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I have a very basic question but apparently I can't find the solution. I have initialized an array of String of size 100 rows and 100 columns. 

String[][] array1 = new String[100][100];

However most of the elements are empty. I want to remove those empty string but not using the different array. Let's say the non empty strings are from row 1 to 16, and from column 1 to 10. So the final output size should be

String[][] array1 = new String [16][10];

How to find and remove the strings from the first array and reduce the size of the array at the same time?

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  • 2
    No, you can't change the size of an array. Commented Nov 13, 2018 at 2:23
  • so, I will always need to use a different array? Commented Nov 13, 2018 at 2:24
  • Why is “Is it possible to…” a poorly worded question? Commented Nov 13, 2018 at 2:25

5 Answers 5

Reset to default
1

Here is a neat way of doing it with Streams

array = (String[][]) Arrays.asList(array).stream()
                            // Filters out empty arrays
                            .filter(checkEmptyArrays())
                            // Filters out empty strings
                            .map(x -> cleanUpEmptyStrings(x))
                            // Collects it all back into the array matrix
                            .collect(Collectors.toList()).toArray(new String[0][0]);


private String[] cleanUpEmptyStrings(String[] x) {
    return Arrays.asList(x).stream().filter(y -> y != null && !y.equals("")).collect(Collectors.toList()).toArray(new String[0]);
}

private Predicate<String[]> checkEmptyArrays() {
    return k -> Arrays.stream(k).filter(l -> l != null && !l.equals("")).count() != 0;
}

Here's a test 

@Test
public void test() {
    String[][] array = new String[100][100];
    for (int i=0;i< 10; i++) {
        for (int j=10; j< 16; j++) {
            array[i][j] = "abcd";
        }
    }

    array = (String[][]) Arrays.asList(array).stream()
                            // Filters out empty arrays
                            .filter(checkEmptyArrays())
                            // Filters out empty strings
                            .map(x -> cleanUpEmptyStrings(x))
                            // Collects it all back into the array matrix
                            .collect(Collectors.toList()).toArray(new String[0][0]);

    for (String[] a: array) {
        System.out.println(a.length);
    }

}
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1 Comment

This is what I wanted exactly! Thanks a lot!
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Arrays' size cannot be changed. However, you can do this by reassigning the variable:

int x, y = size that you have determined the final array should be
String[][] temp = new String[x][y]
//Copy the values that are not empty into temp
array1 = temp

This will replace array1 with a properly sized version, without having to introduce another permanent variable (x, y, and temp can be ignored after this)

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4 Comments

what will happen if I don't know the value of x and y?
I can't tell from your question, are all of the non-empty cells in a corner of the matrix or ar they spread out randomly?
they are in a corner of a matrix
then you could determine x by iterating through the first row of the array and counting how many strings you see, and determine y by iterating through the rows and counting how many of them have anything in the first position.
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You cannot reduce the size of an array. However, you can create a method that returns a trimmed version of the old array, such as

static String[][] trimArr( String[][] oldArr, int rows, int cols)
{
    String[][] trimmedArr = new String[rows][cols];
    for (int i = 0; i < trimmedArr.length; i++)
         for (int j = 0; j < trimmedArr[i].length; j++)
             trimmedArr[i][j] = oldArr[i][j];            
    return trimmedArr;
}
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2 Comments

what will happen if I don't know the value of rows and cols?
In order to create a new array of a smaller length, you'd have to know that. You can find the desired lengths by iterating through the initial array and going until you find the index where array[index-row][index-col] == null.
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Array has fixed length. You have to replace it with another array that fit your requirement

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0

As other answers and comments have advised, you can't change the size of array, you'll have to create a new one.
However, it's best to use a Collection, which will automatically resize itself.

If you still want to do it using a new array, here is how you can do it:-
I have taken an array of size 3x3, you can use the code for 100x100

{"row0 col0", "", "row0 col2"},
{"row1 col0", "", ""},
{"", "", ""}

In this example, since the max number of columns used by any row is 2, we can have the new array with just 2 columns instead of 3.

Similarly, last row is virtually empty with no non-empty strings, we can have the new array with just 2 rows instead of 3.

public static void main(String[] args) {
    String[][] array1 = new String[][]{
            {"row0 col0", "", "row0 col2"},
            {"row1 col0", "", ""},
            {"", "", ""}
    };

    long maxNoOfColumnsInAnyRow = Arrays.stream(array1)
            .map(strings -> Arrays.stream(strings)
                    .filter(s -> s != null && !s.isEmpty())
                    .count())
            .max(Long::compareTo)
            .orElse((long) array1[0].length);

    long noOfNonEmptyRows = Arrays.stream(array1)
            .filter(strings -> Arrays.stream(strings)
                    .anyMatch(s -> s != null && !s.isEmpty()))
            .count();

    String[][] array2 = new String[(int) noOfNonEmptyRows][(int) maxNoOfColumnsInAnyRow];

    int i = 0, j;
    for (String[] strings : array1) {
        j = 0;
        for (String str : strings) {
            if (str != null && !str.isEmpty()) {
                array2[i][j] = str;
                j++;
            }
        }
        if (j != 0) i++;
    }

    System.out.println(Arrays.deepToString(array2));
}

Output is [[row0 col0, row0 col2], [row1 col0, null]], which is a 2x2 array.

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