我需要对当前代码执行什么操作才能使程序运行乘法表（使用二维数组）？

Question

I'm trying to make a Java program that uses the input value from a user to calculate and list the products of two numbers up to the entered number. Like if a user enters 2, the program should calculate the products between the two numbers (1 *1, 1*2, 2*1, 2*2) stores the products in a two-dimensional array, and list the products. I'm not sure that I totally understand arrays and so I feel as though my code is problem not right in many instances, can someone please tell me what I should do to my current code to make it work properly. Thanks in advance! :)

import java.util.Scanner; 

public class ProductTable {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    String inputString;
    char letter = 'y'; 


    // Prompt the user to enter an integer
    while(letter != 'q') {
    System.out.print("Enter a positive integer: "); 
    int integer = input.nextInt();

   // Create an two-dimensional array to store products 
   int[][] m = new int[integer][3];
   for (int j = 1; j <= m.length; j++) {
      m[j][0] = input.nextInt();
      m[j][1] = input.nextInt();
      m[j][2] = input.nextInt();
   }

   // Display the number title
   System.out.print("   ");
   for (int j = 1; j <= m.length; j++)
      System.out.print("   " + j);

   System.out.println("\n--- ");

   // Display table body
    for (int i = 1; i <= m.length + 1; i++) {
       System.out.print(i);
       for (int j = 1; j <= m.length + 1; i++) {
          System.out.printf("%4d", i * j); 

       }
       System.out.println();
    }



        // Prompt the user to either continue or quit
    System.out.print("Enter q to quit or any other key to continue: ");
    String character = input.nextLine();
    inputString = input.nextLine();
    letter = inputString.charAt(0);
    }  

  }
}     

Answer 1

You can achieve your multiplication table by iterating over 2 counter variables as you already did in your output

// Display table body
// loops running out of bounds (until m.length + 1 instead of m.length-1)
for (int i = 1; i <= m.length + 1; i++) {
   System.out.print(i);
   for (int j = 1; j <= m.length + 1; i++) { // missed to increment j here
      System.out.printf("%4d", i * j); 

   }
   System.out.println();
}

Arrays should be based on 0, that means an array with 3 fields has the indexes 0, 1, 2. So the last index is length-1. Your condition <=m.length+1 runs out of bounds. index<length will work since 2 is less than 3, but not 3 less than 3.

You have also a typo: In the inner loop you are doing an increment of i instead of j , so the loop will run infinite.

Try to create an outer and inner loop as you did, but with start index 0 and end condition index<inputValue . Calculate multiTable[index1][index2] = (1+index1)*(1+index2) .

Then do a similar loop and print the array fields. You don't need to use an array, you could output directly as you did. But you wanted to practice handling arrays.

---

Since you want to learn and understand, I don't like just to write the code. imagine an array with 3 fields having the indexes 0, 1, 2:

index 0 | 1 | 2
value 1 | 2 | 3

Now you would check the length at first, that's 3. You start with 0 and count while the counter does not reach the length since the zero based index is 1 below our natural order (0,1,2 vs. 1,2,3). That is the case as long the index is below the length, meaning index<length .

---

Try this logic (pseudo code). To simplify the problem we include the 0 in our product table. So we get 0*0, 0*1, 0*2... Doing so, we do not have to ignore index 0 or to calculate between index 0 should represent value 1.

maxNumber = userInput()

outer loop idx1 from 0 to maxNumber
  inner loop idx2 from 0 to maxNumber
    array[idx1][idx2] = (idx1) * (idx2)
  end loop
end loop

Do the same to dump your generated array to screen.

First get it running . Afterwards you could try to alter the logic, so that it shows you only numbers from 1 to max.