Strings in Go

For the standard Go compiler, the internal structure of any string type is declared like:

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.

We have learned the following facts about strings from previous articles.

• String values can be used as constants (along with boolean and all kinds of numeric values).
• Go supports two styles of string literals, the double-quote style (or interpreted literals) and the back-quote style (or raw string literals).
• The zero values of string types are blank strings, which can be represented with "" or `` in literal.
• Strings can be concatenated with + and += operators.
• String types are all comparable (by using the == 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.

Example:

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.

• Like Java, the contents (underlying bytes) of string values are immutable. The lengths of string values also can't be modified separately. An addressable string value can only be overwritten as a whole by assigning another string value to it.
• The built-in string type has no methods (just like most other built-in types in Go), but we can
  • use functions provided in the strings standard package to do all kinds of string manipulations.
  • call the built-in len function to get the length of a string (number of bytes stored in the string).
  • use the element access syntax 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.
  • use the subslice syntax aString[start:end] to get a substring of aString. Here, start and end are both indexes of bytes stored in aString.
• For the standard Go compiler, the destination string variable and source string value in a string assignment will share the same underlying byte sequence in memory. The result of a substring expression aString[start:end] also shares the same underlying byte sequence with the base string aString in memory.

Example:

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。

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:

1. a string value can be explicitly converted to a byte slice, and vice versa. A byte slice is a slice with element type's underlying type as []byte.
2. a string value can be explicitly converted to a rune slice, and vice versa. A rune slice is a slice whose element type's underlying type as []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,

• illegal UTF-8 encoded bytes are allowed and will keep unchanged.
• the standard Go compiler makes some optimizations for some special cases of such conversions, so that the deep copies are not made. Such cases will be introduced below.

Conversions between byte slices and rune slices are not supported directly in Go, We can use the following ways to achieve this goal:

• use string values as a hop. This way is convenient but not very efficient, for two deep copies are needed in the process.
• use the functions in unicode/utf8 standard package.
• use the 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.

Example:

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
}

Above has mentioned that the underlying bytes in the conversions between strings and byte slices will be copied. The standard Go compiler makes some optimizations, which are proven to still work in Go Toolchain 1.22.n, for some special scenarios to avoid the duplicate copies. These scenarios include:

• a conversion (from string to byte slice) which follows the range keyword in a for-range loop.
• a conversion (from byte slice to string) which is used as a map key in map element retrieval indexing syntax.
• a conversion (from byte slice to string) which is used in a comparison.
• a conversion (from byte slice to string) which is used in a string concatenation, and at least one of concatenated string values is a non-blank string constant.

Example:

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.

Another example:

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
}

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.

Example:

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))
}

The output of the above program:

 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

From the output result, we can find that

1. the iteration index value may be not continuous. The reason is the index is the byte index in the ranged string and one code point may need more than one byte to represent.
2. the first character, é, is composed of two runes (3 bytes total)
3. the second character, क्षि, is composed of four runes (12 bytes total).
4. the English character, a, is composed of one rune (1 byte).
5. the character, π, is composed of one rune (2 bytes).
6. the Chinese character, 囧, is composed of one rune (3 bytes).

Then how to iterate bytes in a string? Do this:

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).

Besides using the + operator to concatenate strings, we can also use following ways to concatenate strings.

• The Sprintf/Sprint/Sprintln functions in the fmt standard package can be used to concatenate values of any types, including string types.
• Use the Join function in the strings standard package.
• The 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.
• Since Go 1.10, the 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 all of the concatenated strings may present in a concatenation statement.

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.

Example:

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))
}

Above has mentioned that comparing two strings is comparing their underlying bytes actually. Generally, Go compilers will make the following optimizations for string comparisons.

• For == 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).
• If their underlying byte sequence pointers of the compared two strings are equal, then the comparison result is the same as comparing the lengths of the two strings.

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)
}

Output:

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.

• About Go 101 - why this book is written.
• Acknowledgments

• An Introduction of Go - why Go is worth learning.
• The Go Toolchain - how to compile and run Go programs.

• Become Familiar With Go Code
  • Introduction of Source Code Elements
  • Keywords and Identifiers
  • Basic Types and Their Value Literals
  • Constants and Variables - also introduces untyped values and type deductions.
  • Common Operators - also introduces more type deduction rules.
  • Function Declarations and Calls
  • Code Packages and Package Imports
  • Expressions, Statements and Simple Statements
  • Basic Control Flows
  • Goroutines, Deferred Function Calls and Panic/Recover

• Go Type System
  • Go Type System Overview - a must read to master Go programming.
  • Pointers
  • Structs
  • Value Parts - to gain a deeper understanding into Go values.
  • Arrays, Slices and Maps - first-class citizen container types.
  • Strings
  • Functions - function types and values, including variadic functions.
  • Channels - the Go way to do concurrency synchronizations.
  • Methods
  • Interfaces - value boxes used to do reflection and polymorphism.
  • Type Embedding - type extension in the Go way.
  • Type-Unsafe Pointers
  • Generics - use and read composite types
  • Reflections - the reflect standard package.

• Some Special Topics
  • Line Break Rules
  • More About Deferred Function Calls
  • Some Panic/Recover Use Cases
  • Explain Panic/Recover Mechanism in Detail - also explains exiting phases of function calls.
  • Code Blocks and Identifier Scopes
  • Expression Evaluation Orders
  • Value Copy Costs in Go
  • Bounds Check Elimination

• Concurrent Programming
  • Concurrency Synchronization Overview
  • Channel Use Cases
  • How to Gracefully Close Channels
  • Other Concurrency Synchronization Techniques - the sync standard package.
  • Atomic Operations - the sync/atomic standard package.
  • Memory Order Guarantees in Go
  • Common Concurrent Programming Mistakes

• Memory Related
  • Memory Blocks
  • Memory Layouts
  • Memory Leaking Scenarios

• Some Summaries
  • Some Simple Summaries
  • nil in Go
  • Value Conversion, Assignment and Comparison Rules
  • Syntax/Semantics Exceptions
  • Go FAQ 101

• More Go Related Topics