Print hollow rectangle with a 2 dimensional array

Question

I have this code that prints a rectangle using a 2 dimensional array. As you can see I hand-coded the whole array instead of using a loop. What I am looking for is:

How to use a loop to print exactly the same rectangle (with the stars, minuses...)
After this template is created, if I want to place, let's say a char '?' inside the rectangle, I can, for example, call the coordinates tab[5][4], and this will print it.

The problem is a whole column of '?' is printed outside the template. How can I fix this?

NOTE: I don't want to use any of java's Swing or AWT Libraries.

public class HelloWorld{

 public static void main(String []args){
char [][] tab= {
    {'*', '-', '-', '-', '-','*'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'+', ' ',  ' ',  ' ', ' ',  '+'},
    {'*', '-', '-', '-', '-','*'}


};
int row=8;
int col=6;
for (int i=0; i< row; i++){
    for(int j=0; j< col; j++){
        System.out.print(tab[i][j]+" ");

    }
    System.out.print(tab[5][4]='?');
    System.out.println("");

}

This is my output:

* - - - - - * ?
+           + ?
+           + ?
+           + ?
+           + ?
+        ?  + ?
+           + ?
* - - - - - * ?

Thanks for your help

score 1 · Accepted Answer · answered Oct 16 '16 at 13:53

You can do it like this - Declare an array with the right size and fill it with ' ':

final int row=8;
final int col=6;
char[][] tab = new char[row][col];
for (int i=0; i< row; i++){
    for(int j=0; j< col; j++){
        tab[i][j] = ' ';
    }
}

Place the stars at the corners:

tab[0][0] = '*';
tab[0][col-1] = '*';
tab[row-1][0] = '*';
tab[row-1][col-1] = '*';

For the first and last row:

for (int i=1; i<col-1; i++) {
    tab[0][i] = '-';
    tab[row-1][i] = '-';
}

And for all the other rows:

for (int i=1; i < row-1; i++) {
    tab[i][0] = '+';
    tab[i][col-1] = '+';
}

Now you can place the '?' wherever you want and print the array.

score 0 · Answer 2 · answered Oct 16 '16 at 13:17

Here comes your refactored code, first thing. if you concat something to your matrix like you're doing here System.out.print(tab[i][j]+" ");you will not print exactly same matrix.

Second point, this line System.out.print(tab[5][4]='?'); is inside your external for loop, so this is why you're printing it outside the matrix.

Debug your code and you will understand what is happening. Here comes the refactored and fixed version:

public class NewOne {

    public static void main(String[] args) {
        char[][] tab = { { '*', '-', '-', '-', '-', '*' }, 
                         { '+', ' ', ' ', ' ', ' ', '+' },
                         { '+', ' ', ' ', ' ', ' ', '+' }, 
                         { '+', ' ', ' ', ' ', ' ', '+' }, 
                         { '+', ' ', ' ', ' ', ' ', '+' },
                         { '+', ' ', ' ', ' ', ' ', '+' }, 
                         { '+', ' ', ' ', ' ', ' ', '+' },
                         { '*', '-', '-', '-', '-', '*' }

        };
        printArray(tab);
        // then we change a value
        tab[5][4] = '?';
        printArray(tab);
    }

    /**
     * this Method Prints any given matrix
     * 
     * @param tab a char matrix
     */
    private static void printArray(char[][] tab) {
        for (int i = 0; i < tab.length; i++) {
            for (int j = 0; j < tab[i].length; j++) {
                System.out.print(tab[i][j]);
            }
            System.out.println();
        }
    }
}

Thanks Victor for your answer. That effectively answers my second point. I was wondering if there's a way to print the whole thing without having to enter * - - - - * . for example what if I have 100 rows and 200 columns of stars and '-'. it's very hard and error-prone to write the whole array. — omar, Oct 16 '16 at 13:23

Print hollow rectangle with a 2 dimensional array

2 Answers2