Can anyone tell me what's wrong with my program?

Nala

Hi all;
I've repeat Computer Science exams in 2 weeks and I'm struggling. Can anyone tell me whats wrong with this program?
All I'm trying to do is

-Initialise an array of 10 randomly generated integers between 1-20.

1. //This program will randomly generate integers in the range
2. // 1-20 and store them in an array. The program will then
3. //ask the user to enter an integer and see if that integer is in
4. //the array, and display its position if it is in the array,
5. //and an error message if it isn't.
6.
7. import java.util.*;
8. class prac9q3
9. {
10. public static void main(String[]args)
11. {
12.
13. //Initialise variables
14. int userNum;
15. int START = 1;
16. int END = 20;
17. int i;
18.
19. //Initialise array
20. int [] arrayNums = new int [10];
21. for(i = 0; i < arrayNums.length; i++)
22.
23. arrayNums= (int)(Math.random()*10);
24. while (arrayNums >= START && arrayNums =< END) {
25. System.out.println(arrayNums); }
26.
27. }
28. }
29. }
30.

When I compiled I got:

19: illegal start of expression
20: ';' expected
24: 'class' or 'interface' expected
24: 'class' or 'interface' expected


I know it probably looks so simple but I really am on the verge of just giving up and cancelling my exams.

aidan_walsh

Your first for loop is missing an opening brace. And you're not taking in input anywhere... But you knew that much already...

briano

What errors are you getting?

Hobbes

I think the problem is you didn't study enough.

You have an extra "}". Remove one or fix it.
You never type "=<" it is always the equates last eg. +=. -=. >=. <=

Also I am not entirely sure what your trying to do with the While loop. Can you explain what you are doing?

Edit: Also loops are more readable if you wrap the command in a brace, even if there is only one command.

Nala

I've edited the post now to include the errors I got.
I changed the last for-loop to a while-loop.

stas

import java.util.*;
class prac9q3
{
	public static void main(String[]args)
	{
	
		//Initialise variables
		int userNum;
		int START = 1;
		int END = 20;
		int i;

		//Initialise array
		int [] arrayNums = new int [10];
		
		for(i = 0; i < arrayNums.length; i++) [b]{[/b]
			arrayNums[i]= (int)(Math.random()*10);
			while (arrayNums[i] >= START && arrayNums[i] [b]<=[/b] END) 
			{
				System.out.println(arrayNums[i]); 
			}
		}
	}
}

In fact you had two errors there:
- No opening clause for the "for" loop (mentioned above)
- There's no such operator as =<, it should be <= instead.

Hobbes

stas wrote:

		for(i = 0; i < arrayNums.length; i++) [b][/b]
			arrayNums[i]= (int)(Math.random()*10);

In fact you had two errors there:
- No opening clause for the "for" loop (mentioned above)
- There's no such operator as =<, it should be <= instead.

I disagree. He should have a brace loop, but it shouldn't close where you have it.

Nala

Hobbes wrote:

I think the problem is you didn't study enough.

You have an extra "}". Remove one or fix it.
You never type "=<" it is always the equates last eg. +=. -=. >=. <=

Also I am not entirely sure what your trying to do with the While loop. Can you explain what you are doing?

Edit: Also loops are more readable if you wrap the command in a brace, even if there is only one command.

The while loop is for making sure the randomly generated number is between 1 and 20.

stas

Spot on Hobbes! Problem was the initial code wasn't formatted properly and I didn't pay much attention to actual logic, rather trying to fix the *compilation* problem. So here goes then:

import java.util.*;
class prac9q3
{
	public static void main(String[]args)
	{
	
		//Initialise variables
		int START = 1;
		int END = 20;

		//Initialise array
		int [] arrayNums = new int [10];
		
		for(int i = 0; i < arrayNums.length; i++)
			arrayNums[i]= (int)(Math.random()*10);
		
		while (arrayNums[i] >= START && arrayNums[i] <= END) 
			System.out.println(arrayNums[i]); 
	}
}

Hobbes

neev wrote:

The while loop is for making sure the randomly generated number is between 1 and 20.

read it again and tell me what the code is doing.

stas

neev wrote:

The while loop is for making sure the randomly generated number is between 1 and 20.

Well, it doesn't look quite right. Imagine the first number in your array is greater than value of END or lesser than START. What happens is it will break right on the first one and nothing will be printed in the console.

Nala

Stas I tried your code but it c"couldnt resolve symbol" for i so I initialised i and re-comiled without errors but when I ran it it threw and ArrayIndexOutOfBoundsException at line 18(the while-loop).

stas

Sorry :-) Try this.

import java.util.*;
class prac9q3
{
	public static void main(String[]args)
	{
	
		//Initialise variables
		int START = 1;
		int END = 20;

		//Initialise array
		int [] arrayNums = new int [10];
				
		for(int i = 0; i < arrayNums.length; i++)
			arrayNums[i]= (int)(Math.random()*10);
		
		
		for(int i = 0; i < arrayNums.length; i++) 
			if (arrayNums[i] >= START && arrayNums[i] <= END) 
				System.out.println(arrayNums[i]); 
	}
}

Hobbes

Stas you shouldn't just be giving him the code.

I asked him to explain what the while loop was doing as it is clear he doesn't know. If you can't explain exactly what your code is doing then your not ready for the exam.

Edit: removed the last part as it appears that Stas gave you the answer to your exam question(well almost). Lets hope you can explain what it does.

Btw, any IDE would of told you exactly what your problems were with the code. Which leads me to believe you haven't studied at all.

stas

Well, Hobbes you maybe right. But guy asked for help with his particular problem. He didn't ask to teach him Java. And I'm not a teacher nor I want to be.

Nala

Hobbes wrote:

If you can't explain exactly what your code is doing then your not ready for the exam.

Believe me I'm not in any way ready for the exam!

Hobbes wrote:

Also Neev, you code does not do what the test tells you what it should do. All stas has done is make your code compile.

It compiled alright and printed out 8 numbers instead of 10, dont know why.

P.S. I'm a girl!

Hobbes

neev wrote:

Believe me I'm not in any way ready for the exam!



It compiled alright and printed out 8 numbers instead of 10, dont know why.

Write out exactly what you think each part is doing.

P.S. I'm a girl!

Which means what exactly?

Nala

Hobbes wrote:

Write out exactly what you think each part is doing

OK...

import java.util.*;
Imports the Java utility package so I can use the Math class

class prac9q3 Creates the class file
{
public static void main(String[]args) Main method
{

//Initialise variables
int START = 1; Initialises 2 integers for the limits 1 and 20, i.e. the range the array's elements must fall into
int END = 20;

//Initialise array
int [] arrayNums = new int [10];Creates a 1D array with 10 elements of type int

for(int i = 0; i < arrayNums.length; i++) Goes through each element of the array
arrayNums= (int)(Math.random()*10);Randomly generates the array's elements. The "*10" bit is because math.random only generates numbers between 0 and 1 which are not integers.


for(int i = 0; i < arrayNums.length; i++) Same as the previous for-loop
if (arrayNums >= START && arrayNums <= END) Checks the numbers are between 1 and 20 and prints them if so.
Now I think I know why it only printed 8 numbers when I ran the program.
System.out.println(arrayNums); //Prints the array.



} Ends main
} Ends class

stas

Hobbes wrote:

Which means what exactly?

Means i shouldn't have been calling her a guy :-)

I guess I know understand what this program should be doing, but I won't post the code here. Let's have it your way Hobbes.

stas

Neev, you don't need java.util there as Math is located in java.lang package which is imported automatically by a compiler.

Nala

Oh ok. I'm just trying to figure out how to make sure the number is between 1-20 without using an if-statement so all 10 numbers are printed every time.

Hobbes

Imports the Java utility package so I can use the Math class

Math is part of java.lang.* and because of this does not require to be imported.

Randomly generates the array's elements. The "*10" bit is because math.random only generates numbers between 0 and 1 which are not integers.

Correct, but your exam says it must be a number between 1 and 20. Also with random, your number will be 0-10 (0-20 when fixed) as Random can return 0.

Checks the numbers are between 1 and 20 and prints them if so.
Now I think I know why it only printed 8 numbers when I ran the program.

Great, but also note that the random portion will never fall outside of the check you are making here, so it is kind of not needed. It will be later when you add the inputting section of the question.

Another thing to note about the last IF statement, is you are using a short circut AND ( && ). What this does if the first statement is false it will automatcally ignore the second statement. So try to right the if statement where the first statement is more efficent.

eg. Testing for null before other test which would fail on a null, or first statement is more likely to be false then the second statement.

naitkris

ok, just spent a little time on this (repeating 3 subjects in 2nd year CS myself btw - didn't study for the exams). anyways, the code below does no error checking and is very inefficient (put it together very quickly) and also does not compile as it is not finished (may finish it later when i get home if there's nothing on tv). i should note i would use a Comparator (or something similar) for the last part but seeing as i have a bus to catch i don't have that extra time to do that or fix the code up.

import java.util.*;
import java.io.*;

/**
* This program will randomly generate integers in the range 1-20 and store them
* in an array. The program will then ask the user to enter an integer and see if
* that integer is in the array, and display its position if it is in the array,
* and an error message if it isn't.
*/

public class prac9q3 {

   public static void main(String[] args) {

      //Initialise variables
      int userNum;
      int START = 1;
      int END = 20;
      int i;

      //Initialise array
      int [] arrayNums = new int [10];

      // Loop to add integers to the array until it is full
      for(i = 0; i < arrayNums.length; i++) {
         // Generate random number between 1 and 20
         arrayNums[i] = (int)(Math.random()*20);
         // Print out on a new line that number
         System.out.println(arrayNums[i]);
      }

      InputStreamReader in = new InputStreamReader(System.in);
      BufferedReader bufReader = new BufferedReader(in);

      String line = null;
      
      while (true) {
         System.out.println("Enter an integer to check if it is in the array:");
         line = bufReader.readLine();
         if (line == null) {
            break;
         }
         for(int j = 0; j < arrayNums.length; j++) {
            String temp = "" + arrayNums[j];
            if(temp == line) {
               System.out.println("True");
            }
            // not implemented below, will loop through until all array length checked though
            else if() {
               System.out.println("False");
            }
         }
      }
   }
}

Hobbes

if (line == null) {

Will never happen.

if(temp == line) {

If I was to take a guess, that would work but it is VERY BAD and should never be done.

Nala

Thanks for all your help guys. I'm sure I will be in a muddle all over again tomorrow but hopefully it will all come right in the end.

40%...thats all I want...just 40%!!!

stas

Hobbes wrote:

if(temp == line) {

If I was to take a guess, that would work but it is VERY BAD and should never be done.

You sure it would? What are the odds those objects are the same? My bet would go for temp.equals(line) instead :-)

Hobbes

stas wrote:

You sure it would? What are the odds those objects are the same? My bet would go for temp.equals(line) instead :-)

Yea I was taking a guess.

String a= "Hello";
String b= "Hello";
String c = new String "Hello";

a == b // true
a == c // false
a.equals(c) // true

So as the string was the same contents and it wasn't generating a new string it would probably be the same, but as you said equals() is the way to go.

OfflerCrocGod

Try this...may work

class some{
 public static void main(String[] args){
   int[] x = new int[10];
   int start=0;
  
   while(x[9]==0){
    int temp = (int)(Math.random()*21);
    
    if(1 <= temp && temp <= 20){
     x[start]=temp;
     start++;   
    } 
   }
   
   for(int i=0; i<x.length; i++){
    System.out.println(x[i]);
   }   
 }
}

stevenmu

(int)(Math.random()*21);

Will that not give 0 to 21 ?
There is a really easy, obvious way of making sure it creates 1 to 20 in that line, it's one of those things that's so obvious you feel like putting your head through a monitor when you find out ......

The Corinthian

Hobbes wrote:

Which means what exactly?

It means she can't program. Or drive. Or go to a toliet without a support group. Everyone knows that.

Hobbes

person wanted help for repeat exam not answer to exam or homework, so code should be allowed

There is no point giving someone working code, because it stops them from learning and building a 'cut and paste' mentality to exams.

If they can explain what the code is doing even if it is wrong is better then just handing over code. That way you can explain what parts are wrong.

Nala

The Corinthian wrote:

It means she can't program. Or drive. Or go to a toliet without a support group. Everyone knows that.

I'm actually the ONLY girl I know who goes to the toilet by herself!

Nala

Hobbes wrote:

There is no point giving someone working code, because it stops them from learning and building a 'cut and paste' mentality to exams.

If they can explain what the code is doing even if it is wrong is better then just handing over code. That way you can explain what parts are wrong.

That's a good point but sometimes I need a bit of code, like methods.

Nala

OK...

This is the finished program. Ot will compile but throws an ArrayIndexOutOfBoundsException when I run it. Has anyone any ideas on how to fix this?

1. //Write a Java program that randomly generates 10 integers
2. //in the range 1 – 20 and stores them in an array.
3. //You should then ask the user to enter a number and search
4. //the array to determine if the number entered is in the array.
5. //If the number is found in the array display the location
6. //the number was found at, otherwise display a message
7. //stating the number entered is not in the array.
8.
9. class sec6q1
10. {
11.
12. //Initialise read only variables
13. final static int SIZE=10;
14. final static int END=20;
15.
16. public static void main(String[]args)
17. {
18.
19. //Initialise variables
20. boolean valid = false;
21. boolean found = false;
22. int number = 0;
23. int index = 0;
24. int i = 0;
25.
26. //Initialise the array
27. int numArray[] = new int[SIZE];
28. for (i=0; i<numArray.length; i++);
29.
30. //Randomly generate 10 numbers in the range for the array
31. for (i = 0; i<numArray.length; i++); {
32. numArray = (1 + (int) (Math.random() * END) );
33. }
34.
35. //Get user input, ensuring the number is between 1-20,
36. // i.e. it is valid
37.
38. //Use a while-loop to ensure the number is valid.
39. //If !valid = false, the number is not in the range.
40.
41.
42. while (!valid) {
43.
44. System.out.println("Please enter a number between 1-20");
45. number=SavitchIn.readLineInt();
46.
47. //Determine it is in the range
48. if(number>0 && number<=END)
49. valid = true;
50.
51. }
52. //ends while
53.
54. //Search the array for the user's input while we
55. //are still within bounds and we have not found
56. //what we are looking for.
57. // !found = true because found = false.
58.
59. for(i = 0; i < numArray.length && !found; i++) {
60.
61. if(numArray == number) {
62. index = i;
63. found = true;
64. }
65. } //ends for
66.
67. //Display the results
68. if(found)
69. System.out.println(number + " found at position "+index+ " in the array");
70. else
71. System.out.println(number + " not found in the array");
72. }
73. }

Hobbes

neev wrote:

OK...

This is the finished program. Ot will compile but throws an ArrayIndexOutOfBoundsException when I run it. Has anyone any ideas on how to fix this?

No one can run you code if you put line numbers on it like that. Use the [ C O D E ] tags.

Your Exception should of told you what line number is causing the problem.

ressem

Line 31, you left a semicolon in the wrong place after cutting and pasting the previous line, so at line 32, i = 10.
Therefore out of bounds.

Have you learned about using an IDE, like Eclipse (free download), for your course,
to let you step through your application line by line, and see, as in this exercise, that line 32 wasn't hit till the loop was finished, and the value of i used when the exception occurred on line 32 will be available in the variables window.

That was day 1 of the course when I was learning C back in the day and is essential for finding mistakes easily.

Nala

UPDATE

Got the results this morning............





I PASSED!!!!!!!!!

Can't believe it tbh, exam was tough and I only got 60% of it done.
Thanks to everybody for helping me out!

Neev

Hobbes

Congrats. Free positive rep for you (when the board lets me)