initialize java int 2d array from string

Question

I have a simple 2d array

// Creating and Initializing 2D array
        int a[][] = {{0,1},{2,3},{4,5}};
        for(int i=0; i<3; i++) {
            for(int j=0; j<2; j++) {
                System.out.print(a[i][j]+" ");
            }
            System.out.println();
        }

works everything fine. But how can I initialize the array when the content is dynamic and coming from a variable?

String arrContent = "{{0,1},{2,3},{4,5}}"; //actually a method readContentFromFile(); will deliver the content
int a[][] = arrContent;

Is there any way to do this?

Unfortunatelty this is not possible without parsing the String, which is rather complicated in this case. If you have the possibility to change the file's content to json, using a json parser will be the quickest solution. — f1sh
– f1sh, Commented Sep 21, 2021 at 9:47

Nowhere Man · Accepted Answer · 2021-09-21 10:30:05Z

The input stream can be parsed using String::split and Stream API:

get rows splitting by comma between a pair of },{
remove redundant curly brackets using String::replaceAll
for each row, split by comma, convert numbers to int Stream::mapToInt and get array
collect to multidimensional array with Stream::toArray

String arrContent = "{{0,1},{2,3},{4,5}}";

int[][] arr = Arrays.stream(arrContent.split("\\}\\s*,\\s*\\{")) //1
    .map(s -> s.replaceAll("[{}]", ""))                          //2
    .map(s -> Arrays.stream(s.split("\\s*,\\s*"))                //3
                    .mapToInt(Integer::parseInt).toArray()
    )
    .toArray(int[][]::new);                                      //4
    
System.out.println(Arrays.deepToString(arr));

Output

[[0, 1], [2, 3], [4, 5]]

Another approach could be to use JSON parser, however, initial input string needs to be modified to become a valid JSON array (curly brackets {} have to be replaced with the square ones [] using simpler String::replace(char old, char rep)):

String arrContent = "{{0,1},{2,3},{4,5}}";

ObjectMapper om = new ObjectMapper();

int[][] arr = om.readValue(
        arrContent.replace('{', '[')
                  .replace('}', ']'),
        int[][].class);

System.out.println(Arrays.deepToString(arr));

// output
[[0, 1], [2, 3], [4, 5]]

Pawel Woroniecki · Accepted Answer · 2021-09-21 14:01:32Z

1

Just wanted to provide a little bit modified solution using simple regexp just in case anybody would prefer doing it this way:

String input = "{{0,1},{2,3},{4,5}}";
Pattern pattern = Pattern.compile("\\{[0-9,]+}");
int[][] array = pattern.matcher(input).results()
    .map(matchResult -> matchResult.group().replaceAll("[{}]", ""))
    .map(numberAsText -> Arrays.stream(numberAsText.split(",")).mapToInt(Integer::parseInt).toArray())
    .toArray(int[][]::new);

Just as the others pointed out - if you have the possibility to change shape of input - I would also advice to use JSON's array and parse it using JSON parser which would make it much easier and more clean.

answered Sep 21, 2021 at 14:01

Pawel Woroniecki

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