I am fairly new to java (and most programming languages as a whole) and am trying to get some unfamiliar concepts down, one of which is recursion. To test it out I tried making it so that a number would constantly increase and get displayed.
public static void recursionTest() {
int numb = 0;
System.out.println(numb);
numb += 1;
recursionTest();
}
I tried writing it out like this, but it will only print the number 0 constantly with no increase. I then tried putting before println, but it only produced the number 1. I then tried replacing with a while loop.
public static void recursionTest() {
int numb = 0;
System.out.println(numb);
while (numb != -1) {
numb =+ 1;
}
recursionTest();
}
This ended up printing out just a single 0, I then tried moving it above println like I did before, but it then didn't display anything. Is there something I'm missing? Sorry if my question is stupid.
As you have defined it, numb is a local variable scoped within the method. It's created with the instruction int numb = 0 at the beginning of each call of the method, and destroyed at the end of it. Each time you call the method (even if the call is recursive) is a different variable. With the same name in the code, but it's a different one.
In order to achieve a counter you should either define numb as a static field of the class or pass as parameter to the method. You can do something like the following:
Option 1 (static field):
public class TheClass {
static int numb = 0;
public static void recursionTest() {
System.out.println(numb);
numb += 1;
recursionTest();
}
}
Option 2 (pass as parameter):
public class TheClass {
public static void recursionTest(int numb) {
System.out.println(numb);
numb += 1;
recursionTest(numb);
}
}
In the second attempt you are doing the System.out.println outside the while loop, so you are printing the value only once. Moreover, the recursive call is never being done due to the infinite while loop, but in the case you put a limit to the loop, the new call to the same method will result in the counter being reset because the same reason as before.
You should read up about the scope and lifespan of variables in Java. You declare a local variable inside the function, which is "created" and initiated (to 0) when you write:
int numb = 0;
This variable is only visible inside this current function call. After your function is executed the variable and its value is invalidated and destroyed. When calling the function again, the variable is created and initiated (to 0) again.
If your variable needs to "survive" multiple calls of your function consider to declare it outside of the function:
private static int numb = 0;
public static void recursionTest() {
System.out.println(numb);
while (numb != -1) {
numb =+ 1;
}
recursionTest();
}
Or pass it as a parameter:
public static void main(String[] args) {
recursionTest(0);
}
public static void recursionTest(int numb) {
System.out.println(numb);
while (numb != -1) {
numb =+ 1;
}
recursionTest(numb);
}
Related
I'm trying to check if a number is a square, and if a number is triangular.
The issue is happening at sqrt(num) which is returning 0 for all numbers I test.
I'm using an online compiler, tried several compilers, so it's not a compiling issue. Tried to declare num as a double and as an int, same results.
I'm new to Java, but not new to programming, I searched online, checked my code several times, everything looks fine, it even worked as expected before adding the variables for checking triangular number, but after declaring the variables checkTri and checkTriSqr, this started to happen. I'm sure this have nothing to do with declaring these variables (almost sure), could anyone please help me understand what's going on here?
import static java.lang.Math.sqrt;
import static java.lang.Math.round;
public class Parent{
public static void main(String[] args){
class Number
{
public int num ;
double numSqr = sqrt(num );
double roundNumSqr = round(numSqr) ;
double checkTri = 8 * num + 1 ;
double checkTriSqr = sqrt(checkTri) ;
public void prinTest()
{
System.out.println(num);
System.out.println(numSqr);
System.out.println(roundNumSqr);
System.out.println(checkTri);
System.out.println(checkTriSqr);
}
public boolean isSqr()
{
if (numSqr == roundNumSqr)
{
return true;
}
else
{
return false;
}
}
public boolean isTriangular(){
if (checkTriSqr * checkTriSqr == checkTri )
{
return true;
}
else
{
return false;
}
}
}
Number number = new Number();
number.num = 350;
number.prinTest();
System.out.println(number.isSqr());
System.out.println(number.isTriangular());
}
}
EDIT: The following screen shot is from the tutorial I'm following, concerning declaring classes within methods!
This:
public int num ;
double numSqr = sqrt(num );
initialises num to 0 upon instance construction (the default value for an integer in the absence of assignment), and numSqr is set immediately afterwards (to zero).
You need to recalculate the sqrt() each time you subsequntly set num (perhaps by providing a method setNum() and recalculating everything within that method)
I wouldn't call your class Number, btw. There's already a Number class in the standard Java class set.
numSqr is created in the constructor, whereas number.num = 350;is declared after the construction of your object.
You can use a constructor like this :
public Numer(int num){
this.num=num;
this.numSqr=sqrt(num)
//.... ... ...
}
You can also use an empty constructor and a setter to set the number attribute :
public void setNumber(int num){
this.num=num;
this.numSqr=sqrt(num)
//.... ... ...
}
The values numSqr, roundNumSqr, etc, are all set at the point of the object's creation, however you don't set num to anything until after the object is created. The result is that, for instance,
At creation:
num = 0
therefore
numSqr = 0
roundNumSqr = 0
etc
Then, you set num = 350
But you don't reset the values of numSqr, etc, so this is still the case:
numSqr = 0
roundNumSqr = 0
You need to make a constructor for this class that takes in the value of num and then sets all of the corresponding values, so that they're only set after num has been set (or, add a "calculate" function that updates all the values).
You can modify in this way and compare with technology you have worked on .
import static java.lang.Math.sqrt;
import static java.lang.Math.round;
public class Number {
public int num = 0;
public void prinTest() {
System.out.println(this.num);
System.out.println(this.getSqrt(this.num));
System.out.println(this.getCheckTri());
}
private double getSqrt(double value) {
return sqrt(value);
}
public boolean isSqr() {
if (this.getSqrt(this.num) == round(this.getSqrt(this.num))) {
return true;
} else {
return false;
}
}
private double getCheckTri() {
return 8 * this.num + 1;
}
public boolean isTriangular() {
if (this.getSqrt(this.getCheckTri()) * this.getSqrt(this.getCheckTri()) == this.getCheckTri()) {
return true;
} else {
return false;
}
}
public static void main(String[] args) {
Number number = new Number();
number.num = 49;
number.prinTest();
System.out.println(number.isSqr());
System.out.println(number.isTriangular());
}
}
You should read some basic tutorials as you have added class inside main method,which means you need more time to check out the syntax.
The other answers alreade said, that the field num was not set to the input number, and that the other fields were actually evaluated on object creation, and hence zero too.
The purpose however is achieved by simple functions:
public static boolean isSquare(int num) {
int root = (int) Math.round(Math.sqrt(num));
return root*root == num;
}
public static boolean isCubic(int num) {
int root = (int) Math.round(Math.cbrt(num));
return root*root*root == num;
}
This exploits the cubic root.
As a comparison of doubles, a sqrt result and its rounded long value are still imprecise, I prefer to recalculate the original parameter.
public int num ;
double numSqr = sqrt(num);
By default, declared instance integer variables (variables declared inside class body) are initialized with 0 (zero). Hence, your code does nothing but take a square root of zero, which is zero.
I have declared a default and a parameterize constructor. Using a single object I am calling both the constructor and an function. When I run the program I am getting the output as 0 instead of getting the factorial. I have initialize f=1 still the output is 0.
class Factorial
{
int num, f;
Factorial()
{
f = 1;
}
Factorial(int n)
{
num = n;
}
public int getFactorial()
{
for(int i = 1; i <= num; i++)
{
f = f * i;
}
System.out.println("Factorial= " + f);
return f;
}
public static void main(int m)
{
Factorial obj = new Factorial();
obj = new Factorial(m);
obj.getFactorial();
}
}
This is because the initial value of f remains zero when the class is initialized with the factorial(int n) constructor.
Set f to 1 in the initializer to fix this problem:
int n, f = 1;
Factorial() {
}
Factorial(int n) {
num = n;
}
This leaves your class with a big problem: calling getfactorial multiple times will change the state of the class, increasing the value of the factorial. You can fix this by creating a boolean variable that indicates whether the factorial has been computed or not, and returning f after the computation has been performed.
If you do not need "lazy" computation, make f a local variable in the method.
As a good practice, a variable should have the minimal possible scope (preferred local, then instance). Or in other words: state variables should be used just when you want to share some date between several methods of the same class. But what in your case? Factorial is an operation that may be computed in just one step, from one single parameter: No need to further processing.
So, I recommend you to refactorize your class to:
Drop off the state variables: Convert them to local variables or parameters to the method.
Set the factorial method as static (precisely because it does not need state variables).
Another small detail: Parameters are passed from command line to the main method through an array of Strings - always.
Leave all the inputs and outputs to/from the user in just one method, preferrably the main method.
So it will remain like this:
class Factorial
{
public static int getFactorial(int num)
{
int f=1;
for (int i = 1; i <= num; i++)
{
f = f * i;
}
return f;
}
public static void main(String[] args)
{
int m=Integer.parseInt(args[0]);
int factorial=Factorial.getFactorial(m);
System.out.printf("factorial of %d is %d\n", m, factorial);
}
}
Much simplier, isn't it?
So let's say I have a main class with a while loop:
public class Main {
public static void main(String[] args) throws InterruptedException {
int one = 1;
int counter = 0;
while (one<100){
Thread.sleep(1000);
counter += 1;
Function.Move();
one++;
}
The counter variable in this loop is counting each second elapsed.
There is a separate class called Function:
public class Function {
public static int Move (int result){
result = 1 + counter;
return result;
}
}
So as you can see, inside the Function class's Move method, I want to be able to use the counter variable's new value, which increases with each passing second, to calculate the value of a different variable which will then be returned to the main method.
The problem is that I can't figure out how to pass the value of counter to the Move method inside the Function class to begin with.
I'm not shure if i understand what you want to do correctly, depending on where exactly you will need that result variable later i think your coude should look something like this:
public class Main {
int counter;
public static void main(String[] args) throws InterruptedException {
int one = 1;
counter = 0;
while (one<100){
Thread.sleep(1000);
counter += 1;
one++;
}
}
public int getCounter() {
return counter;
}
}
public class Function {
public static int move (int result, Main main){
result = 1 + main.getCounter();
return result;
}
}
You can use Function.move() anywhere you need it's value in your Programm now.
Beware, though, that you will need your code using the Function.move() to run in a different Thread as the Main Thread. Otherwise it will always return 101 or 1, as the while loop will always be running before or after your call to Function.move(), depending on where you call it (except if you call it from within the while loop, but then you counld just use counter++ without the need to have an extra class)
I am having trouble with an error saying that my int variable final may not have been initialized however I thought I initialized it in the if statement.
import java.util.Scanner;
public class Fibo
{
static int fibNum(int num)
{
int finish;
if(num < 0)
{
finish = -1;
}
else if(num == 0 || num == 1)
{
finish = num;
}
else if(num > 1)
{
finish = fibNum(num-1) + fibNum(num-2);
}
return finish;
}//end fibNum
public static void main(String []args)
{
int num;
Scanner in = new Scanner(System.in);
num = in.nextInt();
fibNum(num);
System.out.println(" is the nth number in the fibbonaci sequence that you are looking for");
}//end main
}//end class
Error:
Fibo.java:20: error: variable finish might not have been initialized
return finish;
While the member fields (variables) can be initialized to default values implicitly, the local variables cannot, so you should initialize it explicitly by changing this:
int finish;
to this:
int finish = 0;
You have to initialize the variable.
Change:
int finish;
To
int finish = 0;
You Must Declare Return Variable. Because if condition can execute only when calling a method.
int finish = 0;
Local variables do not get default values. Their initial values are undefined without assignment by some means. Before you can use local variables they must be initialized.
That's why you have to intialize the local variable.
int finish = 0;
Yes, you must initialize the int variable finish in fibNum method
I need to generate a program that generates the Fibonacci Sequence
Here is what I have so far:
import java.util.Scanner;
public class FibonacciRunner
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.println("Enter n:");
int n = in.nextInt();
EP64 fg = new EP64();
for (int i = 1; i <= n; i++)
System.out.println(fg.nextNumber());
}
}
public class EP64
{
public static void nextNumber(int n)
{
int fold1 = 1;
int fold2 = 1;
int fnew = fold1 + fold2;
fold1 = fnew;
}
}
I get an error on:
System.out.println(fg.nextNumber());
saying:
method nextNumber in class EP64 cannot be applied to given types:
required: int
found: no arguments
reason: actual and formal argument lists differ in length
and can someone also tell me if I am doing this program right? If not, help! I looked at other similar questions but I cannot make much sense of them
Thank you all!
method nextNumber in class EP64 cannot be applied to given types: required: int found: no arguments reason: actual and formal argument lists differ in length
Your
public static void nextNumber(int n)
^^^^^^^
says that any call to the method must provide an integer as argument. But here:
System.out.println(fg.nextNumber());
^^ you need to add an integer argument
you violate this by providing no argument.
As your code reads now, I'd probably drop the int n argument.
and can someone also tell me if I am doing this program right?
Naah, not really...
fold1 and fold2 should probably be member variables (so they don't get reset in every call to the method),
You're forgetting to update fold2 (you only update fold1),
Also, you probably want to return an int from the nextNumber method.
Read up on
Official Java Tutorial: Defining Methods
You are calling a static method to a object reference instead of the class itself.
And
Not passing any argument at all for nextNumber() method.
Make the method non-static as :
public void nextNumber(int n) {}
Pass arg to the method as :
for (int i = 1; i <= n; i++)
System.out.println(fg.nextNumber(n));
And also don't forget to return the processed number from your nextNumber method,which you collecting in System.out.println.
Your declaration of nextNumber says it takes an int argument, but you are calling it with no arguments.
Also, your code isn't going to do what you want. You probably should make fold1 and fold2 members of class EP64 and make the method an instance method rather than a static method. You also need to do fold2 = fold1; before you update fold1.
Finally, you need to declare nextNumber to return an int value, and then actually have it return an int value.
You have two problems. Firstly, your method doesn't return anything, i.e. it is void. You need to make it int and add a return fnew; at the end. The other problem is you are starting from scratch every time, it will return 2 each time. You need to make fold1 and fold2 fields by moving them above the nextNumber line. Oh, and drop the int n argument as it doesn't do anything.
I agree on the diagnostics of the other posts, but don't suggest a member variable, but a rename and local variables.
You can ask for the 5th Fibonacci-Number with 5 calls to
fib.next ();
or with a single call to
fib (5);
Since the fibonacci-sequence increases very rapidly, you have very few calls (54) before hitting the overflow boundary. So if you repeatedly recalc the same sequence, to print the sequence, it's not a big problem. A recursive solution would be fine.
Btw.: EP64 is a very bad name.
I think this is enough:
import java.util.Scanner;
public class Fibnocci
{
public static void main(String []abc)
{
int a=0,b=1,c;
Scanner in=new Scanner(System.in);
System.out.print("Enter the Range: ");
int n= in.nextInt();
System.out.print(a+" "+b);
for(int i=0;i<n-2;i++) //n-2 because we are showing 0,1 initially.
{
c=a+b;
System.out.print(" "+c);
a=b;
b=c;
}
}
}
If you want to call this as a method then:
import java.util.Scanner;
public class Fibnocci
{
public static void main(String []abc)
{
Scanner in=new Scanner(System.in);
System.out.print("Enter the Range: ");
int n= in.nextInt();
callFibonocci(n);
}
public static void callFibonocci(int n)
{
int a=0,b=1,c;
System.out.print(a+" "+b);
for(int i=0;i<n-2;i++) //n-2 because we are showing 0,1 initially.
{
c=a+b;
System.out.print(" "+c);
a=b;
b=c;
}
}
}
You can call this method out of the class;
// Fibnocci Using c#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace CodeProject
{
class FibnocciSeries
{
public int[] FibonacciArray(int length)
{
int[] fseries = new int[length];
fseries[0] = 0;
fseries[1] = 1;
if (length == 0)
return null;
//Iterating through the loup to add adjacent numbers and create the memeber of series
for (int i = 2; i < length; i++)
{
fseries[i] = fseries[i - 1] + fseries[i - 2];
}
return fseries;
}
}
}
////////////////////
class Program
{
static void Main(string[] args)
{
FibnocciSeries fb = new FibnocciSeries();
Console.WriteLine("Please Enter Integer Length of Fibnocci series");
int length = Convert.ToInt32(Console.ReadLine());
int[] result = fb.FibonacciArray(length);
foreach(int i in result)
Console.Write(i.ToString()+ " ");
Console.ReadLine();
}
}
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