Diamond with nested for loop in Java

Question

I am trying to display a diamond of asterisks using nested for loops.

Here is my code so far:

public          


        

Answer 1

int n = 9;
for(int i =0;i<n;i++){
    for(int k=n-1;k>i;k--){
        System.out.print(" ");
    }
    for(int j=0;j<2*i+1;j++){
        System.out.print("*");
    }
    System.out.println("");
}

for(int j=0;j<n-1;j++){
    
    for(int k=j;k>=0;k--){
        System.out.print(" ");
    }
    
    for(int i=2*(n-j-1)-1;i>0;i--){
        System.out.print("*");
    }
    
    System.out.println("");
    
}

Answer 2

You can print a diamond of asterisks (mathematical operators) with for loops:

int m = 4;
int n = 4;
for (int i = -m; i <= m; i++) {
    for (int j = -n; j <= n; j++) {
        int val = Math.abs(i) + Math.abs(j);
        System.out.print(val > Math.max(m, n) ? " " : "∗");
        if (j < n) {
            System.out.print(" ");
        } else {
            System.out.println();
        }
    }
}

Output:

        ∗        
      ∗ ∗ ∗      
    ∗ ∗ ∗ ∗ ∗    
  ∗ ∗ ∗ ∗ ∗ ∗ ∗  
∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗
  ∗ ∗ ∗ ∗ ∗ ∗ ∗  
    ∗ ∗ ∗ ∗ ∗    
      ∗ ∗ ∗      
        ∗        

---

You can create a 2d array with a diamond of asterisks using IntStreams:

int m = 4;
int n = 4;
String[][] arr = IntStream.rangeClosed(-m, m)
        .mapToObj(i -> IntStream.rangeClosed(-n, n)
                .map(j -> Math.abs(i) + Math.abs(j))
                .mapToObj(j -> j > Math.max(m, n) ? " " : "∗")
                .toArray(String[]::new))
        .toArray(String[][]::new);

// formatted output
Arrays.stream(arr)
        .map(row -> Arrays.stream(row)
                .collect(Collectors.joining(" ", "[ ", " ]")))
        .forEach(System.out::println);

[         ∗         ]
[       ∗ ∗ ∗       ]
[     ∗ ∗ ∗ ∗ ∗     ]
[   ∗ ∗ ∗ ∗ ∗ ∗ ∗   ]
[ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ]
[   ∗ ∗ ∗ ∗ ∗ ∗ ∗   ]
[     ∗ ∗ ∗ ∗ ∗     ]
[       ∗ ∗ ∗       ]
[         ∗         ]

---

^{See also:
• Empty diamond shape with numbers
• Filling a 2d array with numbers in a rhombus form}

Answer 3

In your first for loop remove = mark and just use < e.g. for (int i = 1; i < size; i += 2).

Full code:

int size = 9;

for (int i = 1; i < size; i += 2) {
    for (int k = size; k >= i; k -= 2) {
        System.out.print(" ");
    }
    for (int j = 1; j <= i; j++) {
        System.out.print("*");
    }
    System.out.println();
}// end loop

for (int i = 1; i <= size; i += 2) {
    for (int k = 1; k <= i; k += 2) {
        System.out.print(" ");
    }
    for (int j = size; j >= i; j--) {
        System.out.print("*");
    }
    System.out.println();
}// end loop

Answer 4

Try this code:

I have changed the first loop:

for (int i = 1; i <= size-1; i += 2) {

---

int size = 9;

for (int i = 1; i <= size - 1; i += 2) {
    for (int k = size; k >= i; k -= 2) {
        System.out.print(" ");
    }
    for (int j = 1; j <= i; j++) {
        System.out.print("*");
    }
    System.out.println();
}// end loop

for (int i = 1; i <= size; i += 2) {
    for (int k = 1; k <= i; k += 2) {
        System.out.print(" ");
    }
    for (int j = size; j >= i; j--) {
        System.out.print("*");
    }
    System.out.println();
}// end loop

Answer 5

java-11

By using String#repeat, introduced as part of Java-11, you can do it with a single loop.

public class Main {
    public static void main(String[] args) {
        int size = 9;
        int midRowNum = size / 2 + 1;
        for (int i = 1 - midRowNum; i < midRowNum; i++) {
            System.out.println(" ".repeat(Math.abs(i)) + "*".repeat((midRowNum - Math.abs(i)) * 2 - 1));
        }
    }
}

Output:

    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

By increasing the amount of space by one character, you can also print a variant of the diamond:

public class Main {
    public static void main(String[] args) {
        int size = 9;
        int midRowNum = size / 2 + 1;
        for (int i = 1 - midRowNum; i < midRowNum; i++) {
            System.out.println("  ".repeat(Math.abs(i)) + "* ".repeat((midRowNum - Math.abs(i)) * 2 - 1));
        }
    }
}

Output:

        * 
      * * * 
    * * * * * 
  * * * * * * * 
* * * * * * * * * 
  * * * * * * * 
    * * * * * 
      * * * 
        * 

Answer 6

Just for fun... :) try my code....

public class Diamond {
    static String sp(int n) {
        String s = "";
        for (int i = 0; i < n; i++)
            s += " ";
        return s;
    }

    static String st(int n) {
        String s = "";
        for (int i = 0; i < n; i++)
            s += "*";
        return s;
    }

    static int abs(int n) {
        if (n < 0)
            return -n;
        else
            return n;
    }

    public static void main(String[] args) {
        int size = 9;
        for (int i = 0; i < size; i++) {
            System.out.println(sp(abs((size - 1) / 2 - i)) +
                    st(abs(9 - 2 * ((i + 5) % (size)))) +
                    sp(abs((size - 1) / 2 - i)));
        }
    }
}