Invert incrementing triangle pattern

user_15309982 0
Viewed 623

I am writing some code that creates a incrementing number right angle triangle that looks something like this:

           1
         1 2
       1 2 3
     1 2 3 4
   1 2 3 4 5
 1 2 3 4 5 6

I am unsure on how to make my code output a triangle to that shape as my code outputs the same thing except inverted.

This is the code I have:

public class Main {
    public static void main(String[] args) {
        int rows = 6;
        for (int i = 1; i <= rows; ++i) {
            for (int j = 1; j <= i; ++j) {
                System.out.print(j + " ");
            }
            System.out.println();
        }
    }
}

My speculation is that instead of incrementing some of the values I would decrement them however the code would run infinite garbage values and not what I wanted.

ascii-art for-loop loops java
6 Answers

It is needed to print some spaces before printing the numbers in each row, and the number of spaces should be decreasing depending on the row:

int rows = 6;

for (int i = 1; i <= rows; ++i) {
    for (int j = rows - i; j >= 1; j--) {
        System.out.print("  ");
    }
    for (int j = 1; j <= i; ++j) {
        System.out.print(j + " ");
    }
    System.out.println();
}

This prefix may be build using String::join + Collections::nCopies:

System.out.print(String.join("", Collections.nCopies(rows - i, "  ")));

Or since Java 11, this prefix may be replaced using String::repeat:

System.out.print("  ".repeat(rows - i));
user_132798310

Instead of two nested for loops, you can use a single while loop with two incrementing variables. The number of iterations stays the same.

int n = 7, i = 0, j = 0;
while (i < n) {
    // element
    if (i + j >= n - 1) {
        // print an element
        System.out.print(i + j + 2 - n);
    } else {
        // print a whitespace
        System.out.print(" ");
    }
    // suffix
    if (j < n - 1) {
        // print a delimiter
        System.out.print(" ");
        j++;
    } else {
        // print a new line
        System.out.println();
        j = 0;
        i++;
    }
}

Output:

            1
          1 2
        1 2 3
      1 2 3 4
    1 2 3 4 5
  1 2 3 4 5 6
1 2 3 4 5 6 7

See also: Optimized Bubble Sort

Admin1

Your approach is almost correct - use two nested for loops, all that remains is to add one if else statement and calculate the sum of coordinates i and j.

Try it online!

public static void main(String[] args) {
    int n = 6;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            System.out.print(" ");
            int sum = i + j;
            if (sum > n)
                System.out.print(sum - n);
            else
                System.out.print(" ");
        }
        System.out.println();
    }
}

Output:

           1
         1 2
       1 2 3
     1 2 3 4
   1 2 3 4 5
 1 2 3 4 5 6

See also: Printing a squares triangle. How to mirror numbers?

Admin1

You have to print spaces before printing the numbers to make the triangle look inverted, the number of spaces depends on the amount of numbers you skip which are rows-i, so you can loop from i to rows and print space in each iteration.

int rows = 6;
for (int i = 1; i <= rows; ++i) {
    for (int j = i; j < rows; j++) {
        System.out.print("  ");
    }
    for (int j = 1; j <= i; ++j) {
        System.out.print(j + " ");
    }
    System.out.println();
}

Output:

          1 
        1 2 
      1 2 3 
    1 2 3 4 
  1 2 3 4 5 
1 2 3 4 5 6
user_120994090

You can use the property that for the row containing i as the greatest number of the row the number of the spaces can be calculated as 2*(rows-i). You can rewrite your program like below:

public class Main {
    public static void main(String[] args) {
        int rows = 6;
        for (int i = 1; i <= rows; ++i) {
            for (int nspaces = 0; nspaces < 2 * (rows - i); ++nspaces) {
                System.out.print(" ");
            }
            for (int j = i; j > 0; --j) {
                System.out.print(j + " ");
            }
            System.out.println();
        }
    }
}

Output:

          1 
        2 1 
      3 2 1 
    4 3 2 1 
  5 4 3 2 1 
6 5 4 3 2 1
user_117137770

You can print an inverted triangle using two nested for-loops as follows:

// the number of rows and the
// number of elements in each row 
int n = 6;
// iterating over rows with elements
for (int i = 0; i < n; i++) {
    // iterating over elements in a row
    for (int j = 0; j < n; j++) {
        // element
        if (i + j >= n - 1) {
            // print an element
            System.out.print(i + j + 2 - n);
        } else {
            // print a whitespace
            System.out.print(" ");
        }
        // suffix
        if (j < n - 1) {
            // print a delimiter
            System.out.print(" ");
        } else {
            // print a new line
            System.out.println();
        }
    }
}

Output:

          1
        1 2
      1 2 3
    1 2 3 4
  1 2 3 4 5
1 2 3 4 5 6

See also: How to draw a staircase with Java?

Admin1
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