Question is from here:

This question appears regularly at beginning-programmer discussion sites:

Write a function that takes a floating point number f and an integer n and rounds the floating point number to n places after the decimal point. For instance, given the floating point number 1000/7, rounding to 3 places would produce 142.857, rounding to 2 places would produce 142.86, rounding to 1 place would produce 142.9, and rounding to 0 places would produce 143. For extra credit, allow n to be negative, indicating rounding before the decimal point; for instance, rounding 1000/7 to -1 places would produce 140.

It’s not fair to use a built-in round function if your language provides one.

Your task is to write the function described above.

My Solution:

Given input “1000/7,3″:

1. calculated the fraction: 142.85714285714286
2. multiplied this number to 10^3, 142857.14285714286
3. result was converted to Long. 142857
4. checked if the last digit is greater than or eqaul to 5
5. if yes add 1 to the number, otherwise subtract last digit from the number, 142858
6. Divided the number by 10^3 and convert it back to double, 142.858

import java.io.*;

public class Floating_Point_Rounding {
	public static void main(String[] args) {
		BufferedReader r = new java.io.BufferedReader (new InputStreamReader (System.in));
		String s;
		try {
			System.out.println("Enter a number");
			s = r.readLine();
			round(s);
			while(true){
				System.out.println("Enter a number");
				s = r.readLine();
				round(s);
			}

		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	private static void round(String s){
		//input is of format xxx/x,x
		String[] parts = s.split(",");
		String[] fraction = parts[0].split("/");

		double d1 = Double.parseDouble(fraction[0]);//numerator
		double d2 = Double.parseDouble(fraction[1]);//denominator
		double ans = d1/d2;//fraction
		double n = Double.parseDouble(parts[1]);//decimal point
		long ans2, ans3; String s1; double finalAns;
		if(n > 0){
			ans2 = (long) Math.pow(10, n + 1);
			ans3 = (long) (ans2 * ans);
			s1 = Long.toString(ans3);
			if(Character.getNumericValue(s1.charAt(s1.length() - 1)) >= 5 ){
				ans3 += (10 - Character.getNumericValue(s1.charAt(s1.length() - 1)));
			} else{
				ans3 -= 1;
			}
			finalAns = (double) ans3 / ans2;
		} else if (n == 0){
			ans2 = 10;
			ans3 = (long) (ans2 * ans);
			s1 = Long.toString(ans3);
			if(Character.getNumericValue(s1.charAt(s1.length() - 1)) >= 5 ){
				ans3 += 10;
			}
			finalAns = ans3 /=10;
		} else {
			ans2 = (long) ans;
			int r = (int) Math.abs(n);
			s1 = Long.toString(ans2);
			if(Character.getNumericValue(s1.charAt(s1.length() - r)) < 5 ){
				String s2 = s1.substring(s1.length() - r, s1.length());
				ans2 -= Integer.parseInt(s2);
			} else {
				String s2 = s1.substring(s1.length() - r, s1.length());
				ans2 += (Math.pow(10, r) - Integer.parseInt(s2));
			}
			finalAns = ans2;
		}
		System.out.println("Answer: " + finalAns);
	}
}

Output:

Enter numbers:
1000/7,3
Answer: 142.857
Enter numbers:
1000/7,2
Answer: 142.86
Enter numbers:
1000/7,1
Answer: 142.9
Enter numbers:
1000/7,0
Answer: 143.0
Enter numbers:
1000/7,-1
Answer: 140.0
Enter numbers:
1000/7,-2
Answer: 100.0