Random Number Generator then Summing

[Kris French]

Hello,

I am brand new to programming and java. I decided I wanted to learn programming so now I'm in a class. I am into my second week and my assignment is to generate 100, 3 digit numbers then add them together. I understand how vast java is, but as of right now we have only studied if/else/switch statements, so naturally I had no idea how to begin this project. After doing a little homework I was able to get the 100, 3 digit numbers generated, but I cannot add then together. I have tried a few things that I found online, but either I didn't understand them or I could not get things to work out. Can someone please help give a nudge (or at my level, a violent shove) in the right direction. I appreciate the help.

import java.util.Random;
import java.util.Math;

public class ThreeDigitGenerator {
	public static void main(String[] args) {
		int sum = 0;
				
		Random rand = new Random();
		for(int i = 1; i <= 100; i++) {		
			int randomNum = rand.nextInt((999 - 100) + 1) + 100;
			System.out.println(randomNum);
			
		}
			
	}
}

[Steve Fahlbusch]

Greetings and Welcome,

ok - first you should use the code tag for your code -- but there is so little that should not be an issue for this post but in the future please use the code tags.

Now - two issues - you are declaring a new int each time you generate a random number - you should move the declaration out of the loop and only set the value in the loop.

So for your question - if you declare a variable ( say: int sum - 0; ) outside the loop, and the right after to generate and set the random number in the loop you add that to sum.



hope this helps

-steve

[Paweł Baczyński]

Steve Fahlbusch wrote:Now - two issues - you are declaring a new int each time you generate a random number - you should move the declaration out of the loop and only set the value in the loop.

Should he? What about Bloch's Minimize the scope of local variables rule?
According to Bloch:

Effective Java (2nd Edition) wrote:The most powerful technique for minimizing the scope of a local variable
is to declare it where it is first used. If a variable is declared before it is used, it’s
just clutter—one more thing to distract the reader who is trying to figure out what
the program does.

[Campbell Ritchie]

Welcome again

I added the code tags because you are new, and doesn't it look better. I think that formula (999 − 100) + 1 will correctly find three‑digit numbers, but it is difficult to understand. I suggest you add a // comment explaining that formula
The () around 999 − 100 are unnecessary. Because + and − both associate to the left you can never get 999 − 101 from that formula.
It makes no difference to the execution where you declare the random number, but I suggest you shorten that loop. Start with
for (int i = 0; i < 1; i++)…
Note the spacing and also start with 0. The < operator is usually easier to understand than <= and you can use a very similar format for the for loops with arrays. And you can be confident that it will never throw an out of bounds exception.
As SF says, you need to declare sum; it needs an initial value because local variables don't default to 0.
If you only run the loop once, it will be easy to verify the addition by looking at the output. Then try 2 iterations and then go back to 100.

[Winston Gutkowski]

Kris French wrote:I understand how vast java is, but as of right now we have only studied if/else/switch statements, so naturally I had no idea how to begin this project. After doing a little homework I was able to get the 100, 3 digit numbers generated...

First: Welcome to JavaRanch, Kris.

Second: Congratulations on getting this far; and especially on discovering the Random class. Many beginners miss it out.

...but I cannot add then together. I have tried a few things that I found online, but either I didn't understand them or I could not get things to work out. Can someone please help give a nudge (or at my level, a violent shove) in the right direction. I appreciate the help.

Well, you've correctly declared a 'sum' field, but you then don't do anything with it. You're incredibly close, and I don't want to just hand you the answer, because you'll be much happier if you work it out for yourself.
So: What do YOU think you should do with it?

Also, a little tip for you:

for(int i = 0; i < 100; i++) { ...

will also perform a loop 100 times, and you'll find it a much more useful way to write it, because Java indexes start at 0.

HIH

Winston

[Winston Gutkowski]

Steve Fahlbusch wrote:you are declaring a new int each time you generate a random number - you should move the declaration out of the loop and only set the value in the loop.

I agree with Pawel on this one. I think Kris's code is perfect in that respect.

Furthermore, the declaration inside the loop does not declare a new variable each time it iterates - even though it admittedly looks like it does.

Winston

[Kris French]

Thank you all for your help; I learned a couple of things from that small bit of code. I am still stuck though. I have a feeling it is something simple that I'm missing. What I have now when the program runs it will still output the 100 numbers, but at the end '4950' shows up each time. I'm starting to think I have no idea what I am doing. Again, thanks for all the help.

import java.util.Random;
import java.util.Math;

public class RandomNumber{
	public static void main(String[] args) {
		int sum = 0;
				
		Random rand = new Random();
		for(int i = 0; i < 100; i++) {		
			int randomNum = rand.nextInt(999 - 100 + 1) + 100;
			System.out.println(randomNum);
                        sum += i;
		}
                System.out.println(sum);
	}
}

[Campbell Ritchie]

If you had followed my suggestion about running the loop once you would probably have worked the problem out sooner.

Look what you are adding to sum.

[Kris French]

Thanks Campbell. I re-read your suggestion, and after scratching my head for a bit, I finally understood what you meant by running the loop once. You were right; once I did that it made getting to the answer quickly. Again thank you to all who helped. I can be pretty dense sometimes. Here is the end result:

import java.util.Random;

public class RandomNumber{
	public static void main(String[] args) {
		int sum = 0;
				
		Random rand = new Random();
		for(int i = 0; i < 100; i++) {		
			int randomNum = rand.nextInt(999 - 100 + 1) + 100;
			System.out.println(randomNum);
                        sum += randomNum;
                        System.out.println(sum);

		}
	}
}

[Campbell Ritchie]

Well done

Maybe better to print sum after the end of the loop.

[Kris French]

Good call; I had the print after the loop originally. Thanks!

[Campbell Ritchie]

Youi're welcome

Minor nit‑pick‑y point. The indentation in that loop is inconsistent.

[Kris French]

It's already been fixed. ;)

[Winston Gutkowski]

Kris French wrote:It's already been fixed. ;)

Well done Kris. And be proud that you did it all yourself. That's how you learn; and it's why we don't like to just hand out answers here.

Winston

[Campbell Ritchie]

Kris French wrote:. . . I can be pretty dense sometimes. . . .

Not at all. Everybody else makes that sort of mistake, too.