对于使用二维数组的每个循环

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);
    }
}