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

In Go language slice is more powerful, flexible, convenient than an array, and is a lightweight data structure. The slice is a variable-length sequence which stores elements of a similar type, you are not allowed to store different type of elements in the same slice.
In the Go slice of bytes, you can find the first index value of the specified instance in the given slice using Index() function. This function returns the index of the first instance of the given value in the original slice of bytes. It returns -1 if the given value is not available in the original slice. It is defined under the bytes package so, you have to import bytes package in your program for accessing Index function.

Syntax:

func Index(ori_slice, sep_ []byte) int

Here, ori_slice is the original slice and sep_ is a slice, whose we want to find the first index value. Let us discuss this concept with the help of the given examples:

Example 1:

// Go program to illustrate the concept
// of the index in the slice of bytes
package main

import (
    "bytes"
    "fmt"
)

func main() {

    // Creating and finding the index
    // of the slice of bytes
    // Using Index function
    res1 := bytes.Index([]byte("****Welcome to GeeksforGeeks****"),
                                                    []byte("eek"))

    res2 := bytes.Index([]byte("Learning how to trim a slice of bytes"),
                                                          []byte("xzx"))

    res3 := bytes.Index([]byte("GeeksforGeeks, Geek"), []byte("eeks"))

    // Display the results
    fmt.Printf("\n\nFinal Value:\n")
    fmt.Printf("\nSlice 1: %d", res1)
    fmt.Printf("\nSlice 2: %d", res2)
    fmt.Printf("\nSlice 3: %d", res3)
}

Output:

Final Value:

Slice 1: 16
Slice 2: -1
Slice 3: 1

Example 2:

// Go program to illustrate the concept of
// the index in the slice of bytes
package main

import (
    "bytes"
    "fmt"
)

func main() {

    // Creating and initializing
    // the slice of bytes
    // Using shorthand declaration
    slice_1 := []byte{'!', '!', 'G', 'e', 'e', 'k', 's',
      'f', 'o', 'r', 'G', 'e', 'e', 'k', 's', '#', '#'}

    slice_2 := []byte{'A', 'p', 'p', 'l', 'e'}

    slice_3 := []byte{'%', 'g', 'e', 'e', 'k', 's', '%'}

    // Displaying slices
    fmt.Println("Original Slice:")
    fmt.Printf("Slice 1: %s", slice_1)
    fmt.Printf("\nSlice 2: %s", slice_2)
    fmt.Printf("\nSlice 3: %s", slice_3)

    // Finding the index of
    // the slice of bytes
    // Using Index function
    res1 := bytes.Index(slice_1, []byte("eek"))
    res2 := bytes.Index(slice_2, []byte("ple"))
    res3 := bytes.Index(slice_3, []byte("xox"))

    // Display the results
    fmt.Printf("\n\nFirst Index:\n")
    fmt.Printf("\nSlice 1: %d", res1)
    fmt.Printf("\nSlice 2: %d", res2)
    fmt.Printf("\nSlice 3: %d", res3)

}

Output:

Original Slice:
Slice 1: !!GeeksforGeeks##
Slice 2: Apple
Slice 3: %geeks%

First Index:

Slice 1: 3
Slice 2: 2
Slice 3: -1