在控制台上用 Java 打印金字塔

Question

How can I print a pyramid in Java like this

1
23
456
78910

My current code looks like this:

public class T {
    public static void main(String[] args) {
        int i, j, num = 1;
        int n = Integer.parseInt(args[0]);

        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.println(num);
                num++;
            }
            System.out.println(" ");
        }
    }
}

If I try this removing declared i & j then it shows an array out of bounds exception However 'i' & 'j' are creating the problem. What should my code look like.

Answer 1

    int val=1;
    for(int i=0;i<6;i++){
        for(int j=1;j<i;j++){
             System.out.print(val);
            val++;
        }
        System.out.print("\n");
    }

initially val is equal to 1 . Inside the first for loop i=0 and j with increase from 1, but when i=0 second for loop doesn't run. then you get the first value as 1. Then it will point to new line.

When i=1,j still 1 so second for loop runs 1 time and print 2, because val has increment(val++). when j=2 in inside for loop it is not running only print the new value (3) of val there.

so on this will work

Answer 2

public static void main(String[] args) {
    int num = 1;
    //i is how many numbers per row
    for(int i = 1; i < 5; i++){
        //prints i numbers because j increases from 0 to i, incrementing num each time
        for(int j = 0; j < i; j++){
            System.out.print(num++);
        }
        System.out.println();
    }
}

This code will work for your purposes.

Now, please read on if you would like to understand Java better and see why the compiler was throwing errors in your code. You shouldn't use stackoverflow to copy in paste someone else's code without understanding it. In your code, you were declaring i and j twice. In Java, you cannot declare a variable twice. You did it first in int i,j, num = 1; and then again in each for loop for (int i = 1; i <= lines; i++) . You could correct this by saying for(i = 1; i <= lines; i++) . Notice how the int is left out in the second version of the for loop. You can simply assign a value to a variable in a for loop rather than creating a new variable as you do when declare the type int i = 1

The syntax of a for loop is:

for(initialization; Boolean_expression; update)

{
   //Statements
}

The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.

As for the array out of bounds error that you receive, you are trying to read in a command line argument in the statement int n = Integer.parseInt(args[0]); Notice how the main method has a parameter String[] args . These are called command line arguments and can be passed in if you manually run the program from the command line. You were trying to read in args[0] which is outside of the bounds of args[] .

In other words, if you run

java MyProgram one two

Then args contains:

[ "one", "two" ]

public static void main(String [] args) {
   String one = args[0]; //=="one"
   String two = args[1]; //=="two"
}

Answer 3

I suppose you give the number of lines as your only argument, so the code would be

public static void main(String[] args) 
{
    int lines = Integer.parseInt(args[0]);
    int num = 1;
    for (int i = 1; i <= lines; i++) {
        for (int j = 1; j <= i; j++) {
            System.out.print(num);
            num++;
        }
        System.out.println("");
    }
}

Answer 4

int l=1;

for (int i=0; i<5; i++)
  {
     for (int k=0; k<5-i; k++)
     {
        System.out.print(" ");          
     }
     for (int j=0; j<(i*2)+1; j++)
     {
        if(j%2!=0){
            System.out.print(l++);
        }else {
            System.out.print(" ");
        }           
     }
     System.out.println("");
  }

Answer 5

public static void pyramid(int max) {
    int num = 1;
    max = 4;
    for (int row = 0; row < max; row++) {
        for (int column = 0; column < max; column++) 
            System.out.print(column <= row ? num++ : " ");
        System.out.println();
    }     
}

Answer 6

import java.util.Scanner;

/**
 *
 * @author shelc
 */
public class PrintNumberPyramid {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the count : ");
        int number = scanner.nextInt();
        //enter the number of rows you want to print
        pyramid(number);

    }

    public static void pyramid(int rows) {
        int count = 1;
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < rows; j++) {
                System.out.print(j <= i ? count++ : " ");
            }
            System.out.println();
        }
    }
}