Unit 2.10: Standard String Algorithms

By combining iteration with the String class methods, we can implement several standard and powerful algorithms for processing strings.

Common String Algorithms

Finding Substrings with a Particular Property

This involves iterating through a string and checking each character or substring for a desired characteristic.

Task: Counting vowels in a string using a loop and indexOf.

public class VowelCounter {
    public static void main(String[] args) {
        String text = "Hello, World!";
        int vowelCount = 0;
        String vowels = "aeiouAEIOU";

        for (int i = 0; i < text.length(); i++) {
            String letter = text.substring(i, i + 1);
            // If the letter is found in the vowels string, it is a vowel
            if (vowels.indexOf(letter) != -1) {
                vowelCount++;
            }
        }
        System.out.println("Number of vowels: " + vowelCount); // Output: 3
    }
}

Counting Substrings that Meet Specific Criteria

This involves iterating through a string to find how many times a specific substring appears.

Task: Counting occurrences of a specific pattern within a string.

public class SubstringCounter {
    public static void main(String[] args) {
        String text = "abababa";
        String sub = "aba";
        int count = 0;
        int index = text.indexOf(sub);

        // Keep searching as long as indexOf finds the substring
        while (index != -1) {
            count++;
            // Start the next search from the character after the last match
            index = text.indexOf(sub, index + 1);
        }
        System.out.println("'" + sub + "' appears " + count + " times."); // Output: 3
    }
}

Creating a Reversed String

This involves iterating through the original string from back to front and building a new string character by character.

Task: Reversing a string using backward iteration.

public class StringReverser {
    public static void main(String[] args) {
        String original = "hello";
        String reversed = "";

        for (int i = original.length() - 1; i >= 0; i--) {
            reversed = reversed + original.substring(i, i + 1);
        }
        System.out.println("Reversed string: " + reversed); // Output: "olleh"
    }
}