Programming Session Day 9–15: Number Property Programs in Java (Using `while` Loops)

Day 9–15: Number Property Programs in Java (Using while Loops)

We’ll implement seven classic checks, each in its own static method within a single Java class. All loops use only while, and no built‐ins beyond basic arithmetic.

1. Armstrong Number

An Armstrong number (for a given number of digits) equals the sum of each digit raised to the power of the number of digits.

public static boolean isArmstrong(int num) {
    int original = num, sum = 0, digits = 0;
    int temp = num;
    // Count digits
    while (temp > 0) {
        digits++;
        temp /= 10;
    }
    temp = original;
    // Sum of powers
    while (temp > 0) {
        int d = temp % 10;
        int power = 1;
        int i = 0;
        while (i < digits) {
            power *= d;
            i++;
        }
        sum += power;
        temp /= 10;
    }
    return sum == original;
}

2. Neon Number

A Neon number is one where the square of the number’s sum of digits equals the number.

public static boolean isNeon(int num) {
    int square = num * num;
    int sum = 0;
    while (square > 0) {
        sum += square % 10;
        square /= 10;
    }
    return sum == num;
}

3. Strong Number

A Strong number equals the sum of factorials of its digits.

public static int factorial(int n) {
    int fact = 1;
    while (n > 0) {
        fact *= n;
        n--;
    }
    return fact;
}

public static boolean isStrong(int num) {
    int temp = num, sum = 0;
    while (temp > 0) {
        sum += factorial(temp % 10);
        temp /= 10;
    }
    return sum == num;
}

4. Perfect Number

A Perfect number equals the sum of its proper divisors (excluding itself).

public static boolean isPerfect(int num) {
    int sum = 0;
    int i = 1;
    while (i < num) {
        if (num % i == 0) {
            sum += i;
        }
        i++;
    }
    return sum == num;
}

5. Prime and Composite

A prime has exactly two divisors (1 and itself). A composite (non-prime greater than 1) has more.

public static boolean isPrime(int num) {
    if (num <= 1) return false;
    int i = 2;
    while (i * i <= num) {
        if (num % i == 0) return false;
        i++;
    }
    return true;
}

6. Emirp Number

An Emirp is a prime that yields a different prime when its decimal digits are reversed.

public static int reverseNumber(int num) {
    int rev = 0;
    while (num > 0) {
        rev = rev * 10 + (num % 10);
        num /= 10;
    }
    return rev;
}

public static boolean isEmirp(int num) {
    if (!isPrime(num)) return false;
    int rev = reverseNumber(num);
    return rev != num && isPrime(rev);
}

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