How to replace a letter at a specific index in a string in Go?

I want to replace a letter at a specific index in a string: aaaaaaa -> aaabaaa. Is there a built in way to do this? I wrote the following helper function to use in the mean time:

func main() {
    input := "aaaaaaa"
    output := replaceAtIndex(input, 'b', 3)
}

func replaceAtIndex(input string, replacement byte, index int) string {
    return strings.Join([]string{input[:index], string(replacement), input[index+1:]}, "")
}

Strings are immutable in Go, you have to convert it to runes then modify it then convert it back to a string.

@chendesheng's solution is semi-correct, except you can use rune instead of byte, that way it will work on unicode as well.

func replaceAtIndex(in string, r rune, i int) string {
	out := []rune(in)
	out[i] = r
	return string(out)
}

<kbd>playground</kbd>

Both of the answers (OneOfOne and Denys Séguret) are correct. I just wanted to show the performance difference between them (which is really noticeable when the string is big).

It turns out that using str[:index] + string(replacement) + str[index+1:] is way faster.

So the benchmark:

package main
import (
"testing"
)
func replaceAtIndex1(str string, replacement rune, index int) string {
out := []rune(str)
out[index] = replacement
return string(out)
}
func replaceAtIndex2(str string, replacement rune, index int) string {
return str[:index] + string(replacement) + str[index+1:]
}
func generateString(n int) string{
s := ""
for i := 0; i < n; i++{
s += "a"
}
return s
}
func BenchmarkSmall1(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkSmall2(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkMedium1(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkMedium2(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkBig1(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkBig2(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func main(){}

shows the following results (thanks Thomasz for spotting a copypasting error):

BenchmarkSmall1-4 	10000000	       228 ns/op
BenchmarkSmall2-4 	10000000	       126 ns/op
BenchmarkMedium1-4	  500000	      2091 ns/op
BenchmarkMedium2-4	10000000	       190 ns/op
BenchmarkBig1-4   	   10000	    209232 ns/op
BenchmarkBig2-4   	  500000	      3629 ns/op

You can concatenate strings with the + operator :

return input[:index] + string(replacement) + input[index+1:]

Be careful that index isn't the index of a "letter" but of the byte.

Just for fun:

package main                                                                                                                                                                                                
import (                                                                                                                                                                                                    
"fmt"                                                                                                                                                                                                   
"reflect"                                                                                                                                                                                               
"syscall"                                                                                                                                                                                               
"unsafe"                                                                                                                                                                                                
)                                                                                                                                                                                                           
// We should do this because by default strings in Go are read-only.                                                                                                                                                                                                                                                                                                                                                                                                                  
func mprotect(ptr uintptr, w bool) {                                                                                                                                                                        
// Need to avoid "EINVAL addr is not a valid pointer,
// or not a multiple of PAGESIZE."                                                                                                                   
start := ptr & ^(uintptr(syscall.Getpagesize() - 1))                                                                                                                                                    
prot := syscall.PROT_READ                                                                                                                                                                               
if w {                                                                                                                                                                                                  
prot |= syscall.PROT_WRITE                                                                                                                                                                          
}                                                                                                                                                                                                       
_, _, err := syscall.Syscall(                                                                                                                                                                           
syscall.SYS_MPROTECT,                                                                                                                                                                               
start, uintptr(syscall.Getpagesize()),                                                                                                                                                              
uintptr(prot),                                                                                                                                                                                      
)                                                                                                                                                                                                       
if err != 0 {                                                                                                                                                                                           
panic(err.Error())                                                                                                                                                                                  
}                                                                                                                                                                                                       
}                                                                                                                                                                                                           
// This function is very, very very very unsafe.                                                                                                                                                            
// Nowhere and never use it!                                                                                                                                                                                
func replaceAtIndex(s string, b byte, i int) {                                                                                                                                                              
h := *(*reflect.StringHeader)(unsafe.Pointer(&s))                                                                                                                                                       
mprotect(h.Data, true)                                                                                                                                                                                  
defer mprotect(h.Data, false)                                                                                                                                                                           
*(*byte)(unsafe.Pointer(h.Data + uintptr(i))) = b                                                                                                                                                       
}                                                                                                                                                                                                           
func main() {                                                                                                                                                                                               
h := "Hello, playground"                                                                                                                                                                                
replaceAtIndex(h, 'x', 0)                                                                                                                                                                               
fmt.Println(h)                                                                                                                                                                                          
}

Never try to use it somewhere in your code.
It's slower than any standard solutions or examples above and much more unsafe. =)

(It is not working in playground, because syscall is not defined there).

I took Salvador's benchmark and added strings.Join() to it. His answer remains correct - str[:index] + string(replacement) + str[index+1:] is the fastest option for large strings assuming you want your original string preserved. strings.Join() is pretty close for small strings and very very close for large strings. I also added tests with even larger strings to see if strings.Join() becomes faster at any point - it doesn't seem to.

I also, just for funsies, hacked together two other implementations with unsafe and reflect

• One makes a copy, so it doesn't modify the original string, and has very interesting performance results - faster for small strings, much faster for medium strings, much much slower for large strings
• The other just does a mutable string and is, unsurprisingly, drastically faster for all purposes and runs in constant time, with the caveat that your original string is modified.

I also had to modify generateString() so it would the largest strings would generate in a reasonable timeframe

Anyway, the code:

<!-- language: go -->

package main
import (
"reflect"
"strings"
"testing"
"unsafe"
)
func replaceAtIndex1(str string, replacement rune, index int) string {
out := []rune(str)
out[index] = replacement
return string(out)
}
func replaceAtIndex2(str string, replacement rune, index int) string {
return str[:index] + string(replacement) + str[index+1:]
}
func replaceAtIndex3(str string, replacement rune, index int) string {
return strings.Join([]string{str[:index], str[index + 1:]}, string(replacement))
}
func strToBytes(str string) []byte {
string_header := (*reflect.StringHeader)(unsafe.Pointer(&str))
bytes_header := &reflect.SliceHeader{
Data : string_header.Data,
Len : string_header.Len,
Cap : string_header.Len,
}
return *(*[]byte)(unsafe.Pointer(bytes_header))
}
func strToBytesCopy(str string) []byte {
bytes_unsafe := strToBytes(str)
bytes := make([]byte, len(bytes_unsafe))
copy(bytes, bytes_unsafe)
return bytes
}
func bytesToStr(bytes []byte) string {
bytes_header := (*reflect.SliceHeader)(unsafe.Pointer(&bytes))
string_header := &reflect.StringHeader{
Data : bytes_header.Data,
Len : bytes_header.Len,
}
return *(*string)(unsafe.Pointer(string_header))
}
func replaceAtIndex4(str string, replacement rune, index int) string {
bytes := strToBytesCopy(str)
bytes[index] = byte(replacement)
return bytesToStr(bytes)
}
func replaceAtIndex5(str string, replacement rune, index int) string {
bytes := strToBytes(str)
bytes[index] = byte(replacement)
return bytesToStr(bytes)
}
func generateString(n int) string{
var b strings.Builder
b.Grow(n)
for i := 0; i < n; i++ {
b.WriteRune('a')
}
return b.String()
}
func BenchmarkSmall1(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkSmall2(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkSmall3(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex3(str, replacement, index)
}
}
func BenchmarkSmall4(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex4(str, replacement, index)
}
}
func BenchmarkSmall5(b *testing.B) {
n := 10
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex5(str, replacement, index)
}
}
func BenchmarkMedium1(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkMedium2(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkMedium3(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex3(str, replacement, index)
}
}
func BenchmarkMedium4(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex4(str, replacement, index)
}
}
func BenchmarkMedium5(b *testing.B) {
n := 100
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex5(str, replacement, index)
}
}
func BenchmarkBig1(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex1(str, replacement, index)
}
}
func BenchmarkBig2(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkBig3(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex3(str, replacement, index)
}
}
func BenchmarkBig4(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex4(str, replacement, index)
}
}
func BenchmarkBig5(b *testing.B) {
n := 10000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex5(str, replacement, index)
}
}
func BenchmarkHuge2(b *testing.B) {
n := 100000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkHuge3(b *testing.B) {
n := 100000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex3(str, replacement, index)
}
}
func BenchmarkHuge4(b *testing.B) {
n := 100000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex4(str, replacement, index)
}
}
func BenchmarkHuge5(b *testing.B) {
n := 100000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex5(str, replacement, index)
}
}
func BenchmarkGargantuan2(b *testing.B) {
n := 10000000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex2(str, replacement, index)
}
}
func BenchmarkGargantuan3(b *testing.B) {
n := 10000000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex3(str, replacement, index)
}
}
func BenchmarkGargantuan4(b *testing.B) {
n := 10000000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex4(str, replacement, index)
}
}
func BenchmarkGargantuan5(b *testing.B) {
n := 10000000
str, index, replacement := generateString(n), n / 2, 'B'
b.ResetTimer()
for i := 0; i < b.N; i++ {
replaceAtIndex5(str, replacement, index)
}
}
func main(){}

And the results:

BenchmarkSmall1-8        	20000000	        99.9 ns/op
BenchmarkSmall2-8        	50000000	        29.5 ns/op
BenchmarkSmall3-8        	20000000	        58.1 ns/op
BenchmarkSmall4-8        	50000000	        32.0 ns/op
BenchmarkSmall5-8        	1000000000	         2.93 ns/op
BenchmarkMedium1-8       	 1000000	      1034 ns/op
BenchmarkMedium2-8       	20000000	        68.4 ns/op
BenchmarkMedium3-8       	20000000	        78.8 ns/op
BenchmarkMedium4-8       	30000000	        49.3 ns/op
BenchmarkMedium5-8       	1000000000	         3.02 ns/op
BenchmarkBig1-8          	   20000	     89557 ns/op
BenchmarkBig2-8          	 1000000	      1204 ns/op
BenchmarkBig3-8          	 1000000	      1257 ns/op
BenchmarkBig4-8          	 1000000	      1200 ns/op
BenchmarkBig5-8          	1000000000	         2.93 ns/op
BenchmarkHuge2-8         	  200000	     10260 ns/op
BenchmarkHuge3-8         	  200000	      9908 ns/op
BenchmarkHuge4-8         	  100000	     13628 ns/op
BenchmarkHuge5-8         	1000000000	         2.99 ns/op
BenchmarkGargantuan2-8   	    2000	    822881 ns/op
BenchmarkGargantuan3-8   	    2000	    807522 ns/op
BenchmarkGargantuan4-8   	    1000	   2148387 ns/op
BenchmarkGargantuan5-8   	1000000000	         2.96 ns/op