JavaAlgorithms/Maths/NumberOfDigits.java

package Maths;

/**
 * Find the number of digits in a number.
 */
public class NumberOfDigits {
    public static void main(String[] args) {
        int[] numbers = {0, 12, 123, 1234, -12345, 123456, 1234567, 12345678, 123456789};
        for (int i = 0; i < numbers.length; ++i) {
            assert numberOfDigits(numbers[i]) == i + 1;
            assert numberOfDigitsFast(numbers[i]) == i + 1;
            assert numberOfDigitsFaster(numbers[i]) == i + 1;
            assert numberOfDigitsRecursion(numbers[i]) == i + 1;
        }
    }

    /**
     * Find the number of digits in a number.
     *
     * @param number number to find
     * @return number of digits of given number
     */
    private static int numberOfDigits(int number) {
        int digits = 0;
        do {
            digits++;
            number /= 10;
        } while (number != 0);
        return digits;
    }

    /**
     * Find the number of digits in a number fast version.
     *
     * @param number number to find
     * @return number of digits of given number
     */
    private static int numberOfDigitsFast(int number) {
        return number == 0 ? 1 : (int) Math.floor(Math.log10(Math.abs(number)) + 1);
    }


    /**
     * Find the number of digits in a number faster version.
     *
     * @param number number to find
     * @return number of digits of given number
     */
    private static int numberOfDigitsFaster(int number) {
        return number < 0 ? (-number + "").length() : (number + "").length();
    }

    /**
     * Find the number of digits in a number using recursion.
     *
     * @param number number to find
     * @return number of digits of given number
     */
    private static int numberOfDigitsRecursion(int number) {
        return number / 10 == 0 ? 1 : 1 + numberOfDigitsRecursion(number / 10);
    }
}