How to reverse a string of numbers in a nested loop

Question

This is a homework question so I would like help, not an answer.

I'm trying to create 2 triangles out of numbers based on a number entered by the user.

"Enter a number between 2-9: "3"
1
12
123

  1
 21
321

IE2:

"Enter a number between 2-9: "5"
1
12
123
1234
12345

    1
   21
  321
 4321
54321

I have been able to get the first triangle complete. But when I add my nested loop it messes up my first triangle with the numbers developed from the nested loop. It also puts all the numbers in a straight vertical line. I've tried variations for different nest loops and even tried messing with a StringBuilder, but was still unsuccessful. Here's what I have in code so far:

import java.util.Scanner;

public class NestedLoops
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter a Number between 2-9: ");
        int width = input.nextInt();

        String r = "";
        for (int i = 1; i <= width; i++)
        {
            r = r + i;
            System.out.println(r);

        }

    }

}

Again, I'm looking for help/understanding and not just an answer.

Please hover over the homework tag and read what it says there. — Xymostech
– Xymostech, Commented Feb 17, 2013 at 3:30
do you need to use a nested loop for the assignment? You can solve this in two loops without nesting them or even 1 with some more complicated string manipulation. — twain249
– twain249, Commented Feb 17, 2013 at 3:32

Stephen C · Accepted Answer · 2013-02-17 04:01:35Z

1

There are two aspects the 2nd part of the question.

You need to generate strings with the numbers in the reverse order:
- You could do this by adding the numbers at the other end.
- You could do this by reversing the strings.
You need to arrange that there are spaces to the left.
- You could do this by adding the required number of spaces to the left end of the string.
- You could do this by using the System.out.format(...) with a template that right aligns the string in a field with the required number of characters. (OK, that's a bit too obscure ...)

Or, you can build the string in a character array or string builder rather than using string concatenation.

The "trick" is to figure out what strategy you are going to use ... before you start cutting code.

answered Feb 17, 2013 at 4:01

Stephen C

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Comments

Evgeniy Dorofeev · Accepted Answer · 2013-02-17 03:51:13Z

0

try

    int width = 5;
    // for all lines; number of lines = width
    for (int line = 1; line <= width; line++) {
        // print numbers from 1 to current line number  
        for (int n = 1; n <= line; n++) {
            System.out.print(n);
        }
        // end of line
        System.out.println();
    }
    // add empty line between triangles
    System.out.println();
    // for all lines; number of lines = width
    for (int line = 1; line <= width; line++) {
        // printing padding spaces, number of spaces = with - line number
        int nSpaces = width - line;
        for (int i = 0; i < nSpaces; i++) {
            System.out.print(" ");
        }
        // print numbers from number of current line to 1
        for (int n = line; n >= 1; n--) {
            System.out.print(n);
        }
        // end of line
        System.out.println();
    }

answered Feb 17, 2013 at 3:51

Evgeniy Dorofeev

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2 Comments

SkyVar Over a year ago

Please disregard my last comment, I didn't see where you set line back to 1 in the 2nd for loop.

Evgeniy Dorofeev Over a year ago

this is Java rule: in for (int line = 1; line <= width; line++) {..}var line is visible only inside block {}. It is different from C++

Bk Santiago · Accepted Answer · 2013-02-17 03:36:05Z

0

Can you just add another loop after your first loop like

String r = "";
String space = "";
    for (int i = width; i >= 1; i--)
    {
        r = r + i;
        System.out.println(r);

    }

Try it. not yet tested

answered Feb 17, 2013 at 3:36

Bk Santiago

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Comments

smk · Accepted Answer · 2013-02-17 03:51:21Z

0

You need to use a queue. eg. http://docs.oracle.com/javase/1.5.0/docs/api/java/util/LinkedList.html

Enque the numbers till you reach the max, and then start dequing them.

And while you dequeue, you need to apply the reverse

Queue<String> q = new LinkedList<String>();
        for (int i = 1; i <= width; i++)
        {
            r = r + i;
            q.add(r);
            System.out.println(r);

        }

        while(!q.isEmpty()){
            String j = q.remove();
             //reverse j
            System.out.println(reverse(j));
        }

I leave the reversing part for you to do :)

edited Feb 17, 2013 at 3:51

answered Feb 17, 2013 at 3:29

smk

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Comments

T I · Accepted Answer · 2013-02-17 04:32:20Z

0

public static void main(String[] args)
{
    int n = 5;

    for(int i=1; i<=n; i++)
    {            
        for (int j=(n*2), k=n; j>1; j--) 
        {
            if (k <= i) 
            {
                System.out.print(k);
            } 
            else 
            {
                System.out.print('*');
            }

            k += (j)-1 > n ? -1 : 1;
        }

        System.out.println();
    }
}

answered Feb 17, 2013 at 4:32

T I

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Comments

Giri · Accepted Answer · 2013-02-21 22:14:59Z

0

Just tried to implement in scala. Ignore if you don't like it..:-)

 class Triangle extends App
{
  val width = Console.readInt()

  if (width < 2 || width > 9)
  {
     throw new RuntimeException()
  }

  var i, j = 1;

  for (i <- 1 to width)
  {
     for (j <- 1 to i)
     {
        print(j)
     }
     print("\n")
  }

  for (i <- 1 to width)
  {
     for (dummy <- 1 to width-i)
     {
        print(" ")
     }
     for (j <- i to 1 by -1)
     {
       print(j)
     }
     print("\n")
  }
}

answered Feb 21, 2013 at 22:14

Giri

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