How do you get the index of the first occurrence of character O in string hello how are you?

Get the Index of a Character in a String #

To get the index of a character in a string, you use the indexOf() method, passing it the specific character as a parameter. This method returns the index of the first occurrence of the character or -1 if the character is not found.

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const str = 'hello world';

const index = str.indexOf('l');
console.log(index); // 👉️ 2

In the example, we use the String.indexOf method to get the index of the first occurrence of the l character in the string hello world.

Indexes are zero-based in JavaScript, which means that the first character has an index of 0 and the last an index of str.length - 1.

If the indexOf method does not find the specified character in the string, it returns -1.

Get the Index of a Character's Last Occurrence in a String #

To get the index of a character's last occurrence in a string, you use the lastIndexOf() method, passing it the specific character as a parameter. This method returns the index of the last occurrence of the character or -1 if the character is not found.

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const str = 'hello world';

const lastIndex = str.lastIndexOf('l');
console.log(lastIndex); // 👉️  9

In the code snippet, we use the String.lastIndexOf method to get the index of the last occurrence of the l character in the string hello world.

The last l is the 10th character in the string, therefore it's at index 9.

The lastIndexOf method returns -1 if the character is not found in the string.

Get the Index of all Occurrences of a Character in a String #

To get the index of all of a character's occurrences in a string:

1. Create an array that will store the indexes.
2. In a for loop, loop for string.length iterations, checking if the character at the current index is equal to the specific character.
3. If the equality check passes, push the character's index to the array.

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const str = 'hello world';

const indexes = [];

for (let i = 0; i < str.length; i++) {
  if (str[i] === 'l') {
    indexes.push(i);
  }
}

console.log(indexes); // 👉️ [2, 3, 9]

We used a for loop because it allows us to keep track of the current index in a simple way.

For each iteration, we check if the character at the current index is equal to the character we're trying to get the indexes of.

If the condition is met, we push the current index to the indexes array.

Further Reading #

• Remove all Spaces from a String in JavaScript
• Replace all Numbers in a String using JavaScript
• Remove all Numbers from a String in JavaScript
• Remove Empty Strings from an Array in JavaScript
• Remove Empty String Values from an Object in JavaScript
• How to compare Strings in JavaScript
• How to get the Length of a String in JavaScript
• Check if Array contains Empty String in JavaScript
• Interpolate Variable in a String in JavaScript
• Check if letter in String is Uppercase or Lowercase in JS
• Check if String contains only Letters and Spaces in JS
• Check if String contains only Letters and Numbers in JS
• Remove all non-alphanumeric Characters from String in JS

There are four variants of indexOf() method. This article depicts about all of them, as follows: 
1.int indexOf() : This method returns the index within this string of the first occurrence of the specified character or -1, if the character does not occur.
 

Syntax:
int indexOf(char ch )
Parameters:
ch : a character.

Java

public class Index1 {

public static void main(String args[])

    {

        String gfg = new String("Welcome to geeksforgeeks");

        System.out.print("Found g first at position : ");

        System.out.println(gfg.indexOf('g'));

    }

}

Output

Found g first at position : 11

2. int indexOf(char ch, int strt ) : This method returns the index within this string of the first occurrence of the specified character, starting the search at the specified index or -1, if the character does not occur.
 

Syntax:
int indexOf(char ch, int strt)
Parameters:
ch :a character.
strt : the index to start the search from.

Java

public class Index2 {

public static void main(String args[])

    {

        String gfg = new String("Welcome to geeksforgeeks");

        System.out.print("Found g after 13th index at position : ");

        System.out.println(gfg.indexOf('g', 13));

    }

}

Output

Found g after 13th index at position : 19

3.int indexOf(String str) : This method returns the index within this string of the first occurrence of the specified substring. If it does not occur as a substring, -1 is returned.
 

Syntax:
int indexOf(String str)
Parameters:
str : a string.

Java

public class Index3 {

public static void main(String args[])

    {

        String Str = new String("Welcome to geeksforgeeks");

        String subst = new String("geeks");

        System.out.print("Found geeks starting at position : ");

        System.out.print(Str.indexOf(subst));

    }

}

Output

Found geeks starting at position : 11

4. int indexOf(String str, int strt) : This method returns the index within this string of the first occurrence of the specified substring, starting at the specified index. If it does not occur, -1 is returned. 
 

Syntax:
int indexOf(String str, int strt)
Parameters:
strt: the index to start the search from.
str : a string.
 

Java

public class Index4 {

public static void main(String args[])

    {

        String Str = new String("Welcome to geeksforgeeks");

        String subst = new String("geeks");

        System.out.print("Found geeks(after 14th index) starting at position : ");

        System.out.print(Str.indexOf(subst, 14));

    }

}

Output

Found geeks(after 14th index) starting at position : 19

Some related applications:
 

• Finding out if a given character (maybe anything upper or lower case) is a vowel or consonant. 
  Implementation is given below: 
   

JAVA

class Vowels

{

    public static boolean vowel(char c)

    {

        return "aeiouAEIOU".indexOf(c)>=0;

    }

    public static void main(String[] args)

    {

        boolean isVowel = vowel('a');

                if(isVowel)

            System.out.println("Vowel");

        else

            System.out.println("Consonant");

    }

}

This article is contributed by Astha Tyagi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to . See your article appearing on the GeeksforGeeks main page and help other Geeks. 
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

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