Arrays and enhanced for loops?

Question

Can someone please explain what is going on in this method?

class test{  
public static void main(String args[])
{
   int[] x = new int[4]; 
   int[] xy = new int[4]; 
  //System.out.println(xy[0]); 
    for(int j : x) { 
       xy[j] += 1;
    }
   System.out.println(xy[0]); }}

So I thought that the enhanced for loop would be doing this

/**for(int j=0; j < x.length(); j++)
xy[j=0] = 1
xy[j=1]=1
xy[j=2]=1
xy[j=3]=1*/

but from what I've been reading, the enhanced for loop is doing for(int element:array) . Still, I don't understand what the for loop in my method is actually doing. I have tried System.out.println() statements to check what was being put into the array xy , but it is either addresses or 0.

Thanks for all the help and apologizes if this is confusing.

Answer 1

For Each loop that you have used in here does assign ja value that is already assigned to array x (which in your case is 0 ).

As according to your case of x and xy array:-

x[0]=0;    xy[0]=0
x[1]=0;    xy[0]=0
x[2]=0;    xy[0]=0
x[3]=0;    xy[0]=0

Describing for each loop according to your program case:-

for(j : x)

1. This implies it will run for 4 times which is the length of your array x.

2. when running first time the following process will happen

    j=x[0] (so j=0 coz x[0] according to your case) xy[0]=xy[0]+1 (so now xy[0] value becomes 1) 

3. Similarly for the second run of for each

    j=x[1] (so j=0 coz x[0] according to your case) xy[0]=xy[0]+1 (so now xy[0] value becomes 2 as in previous for each run xy[0]=1) 

4. So all in all finally you will have xy[0]=4 at the end of 4th run of for each loop.

Finally the print statement will print 4.

Answer 2

int[] x = new int[4]; 

This creates an array of 4 elements. Each element's value is 0.

for(int j : x) { 
   xy[j] += 1;
}

This iterates through all the values of x . At each iteration, j is the next element in x , but since all elements are initialized to 0, j is always 0. So, the 4 iterations increment xy[0] .

Answer 3

Here in the advanced for loop, the int j does not represent the index. Rather it represents the values in the xy[] array. It is not possible to get the index of advanced for loop. If you want index, then you might have to use ordinary for loop. Ex. If you have

xy[]={5,8,3,4};
for(int j:xy)
{
    println(j);
}

then the output would be

5
8
3
4

Answer 4

It should be straight-forward to find out, if you trace through using debugger.

In brief, it is getting each value in x , and use the value as index to increment corresponding value in xy .

It will be more obvious if you initialize both array with meaningful values:

int[] x = {1,1,3,3}
int[] xy = new int[4]; // initialized to 0 by default

after your piece of code, you will find xy containing {0,2,0,2} .

---

You should understand how enhanced-for-loop is expanded for array:

for (T v : arr) {
    // do something 
}

is transformed to

for (int i = 0; i < arr.length; ++i) {
    T v = arr[i];
    // do something 
}

Obviously your understanding on how enhanced for loop being expanded is wrong.

Answer 5

In your case you need only for-loop and not enhanced for-loop:

for(int j = 0 ; x.length > j; j ++) { 
   xy[j] += 1;
}

Problem with your code is that your loop traverse only at index 0 of xy hence you get xy as [4, 0, 0, 0]

int[] x = new int[4];//default values [0, 0, 0, 0]
int[] xy = new int[4]; //default values [0, 0, 0, 0]
for(int j : x) { // j is always 0
   xy[j] += 1;// xy[0] += 1
}

Answer 6

You can use the for-loop instead:

for(int i = 0 ; x.length > i; i ++) { 
xy[i] += 1;
}

If you want to still use for each then here is the example

a[]={1,2,3,4};
for(int i:a)
{
println(i);
}

Hope it helps!