How to Find Index of Substring in Go: Simple Guide

In Go, you can find the index of a substring using the strings.Index function from the strings package. It returns the position of the first occurrence of the substring or -1 if the substring is not found.

Syntax

The strings.Index function has this syntax:

• strings.Index(s, substr string) int

Here, s is the main string to search in, and substr is the substring you want to find. The function returns the starting index of substr in s, or -1 if substr is not present.

go

package main

import (
	"fmt"
	"strings"
)

func main() {
	index := strings.Index("hello world", "world")
	fmt.Println(index) // Output: 6
}

Output

6

Example

This example shows how to find the index of a substring in a string. It prints the index if found, or a message if not found.

go

package main

import (
	"fmt"
	"strings"
)

func main() {
	text := "Go programming language"
	substring := "program"

	pos := strings.Index(text, substring)

	if pos != -1 {
		fmt.Printf("Substring '%s' found at index %d\n", substring, pos)
	} else {
		fmt.Printf("Substring '%s' not found\n", substring)
	}
}

Output

Substring 'program' found at index 3

Common Pitfalls

One common mistake is not checking if the returned index is -1, which means the substring was not found. Using the index without this check can cause errors if you try to slice the string at that position.

Also, strings.Index is case-sensitive, so searching for "Go" and "go" will give different results.

go

package main

import (
	"fmt"
	"strings"
)

func main() {
	text := "Hello World"
	substring := "world"

	pos := strings.Index(text, substring)

	// Wrong: Using pos without checking
	// fmt.Println(text[pos:pos+len(substring)]) // This will panic if pos == -1

	// Right: Check if substring exists first
	if pos != -1 {
		fmt.Println(text[pos : pos+len(substring)])
	} else {
		fmt.Println("Substring not found")
	}
}

Output

Substring not found

Quick Reference

Function	Description	Return Value
strings.Index(s, substr)	Finds first index of substr in s	Index (int) or -1 if not found
strings.LastIndex(s, substr)	Finds last index of substr in s	Index (int) or -1 if not found
strings.Contains(s, substr)	Checks if substr is in s	Boolean (true/false)

Key Takeaways

Use strings.Index to find the first position of a substring in a string.

Always check if the returned index is -1 before using it.

strings.Index is case-sensitive; consider strings.ToLower for case-insensitive search.

If you need the last occurrence, use strings.LastIndex instead.

Use strings.Contains if you only need to check presence without the index.