Question

Write your own versions of String search methods indexof and lastIndexof.

Short answer

Create two functions to find the first and last substring indices by iterating through the string.

Step-by-step solution

Understanding 'indexOf'

The 'indexOf' method in string manipulation is used to find the first occurrence of a substring within a given string. It returns the index of the first character of the found substring or -1 if the substring is not found.

Initialize String Search for 'indexOf'

To implement 'indexOf', first create a function that takes two parameters: the main string and the substring you are searching for. Initialize a loop to iterate through the main string.

Implement Search Logic for 'indexOf'

In the loop, for each character position in the main string, check if the substring exists there by comparing the current segment of the main string to the substring. If they match, return the current index.

Return Result for 'indexOf'

If the loop completes without finding the substring, return -1 to indicate that the substring was not found in the main string.

Understanding 'lastIndexOf'

The 'lastIndexOf' method is used to find the last occurrence of a substring in the main string, returning the index of the first character of this last occurrence, or -1 if the substring is not found.

Initialize String Search for 'lastIndexOf'

Create a function similar to 'indexOf', taking two inputs: the main string and the substring. Initialize a loop that iterates backward from the end of the main string.

Implement Search Logic for 'lastIndexOf'

In the backward loop, for each position, check if the substring occurs by comparing the main string segment to the substring. Return the index if a match is found.

Return Result for 'lastIndexOf'

If no match is detected after the loop concludes, return -1 to imply the substring is absent from the main string.

Key concepts

indexOf method

The `indexOf` method is a powerful tool in Java string manipulation that searches for the first occurrence of a substring within another string. Imagine you have a long book and you're trying to find the first time a character's name is mentioned. The `indexOf` method performs a similar task in your code.

Here's how it works:

• It scans through the given string from the start.
• When it finds the substring, it returns the starting index of that substring within the main string.
• If the substring isn't found, it returns -1.

For example, if you have the string "Java is fun" and you use `indexOf("is")`, it will return `5`, as the word "is" starts at index `5`. This method allows you to pinpoint exactly where a specific pattern occurs, assisting in text parsing and analysis.

Remember, index numbering in Java begins at `0`, so make sure to count from this base when interpreting results. The `indexOf` method is case-sensitive, meaning "Java" and "java" would be treated as different strings.

lastIndexOf method

The `lastIndexOf` method essentially mirrors `indexOf` but with a slight twist—it finds the last occurrence of a specified substring within a string. Consider it like searching a book backwards to see when a character's name appears for the last time.

Here's what happens:

• The method begins checking from the end of the string.
• It returns the index of the first character of the last found substring.
• If the substring is nonexistent in the original string, it returns -1.

For instance, given the string "She sells sea shells by the sea shore" and you search for `lastIndexOf("sea")`, it returns `27`, in this scenario, as the last "sea" starts there.

Like `indexOf`, it is case-sensitive, so be sure to match the casing exactly. By utilizing the `lastIndexOf` method, you gain the ability to pinpoint the last place an identifier appears, which is invaluable for parsing data where the last occurrence is of interest.

substring search

Substring searching is a fundamental operation in Java's string manipulation, aimed at isolating smaller strings within larger textual data. This technique is pivotal when you deal with tasks like locating identifiers, keywords, or even parsing URLs.

The idea is straightforward:

• You have a main string, which is the large block of text.
• Then there's the substring, a smaller sequence you need to locate.
• Methods like `indexOf` and `lastIndexOf` are commonly used to facilitate these searches.

These searches are particularly useful because they can extract meaningful information quickly without the need for complex pattern matching.

Furthermore, understanding the efficiency of substring searches helps in optimizing performance, especially in applications processing extensive datasets or requiring rapid text evaluations. Employing these string methods correctly can significantly enhance your software's ability to process and manage string-based data efficiently.