Why is the result only 81 when I try to use array-CodePudding

Thanks for reading my question:

public class Gugudan_array {
    public static void main(String[] args) {

        for(int i = 2; i<10; i  ) {
            for(int j = 1; j<10; j  ) {
                System.out.println(i * j);
            }
            System.out.println();
        }
        
    }
}

In the multiplication table above, result comes correctly from 2-9, however:

public class Gugudan_array {
    public static void main(String[] args) {

        int[] result = new int[10];
            for(int h = 0; h < result.length; h  ) {
                for(int i = 2; i < 10; i  ) {
                    for(int j = 1; j<10; j  ) {
                        result[h] = (i * j);
                        }
                }
            }
            for(int a = 0; a < result.length; a  ) {
                System.out.println(result[a]);
            }
    }
}

In this code with array, the result comes out only 81. What have I done wrong? Thank you!

CodePudding user response：

I'll first explain why your code doesn't work, and then go over a correct solution:

public class Gugudan_array {
    public static void main(String[] args) {

        int[] result = new int[10];
            for(int h = 0; h < result.length; h  ) {
                for(int i = 2; i < 10; i  ) {
                    for(int j = 1; j<10; j  ) {
                        result[h] = (i * j);
                        }
                }
            }
            for(int a = 0; a < result.length; a  ) {
                System.out.println(result[a]);
            }
    }
}

The main problem lies in this chunk of code:

for(int i = 2; i < 10; i  ) {
   for(int j = 1; j<10; j  ) {
      result[h] = (i * j);
   }
}

Here, you are constantly overwriting the value of result[h], so that once the loop ends and both i = 9, and j = 9, the code will execute result[h] = 9 * 9 and then continue on to the next h.

My solution:

public class Gugudan_array {
    public static void main(String[] args) {

        int[] result = new int[10];
        for(int i = 2; i < 10; i  ) {
            for(int j = 1; j<10; j  ) {
                result[i - 2] = (i * j);
            }
        }
        for(int a = 0; a < result.length; a  ) {
            System.out.println(result[a]);
        }
    }
}

Output:

First, notice how I completely got rid of the h loop. That is because we can make the index in terms of i. When we determine the first number, when i = 2, we want to store that number in the 0th index of our array. Similarly, when we get our second number, when i = 3, we want to store the result in the 1st index of our array.

To summarize, whenever we calculate a result, we will want to store it in the i - 2th index of our array.

Better Solution using 2D arrays:

    int[][] result = new int[8][9];
    for(int i = 2; i < 10; i  ) {
        for(int j = 1; j<10; j  ) {
            result[i - 2][j - 1] = (i * j);
        }
    }
    for(int a = 0; a < result.length; a  ) {
      for(int b = 0; b < result[a].length; b  ){
        System.out.print(result[a][b]   " ");
      }
      System.out.println();
    }

Output:

2 4 6 8 10 12 14 16 18 
3 6 9 12 15 18 21 24 27 
4 8 12 16 20 24 28 32 36 
5 10 15 20 25 30 35 40 45 
6 12 18 24 30 36 42 48 54 
7 14 21 28 35 42 49 56 63 
8 16 24 32 40 48 56 64 72 
9 18 27 36 45 54 63 72 81

Note: If you want the output to match your original code, change System.out.print(result[a][b] " "); to System.out.println(result[a][b])

It would make the most sense to store a multiplication table in a 2D array.

This code works by mapping i * j to the [i - 2][j - 1]'th element of the 2D array, so that 2 * 1, will end up in result[0][0]

I hope this made sense! Please let me know if you need any further help or clarification!

CodePudding user response：

You have to increase h variable after all iteration. Here is whole code:

public static void main(String[] args) {
    int[] result = new int[10];
    for (int h = 0; h < result.length; h  ) {
        for (int i = 2; i < 10; i  ) {
            for (int j = 1; j < 10; j  ) {
                result[h] = (i * j);
                if (h < 9) {
                    h  ;
                }
            }
        }
    }

    for (int a = 0; a < result.length; a  ) {
        System.out.println(result[a]);
    }
}

CodePudding user response：

The issue is that your current setup with 3 nested loops, the outer loop runs 10 times, the middle loop runs 8 times for each other loop, and the inner loop runs 9 times for each middle loop, which gives a total of 10x8x9=720 times... but I suspect you just want 72 results like you show in your first example, so we need to remove the outer loop first for(int h = 0; h < result.length; h ) {.

Now the issue is that we need to store all 72 results but your current array only fits 10 results int[] result = new int[10];, we can solve this with a bigger array:

int[] result = new int[8 * 9]; //72 spaces

Here is a working solution with a long array and a counter that keeps track of where to store the value, note the code comments for extra details:

//Your loops calculate 8 * 9 results, so you need an array with 72 spaces
int[] result = new int[72];

//Use a variable to track the array location
int counter = 0;

//Youn only need two loops
for(int i = 2; i < 10; i  ) {
    for(int j = 1; j< 10; j  ) {
        //Save the result in the correct location
        result[counter] = (i * j);
        //Incriment the counter
        counter  ;
    }
}

//Print the stored results
for(int a = 0; a < result.length; a  ) {
    System.out.println(result[a]);
}

Some further important reading from the official Java guide on Arrays: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html

A better solution would be to use a 2D array.