Enhanced For Loop For Array

[Sankar Raj]

Kindly explain the way the enhanced for loop iterates and the way it varies from the conventional for loop with the below example ?

Source : J.I Inquisition John Meyers SCJP5 Mock Exam

Question :

int []arr = {1,2,3,4};
for ( int i : arr )
{
arr[i] = 0;
}
for ( int i : arr )
{
System.out.println(i);
}

Answer :

Prints 0030

As with the conventional for loop i thought it would print 0000.

To understand the way it works, i tried out as below

for ( int i : arr )
{
System.out.println("New For Index "+ i);
arr[i] = 0;
System.out.println("New For "+ i + " Value " + arr[i]);
System.out.println(" ********* ");
}

for ( int i : arr )
{
System.out.println("New For Output "+i);
}

and got below as output,

New For Index 1
New For 1 Value 0
*********
New For Index 0
New For 0 Value 0
*********
New For Index 3
New For 3 Value 0
*********
New For Index 0
New For 0 Value 0
*********
New For Output 0
New For Output 0
New For Output 3
New For Output 0


Help me out in getting the result

[Ivan Ivanic]

This is quite easy to understand:
For-each loop has internal counter that moves pointer to next element in each iteration.
So:
1st iteration here is how array looks:

1 | 2 | 3 | 4          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
^                          --> this is position of pointer which for-each loop takes care of

So code

   arr[i] = 0  

assigns value of 1 to i because pointer points to that value in arr.
That said code looks like this:

 arr[1] = 0 

and changes arr to look like this:

1 | 0 | 3 | 4          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
^                          --> this is position of pointer which for-each loop takes care of

2nd iteration:
Pointer moves to the next value:

1 | 0 | 3 | 4          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
    ^                       --> this is position of pointer which for-each loop takes care of

So code

   arr[i] = 0  

assigns value of 0 to i because pointer points to that value in arr.
That said code looks like this:

 arr[0] = 0 

and changes arr to look like this:

0 | 0 | 3 | 4          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
    ^                          --> this is position of pointer which for-each loop takes care of

3rd iteration:
Pointer moves to the next value:

0 | 0 | 3 | 4          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
        ^                  --> this is position of pointer which for-each loop takes care of

So code

   arr[i] = 0  

assigns value of 3 to i because pointer points to that value in arr.
That said code looks like this:

 arr[3] = 0 

and changes arr to look like this:

0 | 0 | 3 | 0          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
        ^                  --> this is position of pointer which for-each loop takes care of

4th iteration:
Pointer moves to the next value:

0 | 0 | 3 | 0          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
            ^              --> this is position of pointer which for-each loop takes care of

So code

   arr[i] = 0  

assigns value of 0 to i because pointer points to that value in arr.
That said code looks like this:

 arr[0] = 0 

arr[0] is changed but to the same value and looks like this:

0 | 0 | 3 | 0          --> these are values in arr
0 | 1 | 2 | 3          --> this is index of arr
            ^              --> this is position of pointer which for-each loop takes care of

I hope this helps

[Sankar Raj]

Thank You. I was confused & tensed as my exam date is nearing.. thanks for helping me out..

[Ivan Ivanic]

On The Exam you will have paper and pen or whiteboard and marker. Always use them to visualize things that are hard for you to follow: execution of loops, creation/garbage collection of objects, inheritance... Draw, write, sketch use lines, rectangles, dots - anything to keep your mind working.
Take your time to actually think, that way you will not get stuck in a place.
All the best wishes for your exam,
Ivan.

[Chaitanya Lele]

Consider the following code

 public class Test {

    public static void main(String[] args)
    {
        int[] arr = {3,0,1,2};
        for(int i:arr)
        {
            arr[i]=0;
        }
        for(int i:arr)
        {
            System.out.println("arr[i] = "+ arr[i]);

        }
    }
}   

The output produced is 0 0 0 0

But according to Ivan's explanation above the output should be 0 0 1 0

Really confused here...

[Vishwanath Krishnamurthi]

Wow. This is a tricky question.

The thing is that the elements of the array are of course { 0, 0, 1, 0}

But when you're printing you are not printing i but arr[i]... See the difference?


Vishwa