I have this code:
package math;
import java.io.IOException;
import java.util.Scanner;
public class Main
{
public static void main(String[] args) throws IOException
{
System.out.println("Hi, I will beat Java's Math.sqrt(double) method");
System.out.println("Both ways of calculation will be done");
System.out.println("I will time how long they took to calculate");
System.out.println("Random doubles will be generated");
System.out.println();
System.out.println("Please give the number of sqrt-calculation will be done");
int calcs = new Scanner(System.in).nextInt();
boolean output = true;
if (calcs > 10000)
{
System.out.println("You're asking much calculations");
System.out.println("Disabling output is recommend");
System.out.println("Disable output? (y/n)");
char a = (char) System.in.read();
if (a == 'y')
{
output = false;
}
}
System.out.println("Press enter to start");
System.in.read();
test(calcs, output);
System.out.println();
System.out.println("I was much faster I think");
System.out.println("Now you can check my precision");
System.out.println("Please give a complex double");
double x = Double.parseDouble(new Scanner(System.in).next());
System.out.println();
System.out.println("Math.sqrt(" + x + ") = " + Math.sqrt(x));
System.out.println("SqrtCalculator.sqrt(" + x + ") = " + sqrt(x));
System.out.println("------------------------");
System.out.println("Now please make your conclusion");
System.out.println("Thanks for trying");
}
public static void test(int calculations, boolean output)
{
double factor = Math.random() / 2;
// Math
long mathStart = System.currentTimeMillis();
for (int i = 1; i <= calculations; i++)
{
double x = i * factor;
double result = Math.sqrt(x);
if (output)
{
System.out.println("Math.sqrt(" + x + ") = " + result);
}
}
long mathStop = System.currentTimeMillis();
long mathTime = mathStop - mathStart;
// My Method
long myStart = System.currentTimeMillis();
for (int i = 1; i <= calculations; i++)
{
double x = i * factor;
double result = sqrt(x);
if (output)
{
System.out.println("SqrtCalculater.sqrt(" + x + ") = " + result);
}
}
long myStop = System.currentTimeMillis();
long myTime = myStop - myStart;
System.out.println();
if (output)
System.out.println("---------------------------");
System.out.println("Here are the results:");
System.out.println("Math and SqrtCalculator did each " + calculations + " of the same sqrt-calculations");
System.out.println();
System.out.println("Math: " + mathTime + " milliseconds");
System.out.println("I: " + myTime + " milliseconds");
}
public final static double sqrt(double x)
{
double previous = 1;
double now = 0;
for (;;)
{
now = (x / previous + previous) / 2;
if (previous == now)
{
return now;
}
previous = now;
}
}
}
This sqrt method is called "heroon".
If I run my program and I ask 80000 calculations and I disable the output, Math.sqrt() is much faster than my method. If I ask 80000 calcs and I enable the output, My method is much faster.
Can someone explain this?
Thanks
Sorry for bad English.
I could not reproduce your results. Tried some times using Eclipse Galileo and JDK 1.6.0.
For 80000, output disabled, I got something like:
Math: 15 milliseconds
I: 32 milliseconds
small times, It would be better to use System.nanoTime() or more interactions.
For 80000, output enabled:
Math: 3609 milliseconds
I: 4906 milliseconds
So probably the problem is the way that output is handled (scrolling, buffering, ...)
The Math.sqrt method defers to StrictMath.sqrt, which is done in hardware or native code. (Look at the source for the JDK - you'll see that it's a native method.) This is certainly faster than anything you'll write. It might even be using the same algorithm that you coded. It's well known. Your method is simply Newton's method for calculating square roots. It's been known since Babylon; Newton simply rederived it using calculus. Quadratic convergence is good.
Whatever you've done, it's unlikely that you've discovered anything new or noteworthy. Sounds like something having to do with IO is artificially biasing the results.
You're probably overwhelming the actual calculation time with the output time, and running into a fluke of the buffering. A profiler would show you what's actually consuming the time.
Kudos for trying to improve on an existing implementation; even if you fail, you can learn a lot about algorithms in the process. Naturally, you have to test your alternative using this kind of micro-benchmark. Unfortunately, there are numerous pitfalls. In particular, don't mix irrelevant code, e.g testing and output, with your calculation; do warm up the JVM early in your test. There's more in this article on bechmarking. Also, when comparing floating point values, consider these Guidelines for comparing floating point numbers.
Related
I'm sorry, I know this question is probably asked a million different times every day, but I truly can't find the answer I'm looking for. I'm a beginner in Java (I'm in college and learning a bunch of new languages), and my while loop is printing out the same thing every time.
import java.util.Scanner;
public class Main
{
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("What is the loan amount? ");
int amount = scanner.nextInt();
int x = 1;
//your code goes here
while (x < 6){
System.out.println("Month " +x+ ":");
int percent = (amount / 10);
System.out.println("Payment: 10 percent of " +amount+ " = " +percent);
int rAmt = amount - percent;
System.out.println("Remaining amount: " +rAmt);
x++;
}
}
}
So the issue is that you never actually change amount after doing your calculations inside the while loop. What I think you want to do, is to set amount = rAmt;, which would produce the following code. This will cause the amount to be decreased by 10% each iteration, and this new value carried forward.
...
//your code goes here
while (x < 6){
System.out.println("Month " +x+ ":");
int percent = (amount / 10);
System.out.println("Payment: 10 percent of " +amount+ " = " +percent);
int rAmt = amount - percent;
System.out.println("Remaining amount: " +rAmt);
amount = rAmt;
x++;
}
...
This is the basic codechef question: Pooja would like to withdraw X $US from an ATM. The cash machine will only accept the transaction if X is a multiple of 5, and Pooja's account balance has enough cash to perform the withdrawal transaction (including bank charges). For each successful withdrawal, the bank charges 0.50 $US. Calculate Pooja's account balance after an attempted transaction.
''
This is my code:
import java.util.Scanner;
public class Main
{
public static void main(String args[]){
Scanner s= new Scanner(System.in);
double withdraw = s.nextDouble(); double balance = s.nextDouble();
if((withdraw%5==0) && balance>=(withdraw+0.50))
System.out.println("Balance: "+(balance-withdraw-0.50));
else
System.out.println("Balance: "+balance);
}
}
It's giving the correct output still I'm getting a wrong answer
I think you need to check if withdraw > 0.
Beware modulus, floating-point, and comparing floating-point numbers with "=". It is safer to convert the inputs to long (by multiplying by 100 and using Cents instead of $s). That can avoid some nasty rounding errors with >= 0.50, which would now always be exact and look like >= 50. When producing output, you would have to convert to dollars again:
static long dollarsToCents(double dollars) { return (long)(dollars*100); }
static String centsAsDollars(long cents) { return "" + (cents/100) + "." + (cents%100); }
It is unlikely that the problem is due to this, but look at the following example:
double d = 5_000_000_000_000_000.0; // 5 * 10^15
double e = 0.5;
double f = d + e;
System.out.println("" + d + " + " + e + " = " + f);
System.out.println("both are equal?: " + (d == f)); // true
long a = dollarsToCents(d);
long b = dollarsToCents(e);
long c = a + b;
System.out.println("" + a + " + " + b + " = " + c);
System.out.println("both are equal?: " + (a == c)); // false
For increased safety (and if you ever have to handle money in a program), use BigDecimal. However, those are much slower than fixed-size integers such as long.
I am very new to Java Programming. For example, even if I roll the same number i still lose the bet. If I roll like one and fine, I still win the bet amount. I am trying to fix that problem for hours. But can't figure it out. Please, someone, help me. Thanks in advance.
Here is my code.
public class Dice {
private int dice;
public Random number;
//default constructor
public Dice() {
number = new Random();
}
//To generate random number between 1 to 6. random starts from 0. there is no 0 on dice.
//By adding one, it will start at 1 and end at 6
}
//Method to check two dice
public boolean isequal(int dice1,int dice2) {
}
else
}
public class Playgame
{
//
}
public static void main(String[] args) {
//
}
}
{
return false;
}
}
userinput.close();
}
}
At least one problem is here (there may be others) :
if(obj1.isequal(obj1.play(), obj1.play()) == true)
{
System.out.println("You rolled a " + toString(obj1.play()) + " and a "
+ toString(obj1.play()) );
When you print the message, you are calling obj1.play() again and generating 2 new random numbers. If you need to use the value twice (once for comparison and once for printing) then you should store it in a variable.
int firstRoll = obj1.play();
int secondRoll = obj1.play();
if(obj1.isequal(firstRoll, secondRoll) == true)
{
System.out.println("You rolled a " + toString(firstRoll) + " and a "
+ toString(secondRoll) );
//...
Each call to obj1.play() return a different values.
Hence your test: obj1.isEqual(obj1.play(), obj1.play()) will mostly not return true.
no need for the dice class if it is to generate the random number and checks whether two number is equal or not. try the code below it will work
Random random = new Random();
int n1 = random.nextInt(6) + 1;
int n2 = random.nextInt(6) + 1;
System.out.println("You rolled a " + toString(n1)+ " and a " +toString(n2));
if (n1 == n2) {
double win = betmoney * 2;
System.out.println("You win $" + win);
startmoney += win;
} else {
startmoney -= betmoney;
System.out.println("You lose $" + betmoney);
System.out.println("You left only $" + startmoney);
}
problem with your code is your generating random numbers two times 1.during condition check and 2. in the sysout statement. your program is working fine only. but due to this your confusing yourself that it.
Each time you call ob1.play() method, it will give you different numbers.
in if clause:
if(obj1.isequal(obj1.play(), obj1.play()) == true)
will give you two random values that different from two random values in if block:
System.out.println("You rolled a " + toString(obj1.play()) + " and a " + toString(obj1.play()) );
Ok, I'm a beginner in java, learning on my own through websites and books. I tried a simple square root calculator with a for loop and a while loop (I've included what I tried below). Sadly, all my code does when I enter a number is terminate. Any help would be appreciated!
import java.util.Scanner;
public class The2RootProdject {
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
double rootIt = input.nextDouble();
double dummy = 0.0000000;
while (dummy != dummy * dummy) {
dummy += 0.0000001;
if (rootIt == dummy * dummy) {
System.out.println("the squar root of " + rootIt + " is "
+ (dummy * dummy));
}
}
}
}
You have a couple of problems here:
1) Logical bug: 0 == 0 * 0
<= This means while (dummy != dummy * dummy) {..} will never be untrue, and you'll never even enter the loop
2) Floating point numbers are inexact, so your algorithm (which relies on "==") might not work anyway
Look here for more details on floating point imprecision:
http://www.lahey.com/float.htm
This is true for ANY language - your algorithm for square root must take this into account.
Try to use this algorithm which use Newton's iteration:
import java.util.Scanner;
public class Main
{
public static void main(String args[])
{
double number, t, squareRoot;
Scanner input = new Scanner(System.in);
number = input.nextDouble();
squareRoot = number / 2;
do
{
t = squareRoot;
squareRoot = (t + (number / t)) / 2;
}
while ((t - squareRoot) != 0);
System.out.println(squareRoot);
}
}
Newton's iteration is an algorithm for computing the square root of a number via the recurrence equation:
X(n+1) = (X(n) + number/X(n))/2
I think the while condition is supposed to be =
while(rootIt != dummy * dummy) {}
Your current condition will only ever be true if you initialized dummy as 1; but I don't that would be what you want anyways.
I having some trouble with an APCS assignment. The program is supposed to read strings with a length of two from a text file - test1.txt - and print out percentages of: a) girl-girl, boy-boy, boy-girl or girl-boy combinations, and b) the total number of individual groups.
I've been trying for an hour to figure this out! Although I'm suspicious of the String declaration in line 25, I don't have a way to confirm that. Furthermore, I'm worried that I messed up my if-else-if-else loop without prompting a compiler error.
The source code is attached for your reference. If you need any additional information, please don't hesitate to ask.
Since I'm a new user with a reputation < 10, please see the attached image:
For elaboration on what isn't working. I took a screenshot and wrote relevant comments on it!
/**
* Family takes user input of sets of boys, girls, and boys + girls. Results are then
* tabulated and displayed in a percentage form to the user. The total number of
* individuals are also displayed.
*
* #E. Chu
* #Alpha
*/
import java.util.Scanner;
import java.io.File;
import java.io.IOException;
public class Family {
public static void main (String[] args) throws IOException {
int boyCount = 0;
int girlCount = 0;
double boyGroupCount = 0.0;
double girlGroupCount = 0.0;
int mixedGroupCount = 0;
int totalPersonCount = 0;
double totalGroupCount;
String currentToken = " ";
Scanner inFile = new Scanner (new File ("test1.txt"));
while (inFile.hasNextLine()) {
currentToken = inFile.nextLine( );
if (currentToken == "BG") {
boyCount++;
girlCount++;
mixedGroupCount++; }
else if (currentToken == "GB") {
boyCount++;
girlCount++;
mixedGroupCount++; }
else if (currentToken == "BB") {
boyCount += 2;
boyGroupCount++; }
else {
girlCount += 2;
girlGroupCount++; }
}
inFile.close();
totalPersonCount = boyCount + girlCount;
totalGroupCount = boyGroupCount + girlGroupCount + mixedGroupCount;
System.out.println("Sample Size: " + totalPersonCount);
System.out.println("Two Boys (%): " + boyGroupCount / totalGroupCount + "%");
System.out.println("One Boy, One Girl (%): " + mixedGroupCount + "%");
System.out.println("Two Girls (%): " + girlGroupCount / totalGroupCount + "%");
} // End of main method.
} // End of class Family.
currentToken == "BB" should be currentToken.equals("BB")
Don't use == use the method equals instead
Hint: you don't want to compare strings using ==, look into the equals method.