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 i         


        

Answer 1

For Each loop that you have used in here does assign j a 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

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 3

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 4

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 5

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 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!