How can you convert a 2 dimensional array into a 1 dimensional array in Java

Question

I would like to know how to convert a 2 dimensional array into a 1 dimensional array. I have come up with some code but it doesn't exactly seem to work. Can someone please help me? Thanks.

public class TESTER1 {

    /**
     * @param args
     */

    static String[][] data = new String[][] {{"Dum","Dumer","Dumbest"}};

    public static void main(String[] args) {

        convertData(data);
    }

    public static void convertData(String[][]data) {
        String[] toReturn = new String[data.length];
        for(int i = 0;i<data.length;i++) {
            for(int j = 0;j<3;j++){
                toReturn[i] = data[i][j];
            }
        }
        for(String s:toReturn) {
            System.out.println(s);
        }
    }
}

[edit]Thank you very much. Is it possible to convert each row in the String[][] into a index in a String[] for example if we convert the String[][] (above code), then when i print out array[0] it should print out dum,dummer,dumbest [edit]

Answer 1

public static String[] flatten(String[][] data) {
    ArrayList<String> list = new ArrayList<String>();

    for(int i = 0; i < data.length; i++) {
        for(int j = 0; j < data[i].length; j++){
            list.add(data[i][j]);
        }
    }

    return list.toArray(new String[0]);
}

Or add whole rows at one time:

    for(int i = 0; i < data.length; i++) {
        list.addAll( Arrays.asList(data[i]) );
    }

Edit: From comments on my answer it seems like this is what the OP wanted (ie converting each row of 2d array to some string representation of it):

public static String[] rowsToString(String[][] data) {
    ArrayList<String> list = new ArrayList<String>();

    for(int i = 0; i < data.length; i++) {
        String row = Arrays.toString(data[i]);
        list.add( row.substring(1, row.length()-1) );
    }

    return list.toArray(new String[0]);
}

Answer 2

A cleaner version:

public static String[] flatten(String[][] data) {
    List<String> toReturn = new ArrayList<String>();
    for (String[] sublist : Arrays.asList(data)) {
        for (String elem : sublist) {
            toReturn.add(elem);
        }
    }
    return toReturn.toArray(new String[0]);
}

Answer 3

The length of the 1-dimensional array must be the sums of the lengths of all rows in the 2-dimensional array. Of course, Java doesn't really have "true" 2-dimensional arrays, but arrays of arrays. This code works, and is wrapped in a simple demo program.

public class ArrayFlattening {

public static final String[][] STRINGS2 = {
    {"my", "dog", "has", "fleas"},
    {"how", "now", "brown", "cow"},
    {"short", "row"},
    {"this", "is", "a", "final", "row", "in", "this", "test"},
};

public static String[] flatten(String[][] a2) {
    String[] result = new String[totalSize(a2)];
    int index = 0;
    for (String[] a1 : a2) {
        for (String s : a1) {
            result[index++] = s;
        }
    }
    return result;
}

public static int totalSize(String[][] a2) {
    int result = 0;
    for (String[] a1 : a2) {
        result += a1.length;
    }
    return result;
}

public static void main(String[] args) {
    System.out.println("" + STRINGS2.length + " rows");
    for (String[] strings1 : STRINGS2) {
        System.out.println("" + strings1.length + " strings");
        for (String s : strings1) {
            System.out.print("\t" + s);
        }
        System.out.println();
    }
    String[] strings1 = flatten(STRINGS2);
    System.out.println(strings1.length + " strings");
    for (String s : strings1) {
        System.out.print("\t" + s);
    }
    System.out.println();
}

}

Answer 4

Flatten did become much easier in Java 8 with the stream API. The function can be expressed as:

private static String[] flatten(String[][] data) {
    return Stream.of(data).flatMap(Stream::of).toArray(String[]::new);
}