如何在Java中的命令行中打印十字形？

Question

I have to create a cross shape using methods, and the parameter is the 'size' of the cross. A number is input and the cross is drawn if the number is odd, so if I were to input a 5, the output would look like the screenshot I added to the bottom

The centre line is what's really throwing me off as I've only started methods last week, but so far I have:

import java.util.Scanner;

public class Cross {

    public static void main(String[] arg) {
        Scanner keyboard = new Scanner(System.in);
        System.out.println("Please type a number: ");
        int num = keyboard.nextInt(); 
        drawCross(num);
    }

    public static void drawCross(int num){
        for (int = 1; i <= num; i++) {
            if ((num % 2) != 0) {
                System.out.println(i + "*");
            }
        }
    }

}

I know this is probably way off, but I'm a total newbie to methods.

Answer 1

Analyze the problem before you start programming. Break the task into steps:

1. Check whether num is valid for this problem.
   • num must be positive.
   • num must be odd.
   • Notice that you're checking whether num is valid more than once. Check it at the beginning, and quit with an error message if num is invalid. Or, throw an exception, and have main catch it and report to the user.
     • After you know how to use exceptions, that is.
   • By the way, num is a bad name for the variable. It's too generic. Except for loop indices, try to have your name be descriptive.
2. Compute how many spaces must precede the * on all but the center output line.
3. Compute which line is the center output line.
4. Do a loop for the top half of the output.
   • Don't use System.out.println() for individual characters.
   • That method should be called only once per line.
5. Print the central line.
6. Do a loop for the bottom half of the output.

---

Now, you should try to stop doing everything in main and other static methods. Cross is a class, and you should create an instance of the class and have it do the work.

public class Cross {
    public static void main(String[] arg) {
        Scanner keyboard = new Scanner(System.in);
        System.out.println("Please type a number: ");
        int num = keyboard.nextInt();
        Cross cross = new Cross();
        cross.drawCross(num);
    }

    private void drawCross(int size) {
        // Your turn.
    }
}

Why? So you can test it. One of the most invaluable tools in Java programming is the JUnit library. You will want to offload your logic into tiny methods like:

public boolean validSize(int size) {
    // You fill this in.
}

In a test class, CrossTest , you'll write code like:

@Test
public void negativeSizesAreIllegal() {
    Cross cross = new Cross();
    // Test whether cross.validSize(-13) returns false.
    // Look at the JUnit web site or any book describing the tool.
}

Figure out what your requirements on the method are, and write tests to check each one. You'll have similar tests too for 0, odd integers, and even integers. But if you do all your code in static methods, that's a lot harder. This way, as you change your program, you'll know whether your changes have broken your tests. It's like computing an indefinite integral, and differentiating it to check whether you made a mistake.

Finally, simplify your work. The way I outlined your problem, there will be a lot of code duplication. See if you can write a method to replace the duplicate code in steps 4 and 6, and call it twice. Keep going with it; you'll see lots of chances to shorten your program. Having tests is invaluable for that. You'll also be able to use standard methods in the java.lang.String class to simplify things further.

Did you see that

System.out.println(i + "*");

did not do what you thought it did?

Answer 2

Not a best solution. but this one I came up with in 2 mins time :

public static void drawCross(int num){
    if (num % 2 != 0)  {
     for(int i = 0; i < num; i++) {
            for(int j = 0; j < num; j++) {
                if((i == num / 2) || (j == num / 2)) 
                    System.out.print("*");
                else 
                    System.out.print(" ");
            }
            System.out.println();
        }
    }
}

Answer 3

Here is some code that should work:

public static void drawCross(int num)
{
    if(num % 2 != 0) //if "num" is odd
    {
        for(int y = 0; y < num; y++) //loop through the following code "num" times
        {
            for(int x = 0; x < num; x++) //loop through the following code "num" times
            {
                if(x == (int)(num / 2) || y == (int)(num / 2)) //if "x" or "y" is in the middle
                    /*(note that at this point "num" will always be odd, so "num / 2" will always 
                    be #.5. That's why the "(int)" is there: to change it from "#.5" to "#"*/
                {
                    System.out.print("*"); //print "*"
                }
                else //if "x" or "y" is NOT in the middle
                {
                    System.out.print(" "); //print a space
                }
            }
            System.out.println(); //create a new line
        }
    }
}

Answer 4

public static void drawCross(int num){
    if(num % 2 != 0){
        for(int line = 0;line < num;line++){
            for(int col = 0;col < num;col++){
                if(line == num / 2 || col == num / 2) System.out.print("*");
                else System.out.print(" ");
            }
            System.out.println("");
        }
    }
}