How to Print 2D Boolean Arrays in Java

Question

I'm trying to create a boolean array for prime numbers. I'm supposed to end up with a result like:

Java PrimeArray 9

   1 2 3 4 5 6 7 8 9

1    T T T T T T T T

2 T    T F T F T F T

3 T T    T T F T T F

4 T F T    T F T F T

5 T T T T    T T T T

6 T F F F T    T F F

7 T T T T T T    T T

8 T F T F T F T    T

9 T T F T T F T T

I'm not getting the correct result. It isn't printing in rows and columns. I'm new to programming, but I tried playing around with different features of the array with no luck. Can someone please give me some tips on this?

public class PrimeArray {
    public static void main(String[] args) {

        int n = Integer.parseInt(args[0]);

        boolean[][] rpa = new boolean[n + 1][n + 1];

        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++)

                if (i == j)
                    System.out.print(" ");
                else if ((i % j == 1))
                    System.out.print("T");
                else
                    System.out.print("F");

            System.out.print(i);
        }
    }
}

System.out.print will print without causing a line feed. – Gilbert Le Blanc Sep 18 '21 at 21:05 — Gilbert Le Blanc, Sep 18 '21 at 21:05

score 0 · Answer 1 · answered Sep 18 '21 at 21:06

0

As I can see you do not use System.out.print(i) instead of System.out.println(i) to omit line break.
Also you are not using parentheses for the inner loop, it is not so obvious to understand the loop context.
You can read more on error "can't find the symbol" here: What does a "Cannot find symbol" or "Cannot resolve symbol" error mean?

answered Sep 18 '21 at 21:06

Timur Efimov

358
2
10

Look at this part of the question: " I realize I need to print rpa[i][j], but everytime I try to do that, it tells me it can't find the symbol for i or j". (but I could misunderstand) – Timur Efimov Sep 18 '21 at 22:33

score 0 · Answer 2 · answered Sep 18 '21 at 21:18

Try this.

static int gcd(int a, int b) {
    while (b != 0) {
        int bb = b;
        b = a % b;
        a = bb;
    }
    return Math.abs(a);
}

public static void main(String[] args) {
    int n = 9;
    System.out.print(" ");
    for (int j = 1; j <= n; j++)
        System.out.print(" " + j);
    System.out.println();
    for (int i = 1; i <= n; i++) {
        System.out.print(i);
        for (int j = 1; j <= n; j++)
            if (i == j)
                System.out.print("  ");
            else if (gcd(i, j) == 1)
                System.out.print(" T");
            else
                System.out.print(" F");
        System.out.println();
    }
}

output:

  1 2 3 4 5 6 7 8 9
1   T T T T T T T T
2 T   T F T F T F T
3 T T   T T F T T F
4 T F T   T F T F T
5 T T T T   T T T T
6 T F F F T   T F F
7 T T T T T T   T T
8 T F T F T F T   T
9 T T F T T F T T

Thanks! I put the second part of your solution into my boolean array and tweaked it a little bit. It works great! — gizmo.java, Sep 21 '21 at 18:46

score 0 · Answer 3 · answered Sep 18 '21 at 21:59

Here's a more elaborate version of the code. Thanks to saka1029 for the greatest common denominator code.

Here are the results of one of my test runs.

     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
  1     T  T  T  T  T  T  T  T  T  T  T  T  T  T
  2  T     T  F  T  F  T  F  T  F  T  F  T  F  T
  3  T  T     T  T  F  T  T  F  T  T  F  T  T  F
  4  T  F  T     T  F  T  F  T  F  T  F  T  F  T
  5  T  T  T  T     T  T  T  T  F  T  T  T  T  F
  6  T  F  F  F  T     T  F  F  F  T  F  T  F  F
  7  T  T  T  T  T  T     T  T  T  T  T  T  F  T
  8  T  F  T  F  T  F  T     T  F  T  F  T  F  T
  9  T  T  F  T  T  F  T  T     T  T  F  T  T  F
 10  T  F  T  F  F  F  T  F  T     T  F  T  F  F
 11  T  T  T  T  T  T  T  T  T  T     T  T  T  T
 12  T  F  F  F  T  F  T  F  F  F  T     T  F  F
 13  T  T  T  T  T  T  T  T  T  T  T  T     T  T
 14  T  F  T  F  T  F  F  F  T  F  T  F  T     T
 15  T  T  F  T  F  F  T  T  F  F  T  F  T  T

I used String.format and StringBuilder to create the table. This code works with any positive maximum value, although large numbers will be difficult to display.

I broke my code into methods. Sure, this code could be written to be shorter, but the goal is for people reading the code to understand the code.

public class RelativePrimeNumbers {

    public static void main(String[] args) {
        try {
            int maximum = Integer.valueOf(args[0]);
            RelativePrimeNumbers rpn = new RelativePrimeNumbers();
            System.out.println(rpn.createRelativePrimeTable(maximum));
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }

    }
    
    public String createRelativePrimeTable(int maximum) {
        int length = Integer.toString(maximum).length() + 1;
        String numberFormatter = "%" + length + "d";
        String letterFormatter = "%" + length + "s"; 
        
        StringBuilder builder = new StringBuilder();
        builder.append(createHeaderLine(maximum, numberFormatter, letterFormatter));
        builder.append(System.lineSeparator());
        for (int index = 1; index <= maximum; index++) {
            builder.append(createDetailLine(index, maximum, 
                    numberFormatter, letterFormatter));
            builder.append(System.lineSeparator());
        }
        return builder.toString();
    }
    
    private StringBuilder createHeaderLine(int maximum, String numberFormatter, 
            String letterFormatter) {
        StringBuilder builder = new StringBuilder();
        builder.append(String.format(letterFormatter, " "));
        for (int index = 0; index < maximum; index++) {
            builder.append(String.format(numberFormatter, (index + 1)));
        }
        return builder;
    }
    
    private StringBuilder createDetailLine(int index, int maximum, 
            String numberFormatter, String letterFormatter) {
        StringBuilder builder = new StringBuilder();
        builder.append(String.format(numberFormatter, index));
        for (int jndex = 1; jndex <= maximum; jndex++) {
            if (index == jndex) {
                builder.append(String.format(letterFormatter, " "));
            } else if (calculateGCD(index, jndex) == 1) {
                builder.append(String.format(letterFormatter, "T"));
            } else {
                builder.append(String.format(letterFormatter, "F"));
            }
        }
        return builder;
    }
    
    private int calculateGCD(int a, int b) {
        while (b != 0) {
            int bb = b;
            b = a % b;
            a = bb;
        }
        return Math.abs(a);
    }

}

How to Print 2D Boolean Arrays in Java

3 Answers3