How To Find The First Occurrence of a string

Welcome back, visitors. In this post, I will delve into the concepts of ‘what is string occurrence’ and ‘Find The First Occurrence Index.’ These are frequently posed by interviewers when seeking candidates for junior or internship positions.

‘Find the index of the first occurrence in a string’ or ‘How to find all occurrences of a substring’ are also topics covered on LeetCode and GeeksforGeeks websites. To master solving these problems, continue following the guidance on this page until the end, and you will gain expertise in find the occurrence of a substring in a string.

Find the character that appears most often across all possible substrings of a given string

If you’re a new visitor, this message is for you. If you’re gearing up for a programmer role interview and require assistance, consider joining our Quora community group. It’s free and provides a platform where you can pose questions and receive answers within 24 hours. Additionally, if you’re interested in tackling beginner and intermediate level programming challenges, be sure to follow our dedicated pages for that purpose.

What Is String Occurrence ?

When an interviewer poses this question, your response should clarify that the occurrence of a string refers to how many times a particular string repeats within another string. In simpler terms, the number of times a substring appears in a string is referred to as string occurrence.

Allow me to provide a practical example to illustrate the occurrence of a substring or string. Consider the string ‘interviewspreparation.com,’ which represents my website. In this case, ‘interview’ is the substring we are interested in locating within the entire string ‘interviewspreparation.com.’ The task involves finding how many times the term ‘interview’ appears within the string.

And our answer would be 1 time in 0 index of the string. understood find the first occurrence of substring?

Find Occurrence Of A Substring in A String

First, let’s grasp the logic behind finding the occurrence of a string. To achieve a solution, I’ll employ the pyramid pattern logic. This approach is effective for locating objects within a string or an array.

String is indeed a collection or array of characters, am I correct? Therefore, I will utilize a loop to retrieve all characters from a string and locate the first occurrence. Once I have each character, I will apply the row and column logic of a pyramid to search for the substring within the string.

Do you find this challenging? If so, to grasp the concept of pyramid logic, explore our ‘pyramid‘ tag and ‘star pattern program.’ In these resources, I extensively explain the concepts of rows and columns in pyramid programs.

Program To Find The First Occurrence In A String

Let’s implement the logic into a program to better understand needle and haystack programming aspect and find the index of the first occurrence in a string

Step 1

First, let’s create a method with two parameters named ‘needle’ and ‘haystack’, where the return type is ‘int’, and set the default return value to -1. Then, we’ll create a loop to iterate through each element of ‘haystack’.

static void Main(string[] args)
{
    Console.WriteLine(GetTheIndexOfTheFirstOccurrence("interviewspreparation.com", "interview"));
}

static int GetTheIndexOfTheFirstOccurrence(string haystack, string needle)
{

    for (int i = 0; i < haystack.Length; i++)
    {
        
    }

    return -1;
}

Step 2

In the second step, let’s create a string variable named ‘temp’. We will use this variable to collect characters from the string ‘haystack’. Then, we’ll set up a for loop. This loop will start at the current index of our ‘haystack’ string. The condition for this loop will be that the nested loop index is less than the length of our ‘haystack’ string and must also be less than the length of ‘needle’. See the code below for a clearer understanding:

static int GetTheIndexOfTheFirstOccurrence(string haystack, string needle)
{

    for (int i = 0; i < haystack.Length; i++)
    {
        string temp = "";

        for (int j = i; j < haystack.Length && j <= i + needle.Length - 1; j++)
        {
            temp += haystack[j];
        }

        Console.WriteLine(temp);

        if (temp == needle)
        {
            return i;
        }
    }

    return -1;
}

Following these two steps, you can find the first occurrence of a string. If the substring is not found, then you should return -1.

Summarizing Find the index of the first occurrence in a string

To summarize, understanding how to find the first occurrence of a substring enables you to effectively tackle similar problems on platforms like LeetCode and GeeksforGeeks. By mastering this technique, you’ll be equipped to solve challenges involving locating the first index of occurrence of a string within a larger string.

To find the first occurrence of a substring within a string, follow these steps. First, set up a method that takes two parameters: ‘needle’ (the substring to search for) and ‘haystack’ (the string to search within). Initialize the return value to -1. Next, iterate through each character of the ‘haystack’ string using a loop. Within this loop, create a string variable ‘temp’ to collect characters from the ‘haystack’. Use nested loops to ensure that the indices stay within the bounds of both ‘haystack’ and ‘needle’, adding each character from ‘haystack’ to ‘temp’. After the loop, check if ‘temp’ matches the ‘needle’ string. If it does, return the starting index of the first occurrence. If not, return -1 to indicate that the substring is not found. This method allows you to efficiently locate the first occurrence of a substring within a string.