Strings – FRQ Practice (AP Computer Science A)

String manipulation is one of the most heavily tested concepts in the AP Computer Science A FRQ section. Students must know how to process characters, analyze substrings, validate formats, and build new strings using loops and conditions.

This page provides exam-style FRQ practice questions designed to strengthen your string-processing logic.

Why Practice String FRQs?

AP CSA often includes free-response problems requiring:

• substring extraction
• counting patterns
• validating input formats
• scanning through characters
• building new strings with rules
• solving text-based algorithm problems

Mastering these string skills dramatically improves FRQ performance.

How to Use This Page

• Attempt each problem before checking the solution
• Compare your logic with the sample answer
• Study the Common Mistakes to avoid scoring loss
• Re-practice the problem until you can solve it quickly
• Use these as real exam warm-ups

FRQ 1 — Count Vowels (Easy)

Problem:
Write a method countVowels(String s) that returns how many vowels (a, e, i, o, u) appear in the string. Ignore case.

Example:
Input: "Computer" → Output: 3

Approach:

• Convert string to lowercase
• Loop through each character
• Check whether the character is in "aeiou"

Java Solution:

public int countVowels(String s) {
    String low = s.toLowerCase();
    int count = 0;

    for (int i = 0; i < low.length(); i++) {
        char c = low.charAt(i);
        if ("aeiou".indexOf(c) != -1) {
            count++;
        }
    }
    return count;
}

Common Mistakes:

• Forgetting uppercase vowels
• Using == to compare strings
• Checking only one vowel

FRQ 2 — Remove All Digits (Medium)

Problem:
Write a method removeDigits(String s) that returns a new string with all digits removed.

Example:
Input: "a1b2c3d4" → Output: "abcd"

Approach:

• Loop through the string
• Append non-digit characters only
• Use Character.isDigit()

Java Solution:

public String removeDigits(String s) {
    StringBuilder result = new StringBuilder();

    for (int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);
        if (!Character.isDigit(c)) {
            result.append(c);
        }
    }
    return result.toString();
}

Common Mistakes:

• Rebuilding string with + in loop (slower)
• Returning null for empty result
• Incorrect digit checks

FRQ 3 — Find the Longest Word (Medium)

Problem:
Write a method longestWord(String sentence) that returns the longest word from a sentence.
Assume words are separated by spaces.

Example:
Input: "I love programming in Java" → Output: "programming"

Approach:

• Split using split(" ")
• Track the longest word
• Return longest

Java Solution:

public String longestWord(String sentence) {
    String[] words = sentence.split(" ");
    String longest = "";

    for (String w : words) {
        if (w.length() > longest.length()) {
            longest = w;
        }
    }
    return longest;
}

Common Mistakes:

• Not handling multiple spaces
• Using >= causing different tie results
• Not returning “” when sentence is empty

FRQ 4 — Count Substring Occurrences (Hard)

Problem:
Write a method countOccurrences(String s, String sub) that returns how many times sub appears in s.
Overlapping occurrences must be counted.

Example:
Input: "aaaa" and "aa" → Output: 3

Approach:

• Loop from 0 to s.length() - sub.length()
• Compare using .equals()
• Move window by 1

Java Solution:

public int countOccurrences(String s, String sub) {
    int count = 0;

    for (int i = 0; i <= s.length() - sub.length(); i++) {
        if (s.substring(i, i + sub.length()).equals(sub)) {
            count++;
        }
    }
    return count;
}

Common Mistakes:

• Using == for substring comparison
• Off-by-one loop errors
• Forgetting overlapping matches

FRQ 5 — Validate ID Format (Hard)

Problem:
A valid ID has the pattern:
2 letters + 4 digits
Example valid → "AB1234"
Invalid → "A1234", "ABC123" "AB12C4"

Write a method isValidID(String id) that returns true if valid.

Approach:

• Check length = 6
• First 2 chars must be letters
• Last 4 chars must be digits

Java Solution:

public boolean isValidID(String id) {
    if (id.length() != 6) return false;

    if (!Character.isLetter(id.charAt(0)) ||
        !Character.isLetter(id.charAt(1))) {
        return false;
    }

    for (int i = 2; i < 6; i++) {
        if (!Character.isDigit(id.charAt(i))) {
            return false;
        }
    }

    return true;
}

Common Mistakes:

• Not checking length before accessing chars
• Using try/catch unnecessarily
• Returning true even when first letters are lowercase/uppercase mismatch

More String FRQs Coming Soon

• Reverse each word
• Check for palindrome
• Remove duplicate characters
• Run-length encoding (compression)
• Extract balanced parentheses
• Pattern-based filtering tasks

Related FRQ Pages

• Arrays – FRQ Practice
• ArrayList – FRQ Practice
• 2D Arrays – FRQ Practice
• Classes & Objects – FRQ Practice
• Mixed FRQs
• Mock FRQ Sets

(Add links once pages are published.)

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