How to find the last index value in slice of bytes in Golang?

In Go, a slice is a flexible data structure that stores a variable-length sequence of elements of the same type. The LastIndex() function from the bytes package returns the last index of a specified value in a byte slice, or -1 if the value is not found.

Example

package main
import (
	"bytes"
	"fmt"
)
func main() {
	// Define the slice of bytes
	slice := []byte("GeeksforGeeks, Learning Go")
	
	// Define the byte to search for
	searchByte := []byte("e")

	// Find the last index of the specified byte
	lastIndex := bytes.LastIndex(slice, searchByte)

	// Display the result
	fmt.Printf("\n", searchByte, lastIndex)
}

Syntax

func LastIndex(ori_slice, sep_ []byte) int                  # Using the bytes.LastIndex Function

Using the bytes.LastIndex Function

The simplest way to find the last index of a specified byte slice is to use the LastIndex function from the bytes package.

package main
import (
    "bytes"
    "fmt"
)

func main() {
    originalSlice := []byte("Welcome to GeeksforGeeks")
    searchSlice := []byte("ks")

    lastIndex := bytes.LastIndex(originalSlice, searchSlice)
    fmt.Printf("%s: %d\n", searchSlice, lastIndex)
}

ks: 22

Other method's:

Apart from the simple method mentioned above,there are few more methods which we can use to find the last index value in slice of bytes in Golang.

Manual Iteration through the Slice

You can manually iterate through the slice in reverse order to find the last index of the specified byte value.

package main
import "fmt"

func lastIndexManual(slice []byte, value byte) int {
    for i := len(slice) - 1; i >= 0; i-- {
        if slice[i] == value {
            return i
        }
    }
    return -1
}

func main() {
    slice := []byte("GeeksforGeeks")
    value := byte('e')

    lastIndex := lastIndexManual(slice, value)
    fmt.Printf("%c: %d\n", value, lastIndex)
}

e: 10

Using a Loop with bytes.Equal

You can create a custom function that checks for equality between slices to find the last index.

Example

package main
import (
    "bytes"
    "fmt"
)

func lastIndexEqual(slice []byte, target []byte) int {
    for i := len(slice) - len(target); i >= 0; i-- {
        if bytes.Equal(slice[i:i+len(target)], target) {
            return i
        }
    }
    return -1
}

func main() {
    slice := []byte("Geeks for Geeks")
    target := []byte("for")

    lastIndex := lastIndexEqual(slice, target)
    fmt.Printf("%s: %d\n", target, lastIndex)
}

for: 6

Using strings.LastIndex for String Conversion

Convert the byte slice to a string and use the strings.LastIndex function to find the last index.

package main
import (
    "fmt"
    "strings"
)

func main() {
    originalSlice := []byte("Geeks for Geeks")
    searchString := "for"

    lastIndex := strings.LastIndex(string(originalSlice), searchString)
    fmt.Printf("'%s': %d\n", searchString, lastIndex)
}

'for': 6