記錄用: 106.03.31 Java作業四各種四捨五入

這次過了死線了才PO啊....
原因可能是最近看的一部電影讓我頗憂鬱
《明天，我要和昨天的妳約會》
不說了，進正題吧QuQ

這次作業是讓我們用Babylonian method來求正數平方根並四捨五入到小數點後第二位

演算法不難照公式打就好
我覺得難的是四捨五入 (突然想到高中zero judge的d561:被秒殺的四捨五入
所以本篇主要記錄我查到的各種四捨五入方法

以下程式我改寫成能夠自訂位數

public static double round(double value, int places){}
//value為想四捨五入的數、places為取到小數點下第幾位

一、Math.round

	package rounding;

	public class RoundByMathRound {
	public static double round(double value, int places) {
	if (places < 0) throw new IllegalArgumentException();

	long factor = (long) Math.pow(10, places);
	value = value * factor;
	long tmp = Math.round(value);
	return (double) tmp / factor;
	}
	}

二、DecimalFormat

	package rounding;

	import java.text.DecimalFormat;

	public class RoundByDecimalFormat {
	public static double round(double value, int places) {
	if (places < 0) throw new IllegalArgumentException();

	String format = "#.";
	for(int i = 0; i < places; i++){
	format+="#";
	}
	DecimalFormat df = new DecimalFormat(format);
	return Double.parseDouble(df.format(value));
	}
	}

三、BigDecimal

	package rounding;

	import java.math.BigDecimal;
	import java.math.RoundingMode;

	public class RoundbyBigDecimal {
	public static double round(double value, int places) {
	if (places < 0) throw new IllegalArgumentException();

	BigDecimal bd = new BigDecimal(value);
	bd = bd.setScale(places, RoundingMode.HALF_UP);
	return bd.doubleValue();
	}
	}

四、自幹一個字串處理 (會有bugs，踩雷記錄QuQ)

	package rounding;

	public class RoundByStringHandling {
	public static double round(double value, int places){
	if (places < 0) throw new IllegalArgumentException();

	StringBuilder sb = new StringBuilder(((Double)(value)).toString());
	StringBuilder resultString = null;
	double resultValue = 0D;
	boolean isFindDot = false;
	String avoidOverIndexZero = "";

	for(int i = 0; i < places; i++){
	avoidOverIndexZero += "0";
	}

	for(int i = 0; i < sb.length(); i++){
	if(sb.charAt(i) == '.'){
	isFindDot = true;
	sb = sb.append(avoidOverIndexZero);//if value only 1 or 2 decimal add "00" can avoid StringIndexOutOfBoundsException
	resultString = new StringBuilder(sb.substring(0, i + places + 2));//.toString();//Cut after 3 decimal String
	break;
	}
	}

	if(isFindDot == false){//no dot represent value is integer
	return value;
	}
	else{
	int nextDecimal = Integer.parseInt(resultString.substring(resultString.length()-1));
	//System.out.println(nextDecimal);
	if(nextDecimal >= 5){
	resultString.deleteCharAt(resultString.length()-1);
	resultValue = Double.parseDouble(resultString.toString())+Math.pow(0.1, places);
	}
	else{
	resultString.deleteCharAt(resultString.length()-1);
	resultValue = Double.parseDouble(resultString.toString());
	}
	}
	return resultValue;
	}
	}

實際測試：(精確度都以2位為測試)

	package rounding;

	import java.util.Scanner;

	public class RoundingTest {

	public static void main(String[] args) {
	Scanner scanner = new Scanner(System.in);
	double num = Double.parseDouble(scanner.nextLine());
	System.out.println("Origin:\t"+(num));

	double d1 = RoundByMathRound.round(num, 2);
	System.out.println("d1:\t" + d1);

	double d2 = RoundByDecimalFormat.round(num, 2);
	System.out.println("d2:\t" + d2);

	double d3 = RoundbyBigDecimal.round(num, 2);
	System.out.println("d3:\t" + d3);

	double d4 = RoundByStringHandling.round(num, 2);
	System.out.println("d4:\t" + d4);

	scanner.close();
	}
	}