Loop Algorithms

public String reverse(String str) {
    String reversed = "";
    for (int i = str.length() - 1; i >= 0; i--) {
        reversed += str.substring(i, i + 1);
    }
    return reversed;
}

reverse("Hello")  // "olleH"
reverse("abcd")   // "dcba"

public boolean isPalindrome(String str) {
    String reversed = "";
    for (int i = str.length() - 1; i >= 0; i--) {
        reversed += str.substring(i, i + 1);
    }
    return str.equals(reversed);
}

isPalindrome("racecar")  // true
isPalindrome("hello")    // false

public int countOccurrences(String text, String target) {
    int count = 0;
    for (int i = 0; i <= text.length() - target.length(); i++) {
        if (text.substring(i, i + target.length()).equals(target)) {
            count++;
        }
    }
    return count;
}

countOccurrences("banana", "an")  // 2
countOccurrences("hello", "ll")   // 1

public String replaceChar(String str, String oldCh, String newCh) {
    String result = "";
    for (int i = 0; i < str.length(); i++) {
        String ch = str.substring(i, i + 1);
        if (ch.equals(oldCh)) {
            result += newCh;
        } else {
            result += ch;
        }
    }
    return result;
}

replaceChar("hello", "l", "r")  // "herro"

// Sum of first n positive integers
int sum = 0;
for (int i = 1; i <= n; i++) {
    sum += i;
}
double average = (double) sum / n;  // cast to avoid integer division!

// Find the largest digit in a number
public int largestDigit(int num) {
    num = Math.abs(num);        // handle negatives
    int max = 0;
    while (num > 0) {
        int digit = num % 10;   // extract last digit
        if (digit > max) {
            max = digit;
        }
        num /= 10;              // remove last digit
    }
    return max;
}

largestDigit(38274)  // 8
largestDigit(11111)  // 1

public boolean isPrime(int n) {
    if (n <= 1) return false;
    for (int i = 2; i <= Math.sqrt(n); i++) {
        if (n % i == 0) {
            return false;   // found a factor → not prime
        }
    }
    return true;   // no factors found → prime
}

isPrime(7)   // true
isPrime(12)  // false (12 % 2 == 0)
isPrime(1)   // false

public int digitSum(int num) {
    num = Math.abs(num);
    int sum = 0;
    while (num > 0) {
        sum += num % 10;    // add last digit
        num /= 10;          // remove last digit
    }
    return sum;
}

digitSum(1234)  // 1 + 2 + 3 + 4 = 10
digitSum(999)   // 9 + 9 + 9 = 27

// Count uppercase letters in a string
public int countUppercase(String str) {
    int count = 0;
    for (int i = 0; i < str.length(); i++) {
        String ch = str.substring(i, i + 1);
        if (ch.equals(ch.toUpperCase()) && !ch.equals(ch.toLowerCase())) {
            count++;
        }
    }
    return count;
}

countUppercase("Hello World")  // 2 (H, W)

// Keep only the letters (remove digits, punctuation)
public String lettersOnly(String str) {
    String result = "";
    String letters = "abcdefghijklmnopqrstuvwxyz";
    for (int i = 0; i < str.length(); i++) {
        String ch = str.substring(i, i + 1).toLowerCase();
        if (letters.indexOf(ch) >= 0) {
            result += str.substring(i, i + 1);  // keep original case
        }
    }
    return result;
}

lettersOnly("H3llo W0rld!")  // "HlloWrld"