For each loop using 2D array

Question

This is the snippet of Java code:

int[][] uu = new int[1][1];
uu[0][0] = 5;
for(int[] u: uu){
    System.out.println(u[0]);
}

It prints 5. But why does the declaration part of for loop is declared as int[] u, but not as int[][] u?

At the uu you reference 2D array... That is not a homework. I am preparing for Java certification. Cheers

Answer 1

Since your uu is an array of array. So, when you iterate over it, you will first get an array, and then you can iterate over that array to get individual elements.

So, your outer loop has int[] as type, and hence that declaration. If you iterate through your u in one more inner loop, you will get the type int: -

for (int[] u: uu) {
    for (int elem: u) {
        // Your individual element
    }
}

Answer 2

It is because uu is an array of int[] arrays. So every item in it is int[]. In a for loop you declare the type of an item in an array you iterate over.

Answer 3

The loop is iterating on the elements of uu, which are objects of type int[]. (Or in other words - u is an element in uu, thus it is an int[]).

The declaration is always of the type of the objects retrieved by the iteration - in this case - it is int[] -

Same as iterating over an int[] is:

 for (int x : myArray) { ...}

because each element of x is of type int.

Answer 4

"why does the declaration part of for loop is declared as int[] u, but not as int[][] u?"

The array is two-dimensional, so you are dealing with a double-layered iteration. You have an array "inside" another, in the same principle as List<List<Integer>> would work.

To iterate through all the elements, you should consider a rows-elements structure. It's necessary that you get each row from the container, and then each element from each row.

for(int[] u: uu) is simply a for-each iteration rows, with the same principle of for(int row = 0; row < container.length; row++), and u or respectively container[row] are not elements themselves, but rows (arrays) of elements. Meaning you require a second iteration layer to get the elements:

int[][] container = new int[10][10];
//... - Fill elements.
for(int row = 0; row < container.length; row++){
  for(int element = 0; element < container[row].length; element++){
    System.out.printf("Row: %d Element: %d Value: %d\n", row, element, container[row][element]);
  }
}

Answer 5

This is an example of finding the sum in 2d array and print it

public class Array2DForEach {
    public static void main(String[] args) {
        int sum = 0;
        int[][] myFirst2DArray = {
                                    { 3, 5, 1, 9 },
                                    { 10, 15, 3, 0  },
                                    { 1, 11, 31, 90 },
                                    { 2, 51, 1, 9 }
                                };

        for (int[] row:myFirst2DArray) {

            for (int columnElement : row) {
                sum+=columnElement;
            }
            
        }
        System.out.println(sum);
    }
}