Enhanced For Loop Problem

[Charandeep Singh]

We have a prog
class ArrTest
{
public static void main(String[] args)
{
int []arr = {1,2,3,4};

for(int i=0;i<arr.length;i++){// line 1
System.out.println(i);
arr[i]=0;
}

for(int j : arr)

System.out.println(j);
}
}

The output is
0
1
2
3
0
0
0
0
Where as if i use enhanced for loop in place of line 1
the out put is
1
0
3
0
0
0
3
0

I always thought they both worked same way
can any body explain the different usage between these two for loops

Thanks
Charandeep

[anil kumar]

---------------------------------------------------------------------
for(int i=0;i<arr.length;i++){// line 1
System.out.println(i);//line 2
arr[i]=0;//line 3
--------------------------------------------------------------------

At line 2 you are printing the value of varibale i

and at line 3 you are assigning the 0 to every element in the array


if you replace the line 1 like this
for(int j:arr)
System.out.println(j);

you will get

1
2
3
4


But if you place the enhanced for loop after line 3
you will get alll 0's.Because you assigned the array elements to 0.



Thanks
Anil Kumar

[Henry Wong]

What do you mean by this statement?

Where as if i use enhanced for loop in place of line 1
the out put is

Do you mean this?

for(int i : arr){        // line 1
  System.out.println(i);
  arr[i]=0;
}

If you did this, then it translates back to...

for(int temp=0; temp<arr.length; temp++){        // line 1
  i = arr[temp];
  System.out.println(i);
  arr[i]=0;
}

Remember, with the enhanced for loop. The loop variable is not the index, it is the element of the array.

Henry

[Charandeep Singh]

I also always felt that when we use the enhanced for loop
it translates back to orignal for loop only
but

class ArrTest
{
public static void main(String[] args)
{
int []arr = {1,2,3,4};

for(int i: arr){
System.out.println("x" +i);
arr[i]=0;
}

for(int j : arr)

System.out.println("Hello World! x" +j);


}
}

The output is
x 1
x 0
x 3
x 0
Hello World! x 0
Hello World! x 0
Hello World! x 3
Hello World! x 0

But now if you change the program some what like this
class ArrTest
{
public static void main(String[] args)
{
int []arr = {1,2,3,4};

for(int i=0;i<arr.length;i++){
System.out.println("x" +i);
arr[i]=0;
}

for(int j : arr)

System.out.println("Hello World! x" +j);

}
}

The output is
x 0
x 1
x 2
x 3
Hello World! x 0
Hello World! x 0
Hello World! x 0
Hello World! x 0

This is the difference I am talkin about
Can you please explain why and wha is the difference between the two for loops.

Thanks
Charandeep

[Henry Wong]

This is the difference I am talkin about
Can you please explain why and wha is the difference between the two for loops.

Charandeep,

Please go back an re-read my response. Basically, the variable "i" in the enhanced for loop is *not* the index, but the value.

This...

for(int i: arr){
  System.out.println("x " +i);
  arr[i]=0;
}

And this...

for(int i=0;i<arr.length;i++){
  System.out.println("x " +i);
  arr[i]=0;
}

Are not the same logic...

Henry
[ May 12, 2007: Message edited by: Henry Wong ]

[Charandeep Singh]

Hi Henry
Thanks for the explanation
I got your point
My question is
when the enhanced for loop is used on line 1
and after that arr[i]
that means value of i will be 1,2,3,4
so when it reaches 4 it should give ArrayIndexOutOfBoundException
which it doesn't
where as if you remove arr[i]=0;
It does give that
Can you clarify it further

Thanks
Charan

[megha joshi]

Hi,

If you do the following

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

On first iteration
i = 1;
a[i] = 0 => a[1] = 0 => arr = {1,0,3,4}

On second iteration
i = 0;
a[i] = 0 => a[0] = 0 => arr = {0,0,3,4}

On third iteration
i = 3
a[i] = 0 => a[3] = 0 => arr = {0,0,3,0} //this is the step tht prevents the OutOfBoundsexcetion

On forth iteration
i = 0
a[i] = 0 => a[4] = 0 => arr = {0,0,3,0}


This was tricky...I hope the above helps...

Thanks,
Megha

[Charandeep Singh]

Thanks a lot megha
Got it
Logical mistake from my end
thanks a lot for clarifying it