efficient
if the number of the concatenated strings is known at compile time.
From the article arrays, slices and maps,
we have learned that we can use the built-in copy and
append functions to copy and append slice elements.
In fact, as a special case,
if the first argument of a call to either of the two
functions is a byte slice, then the second argument can be a string
(if the call is an append call, then the string argument
must be followed by three dots ...).
In other words, a string can be used as a byte slice for the special case.
package main
import "fmt"
func main() {
hello := []byte("Hello ")
world := "world!"
// The normal way:
// helloWorld := append(hello, []byte(world)...)
helloWorld := append(hello, world...) // sugar way
fmt.Println(string(helloWorld))
helloWorld2 := make([]byte, len(hello) + len(world))
copy(helloWorld2, hello)
// The normal way:
// copy(helloWorld2[len(hello):], []byte(world))
copy(helloWorld2[len(hello):], world) // sugar way
fmt.Println(string(helloWorld2))
}
== and != comparisons,
if the lengths of the compared two strings are not equal,
then the two strings must be also not equal (no needs to compare their bytes).
So for two equal strings, the time complexity of comparing them
depends on whether or not their underlying byte sequence pointers are equal.
If the two are equal, then the time complexity is O(1),
otherwise, the time complexity is O(n),
where n is the length of the two strings.
As above mentioned, for the standard Go compiler, in a string value assignment, the destination string value and the source string value will share the same underlying byte sequence in memory. So the cost of comparing the two strings becomes very small.
Example:package main
import (
"fmt"
"time"
)
func main() {
bs := make([]byte, 1<<26)
s0 := string(bs)
s1 := string(bs)
s2 := s1
// s0, s1 and s2 are three equal strings.
// The underlying bytes of s0 is a copy of bs.
// The underlying bytes of s1 is also a copy of bs.
// The underlying bytes of s0 and s1 are two
// different copies of bs.
// s2 shares the same underlying bytes with s1.
startTime := time.Now()
_ = s0 == s1
duration := time.Now().Sub(startTime)
fmt.Println("duration for (s0 == s1):", duration)
startTime = time.Now()
_ = s1 == s2
duration = time.Now().Sub(startTime)
fmt.Println("duration for (s1 == s2):", duration)
}
duration for (s0 == s1): 10.462075ms
duration for (s1 == s2): 136ns
1ms is 1000000ns! So please try to avoid comparing two long strings if they don't share the same underlying byte sequence.
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