I am learning Java and discovering that a little bit of knowledge is confusing.
The goal is to write a method that is the equivalent of the n! function. I am using a for loop to multiply a variable declared outside the method. All I get back is 0.
What am I doing wrong?
//
// Complete the method to return the product of
// all the numbers 1 to the parameter n (inclusive)
// # param n
// # return n!
public class MathUtil
{
public int total;
public int product(int n)
{
for (int i = 1; i == n; i ++)
{
total = total * i;
}
return total;
}
}
There is actually a lot of problems in your code:
It does not make sense to make it an instance method.
You have not initialize your total to a reasonable value.
The condition in your for loop is wrong
Method is not given a meaningful name
Messy indentation
(The list keeps growing...)
So here is a slightly improved version
public class MathUtil
{
//
// Complete the method to return the product of
// all the numbers 1 to the parameter n (inclusive)
// # param n
// # return n!
public static int factorial(int n)
{
int total = 1;
for (int i = 1; i <= n; i ++)
{
total = total * i;
}
return total;
}
}
so that you can call it as MathUtil.product(123) instead of some weird new MathUtil().product(123)
Personally I would rather do something like
result = n;
while (--n > 0) {
result *= n;
}
return result;
You are missing the initialization. Now I added the default value to 1. And you also have to change the condition. The for loop has to go on as long as the var i is smaller or equal to n.
public class MathUtil
{
public int total = 1;
public int product(int n)
{
for (int i = 1; i <= n; i ++)
{
total = total * i;
}
return total;
}
}
you didn't initialized total, therefore it's 0. Whenever you multiply 0 with anything, you will get 0 as result.
public int total = 1;
public int product(int n) {
for (int i = 1; i <= n; i++) {
total = total * i;
}
return total;
}
you havent initialized total. It defaults to 0. Then when you multiple anything with total, you get 0 as result
Related
I'm doing a task where you input 3 parameters which are size, minimum value and maximum value. It then is meant to return a random number between the minimum and maximum value of the size inputted. There are also other validations such as if the min is more than the max which I've already done.
I am able to do the task using integer instead of short. As soon as I change the data type a bunch of errors come.
Below is what I've done so far, it works as expected but I am pretty sure that there is not meant to be a bottom return null, I get errors when I delete it. On the second loop, it should return the array instead of doing a system print line. The other issue is the data types at the top, it should be short maxVal and short minVal instead of int but I can't get it to work with short.
I would very much appreciate all help. Thanks!
public static ArrayList<Short> RandomArray1(int n, int maxVal, int minVal){
if(n <= 0) {
return null;
}
if(minVal > maxVal) {
return new ArrayList<Short>();
}
ArrayList<Integer> ran = new ArrayList<Integer>();
Random rand = new Random();
for(int i = 0; i < n; i++) {
int result = rand.nextInt(maxVal-minVal) + minVal;
//System.out.println(result);
ran.add(result);
}
for (int i = 0; i < ran.size(); i++) {
System.out.println(ran.get(i));
//return (ArrayList<Short>)ran.get(i);
}
return null;
I would do it like this.
first, method names by convention should start with lower case letters.
Use the method to generate the values and return the list
return interface types as opposed to implementation types (e.g. List)
throw exceptions if the arguments don't satisfy the requirements.
Note, having to cast the arguments to shorts is cumbersome but it prevents errors at compile time. Otherwise you may want to throw an additional run time exception if the values aren't within Short.MIN_VALUE and Short.MAX_VALUE.
public class RandomShorts {
public static void main(String[] args) {
List<Short> shortList = randomList(20, (short)200, (short)99);
shortList.forEach(System.out::println);
}
public static List<Short> randomList(short n, short maxVal,
short minVal) {
if (n <= 0 || minVal >= maxVal) {
throw new IllegalArgumentException(
"\nn must be > 0\nminVal must be < maxVal\n");
}
List<Short> ran = new ArrayList<>();
Random rand = new Random();
for (int i = 0; i < n; i++) {
short result =
(short) (rand.nextInt(maxVal - minVal) + minVal);
ran.add(result);
}
return ran;
}
}
If you just want to return a single random number using the supplied arguments, they you can do it like this.
public static short randomShort(int n, short maxVal, short minVal) {
return (short)((Math.random()*(maxVal - minVal))+minVal);
}
For adding the short to random list since there is not Random.nextShort(), you'll have to substitute the line
int result = rand.nextInt(maxVal-minVal) + minVal;
for
short result = (short) (rand.nextInt(maxVal-minVal) + minVal);
Where then you can add to your ran array instead of the second for loop returning the ArrayList as a short
public static short[] generateRandomShortArray (int sampleSize, short min, short max) {
short[] result = new short[sampleSize];
for (int index = 0; index < result.length; index++) {
result[index] = (short)((Math.random() * (max-min)) + min);
}
return result;
}
If an array list is required just return Arrays.asList(result);
static int count = 0;
static long[] cache = new long[3000];
static {
cache[0] = 1;
}
public static void main(String args[]) {
for (int i = 0; i < 100; i++){
factorial_with_cache(2999);
}
System.out.println(count);
}
public static long factorial_with_cache (int n){
if (cache[n] != 0){
return cache[n];
}
cache[n] = n * factorial_with_cache(n - 1);
count++;
return cache[n];
}
I built a function that calculates factorials using a cache (ignoring overflow).
But its runtime isn't any better compares to non-caching function and I found that caching isn't working correctly.
Because I expected a variable 'count' to be 2999 after loop but I fount it is 293465 which is a lot more than that. (without loop, it prints 2999)
What is to wrong with this function?
It is because the range of long datatype:
long 8 bytes (-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807)
your factorial gives the positive values till you are searching for factorial of 25.
And latter 25 , the value of factorial which are calculated is coming negative (It means you are overflowing long) and your count will work as expected till 65 factorial is calculated (till negative value) and then the value for factorial 66 it reaches to 0..
just try below by printing factorial:
for (int i = 0; i < 100; i++) {
long fact=factorial_with_cache(66);
System.out.println(fact);
}
System.out.println(count);
And the count will be printed as
(forLoopCount*(number-65))+65 , In your case (100*(2999-65))+65 is 293465
becuase the value for factorial it traces back in cache which is not zero is 65th element (at 64 index).
So, let me break it down to you!.
The factorial value of 2999 is 13831198678126180285189556976955373903170397316439366392298225525658528550411773166953335884059334473358394878461205706165250290371348635931517800720356113203873679301399725840687554463393227601721573232498656280819242922032367667908703698446747042372944385912938442812976579204340368770976791467161792908482026536763476497016597962706920837208377639001547222987372530067858391829660217859289638428364594156664891566396944031170356121274762354507422848113071258383496196034678939603033097637499047300169646331468238450491051510529147169986724755958362486789324705690789582425714662973561230539501202732819906031261484507986168459203695503720787715306962036185816958171315101812071237299383615730698142124734425009998777928314671591890471560274735833304904959453223240212984890196653392856493421129589037623689334983609133572374939825462133789731181451373437595533811597516653047843373551034468875167743673579450500731701049557870779579798365517154053116338596649981357437001475641612713380485162624246015285341187194401439315317685276897783748963573748717156161769574677089322363037849660911158458585633121725465474927244487503626338581145799640164089032772344644035725447994450814858680158855133471471566493328118184865933128900946476171137729637721898436949504508364668715924682335777135513931476001478897301228489756952423900549337170685817482869322924594934553551003036605213030431370211238540513634600045019557208087444634144722209572173742863424856277834965975623123264292792298280478857955478985025157988294085407394460004866920398953072534411602822485072110137246001443714741975414563519594304969324628586261528579942898463434127945755294142551435479172585965738348357120600576683876141492098075828585209276254499589065093789637861306274833094913863894376778010733315584027638295254781954610771181735025318637354022424432317128707664886202509105961484149955116054723079614963020498616978124480963271628142440605710424177963844835527047941127068625126602868360296587251294298750977877916818183056200089076530408509618194827562218441437670451251604136520412562026002566902646513949635716489125951469176102624329367970981707309954931234483752478815193819919046940901995441721784302194857041195090070644484327035189466269961364993448728359341757679578928014056475745993488101416681676935545677586672113973814268569600810283522630702882325818379846373279545389063433426048793378977494110923865060198731023168079278694206856962403099309932476468192292446106625814895181431600107247018896474570181676290139870638357579666677214004824737774431290539343185204668200955800140961796618216776009476171707432009598580600540607745366106990480349004177689104965384412262911661517983201465683450782588737131971506423474040743808999564029154506993562074784937792064798377314289371162177187409391071658827235127282051775532062510098826200386957421213481307419200642164870305628383671047981858132767730852384720717147489707456839716365953949812050321036644546531520598864465617319535112877058774715688947419064131580843908826519916664427749093269298823584351338064696733536570891584395455175134404252585951310232510018865069402485833157299381484590749402629481111706181867189617902727630965072766713218125649095472925555254271796579113386470181227145145782044748230689023360927474548519150294185556719080917603439047213543803560461443641342427106320294937680221742056517608545727954154981431896286464615575574564733158523773515339421141910343038451177576269394257673865595696145764470705694837193543322565755533887878425653323007167374034984753189092864395172089482366791478206807334511797067912797698473565007162069130217730303607114778083832632094745399523891591282708699147685012083622118240243657099361565732179240926631053400186908352669170802498149387797404799372830916093992885059492849249732904712828347430127984696856947346045759045222567855013693926840231677378744507230913510338446973920611359605911891548916357205533551076812967546595355169668436624145148563105200443314135704098338484590507017314148504265121657071624074064576429719944333914132659197510167106279294498294744074169106272133146311875939034291810862931519619247593463572388793752634599383049481842295290543310338170244405184924421525345305394766236107745856279305711691002008912764098229027010250648673458812712892369111259770860752452145178192387601343493641928636582411137977634690746620001816089765370131213375846513299799140007308422976207521653093045456515661082438164698099164737736238987887604321282063792704869736900344337373311421647222149710103632831178455467277094227704168859462141968845732336969092143110178250432445572309805015956983080592707178326499816577452815043403343240119997858835910187898983513330297084001603373363523211048122726398082654521129700828750257789083133267241150542349616716479447911942146684493824985807109624549493196517947945459425200477792599605375618154135498044805373036317551108920542553407043626403888641459832178279343412021431660661847004770522200579839513142002891066188195561545147576726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find it yourself
I know its a lot of scrolling :P
Okay, now during your iterations, factorial of 20 will be 2,432,902,008,176,640,000 and factorial of 21 will be 51,090,942,171,709,440,000 and maximum value for long in java is 9,223,372,036,854,775,807 so, here your cache[19] will be -4,249,290,049,419,214,848 and cache[65] will be 0. which makes all values till cache[2999] to 0. So, every time you call the method factorial_with_cache, it's not serving from cache, but calculating every time (because you're checking if(cache[n] != 0) then return from cache else calculate) causing the count value 293465 instead of 2999.
I've modified the code a little for your understanding.
public class Test2 {
static int count = 0;
static long[] cache = new long[3000];
static {
cache[0] = 1;
}
public static void main(String args[]) {
factorial_with_cache(2999);
}
public static long factorial_with_cache (int n){
if (cache[n] != 0){
return cache[n];
}
cache[n] = n * factorial_with_cache(n - 1);
System.out.println("Factorial(" + n + ") is " + cache[n]);
count++;
return cache[n];
}
}
This will print your calculated fact values.
public class Test2 {
static int count = 0;
static long[] cache = new long[3000];
static {
cache[0] = 1;
}
public static void main(String args[]) {
for (int i = 0; i < 100; i++){
factorial_with_cache(2999);
System.out.println("i[" + i + "] count[" + count +"]");
}
}
public static long factorial_with_cache (int n){
if (cache[n] != 0){
return cache[n];
}
cache[n] = n * factorial_with_cache(n - 1);
count++;
return cache[n];
}
}
This will print count value for each iteration.
I want to calculate the factorial of a number with a get method (I must solve a bigger problem). Here's what I tried and it returns 1:
public Sigma() {
n = -1;
}
public Sigma(int n) {
n = n;
}
private int Facto(int n) {
for (int i = 1; i <= n; i++) {
result = result * i;
}
return result;
}
public int getFacto() {
return Facto(n);
}
The problem is that, in your constructor, you type n = n rather than this.n = n. The problem with this is that the local variable inside the constructor is assigned, rather than your class's field. this.n refers to the field n and is what you want.
You are receiving an output of 1 because the default value of all primitive number fields is 0. Using your code, 0! = 1 (which is correct), so that's your output no matter what you pass into the constructor, as the constructor ignores its parameter.
On an unrelated note, please use camelCase rather than UpperCase for method names (and field names). UpperCase should only be used for classes/interfaces/enums/annotations. Also, result = result * n may be simplified to the (almost) equivalent statement result *= n.
For the factorial you need to initialize result in the facto function, like this
private int Facto(int n)
{
int result = 1;
for (int i = 1; i <= n; i++)
{
result = result * i;
}
return result;
}
Fixing up and tidying the converter when I noticed that it somehow gives out incorrect conversions.
For example, when creating a new number to convert using BinaryNumber bn1 = new BinaryNumber("1011"); and then asking it to give out a result with System.out.println(bn1.convertToDecimal()); it prints out 3 instead of the correct result of 11.
I'm almost sure I got the actual conversion wrong but going through it in my head I can't find the mistake.
public class BinaryNumber {
private String n;
public BinaryNumber(String pn) {
n = pn;
}
public String getN() {
return n;
}
// Creating the .convertToDecimal()
public int convertToDecimal() {
int bitPosition = 0;
int sum = 0;
for (int i = n.length() - 1; i >= 0; i--) {
sum = sum + (int) Math.pow(2, bitPosition) * (n.charAt(i) - 48);
}
return sum;
}
// Creating the .add to add the two different binary numbers after
// converting
public int add(BinaryNumber bn2) {
return convertToDecimal() + bn2.convertToDecimal();
}
// Creating the .sub to subtract the two different binary numbers after
// converting
public int sub(BinaryNumber bn2) {
return convertToDecimal() - bn2.convertToDecimal();
}
}
You just need to increment your bitposition variable.
int bitPosition = 0;
int sum = 0;
for (int i = n.length() - 1; i >= 0; i--) {
sum = sum + (int) Math.pow(2, bitPosition++) * (n.charAt(i) - 48);
}
return sum;
First of all, Java has a built in binary system, using the primitive int type. You can use this by putting 0b before the number in binary form.
If you are doing this for learning purposes, then here's what's happening:
On each iteration, the value of bitPosition is always 0, because you never update it. Of course, you want to increase it with each iteration. This can be done simply by changing
Math.pow(2, bitPosition)
to
Math.pow(2, bitPosition++)
with the ++ after the variable to change it after it is referenced.
public class FibonacciGenerator
{
//instance variables
private int recent ; //one values ago
private int previous ; //two values ago
private int n ; // the number of values returned so far
/**
Constructs the generator by setting the instance variables to 1
*/
public FibonacciGenerator()
{
recent = 1 ;
previous = 1 ;
n = 0 ;
}
/**
Produces the next Fibonacci number
#return the next in the Fibonacci sequence
*/
public int next()
{
n ++ ;
if (n == 1) return 1 ;
if (n == 2) return 1 ;
int result = recent + previous ;
//----------------Start below here. To do: approximate lines of code = 3
// 1. Update previous and recent, and 2. return result.
previous++;
recent++;
return result;
//----------------------End here. Please do not remove this comment. Reminder: no changes outside the todo regions.
}
}
import java.util.* ;
public class FibonacciGeneratorTester
{
public static void main(String[] args)
{
System.out.println("The 1st Fibonacci number is: "
+ getFibonacci(1)) ;
System.out.println("The 10th Fibonacci number is: "
+ getFibonacci(10)) ;
}
/**
A static method to return the n'th Fibonacci number
#param n the index of the Fibonacci number
#return the n'th Fibonacci number
*/
public static int getFibonacci(int n)
{
FibonacciGenerator generator = new FibonacciGenerator() ;
int result = 0 ;
//----------------Start below here. To do: approximate lines of code = 4
// 1. Write a for-loop that calls the generator n times 2 . return the last result of the call.
for (int i = 1; i <= n; i++){
generator.next();
return result;
}
}
}
missing return statement second last curly brace highlighted. Is my for loop correct?.......................................................................................................................................................
Your for loop begins:
for (int i = 1; i <= n; i++){
If n was less than 1, it would immediately exit, and there is no return statement between the end of the loop and the end of the method. That is the missing return statement.
Look at this:
public static int getFibonacci(int n)
{
FibonacciGenerator generator = new FibonacciGenerator() ;
int result = 0 ;
//----------------Start below here. To do: approximate lines of code = 4
// 1. Write a for-loop that calls the generator n times 2 . return the last result of the call.
for (int i = 1; i <= n; i++){
generator.next();
return result;
}
}
It must return an int. Look closer at your loop. What is n = 0?
That's right it won't return anything. That is not allowed here.
public static int getFibonacci(int n)
{
FibonacciGenerator generator = new FibonacciGenerator() ;
int result = 0 ;
//----------------Start below here. To do: approximate lines of code = 4
// 1. Write a for-loop that calls the generator n times 2 . return the last result of the call.
for (int i = 1; i <= n; i++){
generator.next();
}
return result;
//put a return statement here, instead of in your loop.
}
TRY THIS CODE:
import java.util.*;
public class Fibonnacci{
public static void main(String args[]){
int num;
Scanner in=new Scanner(System.in);
System.out.println("Enter an integer");
num = in.nextInt();
System.out.println("Fibonacci Series");
int sum=0,a=0,b=1;
for(int i=0;i<num;i++){
System.out.print(" "+sum);
a = b;
b = sum;
sum = a + b;
}
}
}