CODEWITHSUNDEEP
javaarraysarraylist
user8949562
user8949562

Reputation:

How to transfer a matrix into a 2D ArrayList Java

I am trying to accept a multidimensional matrix as input, such as

1 2 3
3 1 5
1 9 7

This is just an example, the one I'm using is a lot larger and different inputs may be different row lengths, row columns. In my input, each row and column will be of equal length. How do I go about storing the input as it appears, perhaps a two dimensional arraylist. I know that I will have to take the input as Strings and later parse them back into integers by Integer.parseInt(string) and I know Scanner's method .split(" ") splits an input by spaces, but how do i seperate lines so that the next row can be extracted.

Upvotes: 1

Views: 867

Answers (2)

OneCricketeer
OneCricketeer

Reputation: 191701

Assuming you have a scanner that reads lines of input, here's one way

List<List<String>> matrix = new ArrayList<>();
while (sc.hasNextLine()) {
    matrix.add(Arrays.asList(sc.nextLine().split(" "));
}

Of course, change datatypes as you want

You could also store a flat Arraylist. Using the width of the matrix, you can translate rows and columns into the index 

matrix[row][col] == list.get(col + row*matrix.width);

Upvotes: 0

janos
janos

Reputation: 124646

Since you talk about splitting strings by spaces and parsing integers, it seems your input is matrix data as a string. You could first split to lines, then to columns, and parse to integers and collect in a 2D array or in a list of list.

String input = "1 2 3\n" +
    "3 1 5\n" +
    "1 9 7";

int[][] matrix = Arrays.stream(input.split("\n"))
    .map(s -> Arrays.stream(s.split(" "))
        .mapToInt(Integer::parseInt)
        .toArray())
    .toArray(int[][]::new);

List<List<Integer>> listOfLists = Arrays.stream(input.split("\n"))
    .map(s -> Arrays.stream(s.split(" "))
        .map(Integer::parseInt)
        .collect(Collectors.toList()))
    .collect(Collectors.toList());

Here's the same thing in good old-fashioned iterative way, if it's easier to understand:

String[] lines = input.split("\n");
int[][] matrix = new int[lines.length][];

for (int i = 0; i < lines.length; i++) {
    String[] columns = lines[i].split(" ");
    matrix[i] = new int[columns.length];
    for (int j = 0; j < columns.length; j++) {
        matrix[i][j] = Integer.parseInt(columns[j]);
    }
}

Upvotes: 2

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