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  1. AP Computer Science A
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Developing Algorithms Using Strings

Emily Wilson

Emily Wilson

5 min read

Next Topic - Nested Iteration

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Study Guide Overview

This study guide covers core string algorithms in Java. It reviews using for loops, indices, and the substring() method for string manipulation. Key algorithms covered include reversing a string, getting all substrings of length N using a sliding window approach, and checking for a substring using equals(). Important string methods like length(), substring(), and equals() are also explained.

String Algorithms: A Last-Minute Review ๐Ÿš€

Welcome! This guide is designed to be your ultimate resource for string algorithms right before the exam. Let's get you feeling confident and ready to ace those questions!

String Algorithms Overview

Key Concept

The core of string manipulation often involves using for loops to iterate through characters and substrings. Understanding how to use indices and the substring() method is crucial.

Exam Tip

When tackling string algorithm problems, always start by tracing the algorithm with a short example string (like "cat" or "dog"). This helps you visualize the process and catch errors early.

Key String Methods

  • length(): Returns the length of the string.
  • substring(int beginIndex, int endIndex): Returns a substring starting at beginIndex (inclusive) and ending at endIndex (exclusive).
  • equals(String other): Checks if two strings have the same sequence of characters.

Reversing a String

String reversal is a classic algorithm that demonstrates how to build a new string by prepending characters from the original string.

Algorithm:

  1. Initialize an empty string called result.
  2. Iterate through the input string s from the beginning to the end.
  3. In each iteration, take the current character and add it to the beginning of result.
  4. Return result.
public static String reverse(String s) {
    String result = "";
    for (int i = 0; i < s.length(); i++) {
        result = s.substring(i, i+1) + result;
    }
    return result;
}

Example:

If s is "hello", the algorithm works like this:

  • i = 0: result becomes "h"
  • i = 1: result becomes "eh"
  • i = 2: result becomes "leh"
  • i = 3: result becomes "lleh"
  • i = 4: result becomes "olleh"
Common Mistake

A common mistake is to append the character to the end of result instead of the beginning, which would not reverse the string.

Getting All Substrings of Length N

This algorithm uses a "sliding window" approach to extract all substrings of a given length. It's a fundamental technique for many string problems.

Algorithm:

  1. Iterate through the input string s using a loop.
  2. The loop condition ensures that the substring of length n does not go out of bounds.
  3. In each iteration, extract a substring of length n starting at the current index i.
  4. Print or process the substring.
public static void printSubstringsLengthN(String s, int n) {
    for (int i = 0; i < (s.length() + 1 - n); i++) {
        System.out.println(s.substring(i, i+n));
    }
}

Analogy: Imagine a window of width n sliding across the string. The loop moves the window one character at a time.

Example:

If s is "abcdef" and n is 3, the substrings are:

  • "abc"
  • "bcd"
  • "cde"
  • "def"

Checking for a Substring

This algorithm checks if a given substring exists within a larger string. It's a common task in text processing.

Algorithm:

  1. Iterate through the input string s using a loop.
  2. In each iteration, extract a substring of the same length as the target substring n.
  3. Compare the extracted substring with n using the equals() method.
  4. If a match is found, return true immediately.
  5. If the loop finishes without finding a match, return false.
public static boolean checkForSubstring(String s, String n) {
    for (int i = 0; i < (s.length() + 1 - n.length()); i++) {
        String currentSubString = (s.substring(i, i+n.length()));
        if (currentSubString.equals(n)) {
            return true;
        }
    }
    return false;
}

Example:

`checkForSubstring("programming

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Question 1 of 11

Given the string "apple", what character is at index 1?

a

p

l

e