How to Create a Matrix from 2 existing ArrayLists (Java)?

Question

I am reading contents from a text file and parsing them into separate ArrayLists.

For example, the text file reads:

Fruit1
Fruit2
Fruit3
Vegetable1
Vegetable2
Vegetable3
Vegetable4

Currently, I have a code that separates each group into its own array

fruits = [Fruit1, Fruit2, Fruit3]
vegetables = [Vegetable1, Vegetable2, Vegetable3, Vegetable4]

How do I make a matrix with n rows and m columns from these two existing ArrayLists?

My goal output is to generate a 3x4 matrix like so

          | Fruit1, Fruit2, Fruit3 
Vegetable1|
Vegetable2|
Vegetable3|
Vegetable4| 
          |

I have seen examples demonstrating initializing a matrix, however, if I update my text file to lets say a 3x20 matrix, or a 5x20 matrix, I want the code to run the same, which is where I am struggling.

Here is the code I've written for the matrix:

List<List<String>> matrix = new ArrayList<List<String>>();
matrix.add(fruits);
matrix.add(vegetables);
System.out.println(matrix);

However, this is the output, which just combines them

[Fruit1, Fruit2, Fruit3, Vegetable1, Vegetable2, Vegetable3, Vegetable4]

How do I create a matrix, making one ArrayList the rows, and the other ArrayList, the columns?

Please follow below link you will get an idea stackoverflow.com/questions/37078635/… — GauravRai1512
– GauravRai1512, Commented Nov 5, 2018 at 18:08
So far you have only the row and column "headers" (at leasts that's what it looks like in your demonstration). What should be the contents? — Twometer
– Twometer, Commented Nov 5, 2018 at 18:20
In the end, I want to compare Fruit1 to Vegetable1, Fruit1 to Vegetable2 and so on... I should have 12 comparisons — Tom Wilson
– Tom Wilson, Commented Nov 5, 2018 at 18:24
I just don't know how to put this into matrix format to later write code that compares each variable in one arraylist to the other equaling 12 comparisons — Tom Wilson
– Tom Wilson, Commented Nov 5, 2018 at 18:25

Twometer · Accepted Answer · 2018-11-05 18:31:46Z

0

Assuming you want the following matrix:

Vegetable1 | Fruit1, Fruit2, Fruit3
Vegetable2 | Fruit1, Fruit2, Fruit3
Vegetable3 | Fruit1, Fruit2, Fruit3
Vegetable4 | Fruit1, Fruit2, Fruit3

You can use the following code to use ArrayLists to do all the comparisons:

List<String> vegetables = new ArrayList<>(); // Fill the lists somehow
List<String> fruits = new ArrayList<>();

for(String vegetable : vegetables) {
    for(String fruit : fruits) {
        System.out.printf("Compare %s to %s%n", vegetable, fruit);
    }
}

If that's all you want, you don't need nested lists. If you want to really have a matrix then you'd require a slight modification of the code:

List<String> vegetables = new ArrayList<>(); // Fill the lists somehow
List<String> fruits = new ArrayList<>();
List<List<String>> matrix = new ArrayList<>();    

for(String vegetable : vegetables) {
    List<String> row = new ArrayList<String>();
    row.add(vegetable);
    for(String fruit : fruits) {
        row.add(fruit);
    }
    matrix.add(row);
}

That would create rows with items VegetableN, Fruit1, Fruit2, Fruit3 where N is the number of the vegetable row.

answered Nov 5, 2018 at 18:31

Twometer

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2 Comments

Tom Wilson Over a year ago

Wow! The first code was exactly what I needed. I am very new to programming so I am unfamiliar with some things. My main goal is to compare strings of different groups (fruits & vegetables as an example) for textual similarity. I believe I can move forward now

Twometer Over a year ago

@TomWilson If my answer helped you, you can mark it as accepted using the green check mark under the voting buttons so that it disappears from the unanswered section.

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