Arrays

Hi Ranchers,

The following question is from John Meyers mock exam:

int[] arr = {1,2,3,4,5};

for(int i : arr)
{
arr[i] = 0;
}

for(int i : arr)
{
System.out.println(arr[i])

}

What wil be the output of the program ?

a) 0030
b) 0000

The coirrect answer is 0030.

Can anyone explain how ?

To be honest, I expected this code to give you an ArrayIndexOutOfBounds exception due to the fact that at some point you will have "arr[5] = 0". Are you sure this is the right code for the answer you provided?

Mihai Fonoage

(offtopic) frankly similar question which made me fall in love with javaranch (offtopic)

PS:I assumed that code
<blockquote>code:

<pre name="code" class="core">System.out.println(arr[i])</pre>

</blockquote>where you leave semicolon is infact your mistake and I inserted it there in my explanation otherwise program won't even compile.

the answer you get here is not correct
examine the code through some debugger
search this forum , there are couple of threads explaining exact question

[b]1st iteration of for loop[b]
i=arr[0]
so i is 1 now
and by this

arr[i]=0
the arr changes to
arr={1,0,3,4,5}

[b]2nd iteration of for loop[b]
i=arr[1]

remember array got updated in previous iteration

so i is 0 now
and by this code

arr[i]=0

the arr changes to
arr={0,0,3,4,5}

[b]3rd iteration of for loop[b]
i=arr[2]

remember array got updated in previous iteration but arr[2] do not

so i is 3 now
and by this code

arr[i]=0

the arr changes to
arr={0,0,3,0,5}

[b]4th iteration of for loop[b]
i=arr[3] // arr[3] current value is 0

remember array got updated in previous iteration

so i is 0 now
and by this code

arr[i]=0

the arr changes to
arr={0,0,3,0,5}

or simple words this iteration do not update the array

[b]5th iteration of for loop[b]
i=arr[4] // arr[4] current value is 5

so i is 0 now
and by this code

arr[i]=0

here we get excetion so probably you wrote this question incorrectly or your source of question is not reliable

I explained because it shows a good point which you might miss

Hi Manpreet,

Check out this.
Doubt in Array

Excellent Vaibhav. Thanks. And thank you Milind for the link..

Hi milind

Thanks for the link.

But I still didn't get it!

How does the enhanced loop work.

What's the initalisation and the condition part.

I thought it's

i=0 and i<arr.length by Default for (int i : arr) (any enhanced loop)

What do you mean by "value will be the same as the current array element"
as mentioned in K&B

The coirrect answer is 0030.

How is that the correct answer..

I am getting
ArrrayIndexOutOfBoundException

Hi, Nabila Mohammad

Please change the array initialization, to {1,2,3,4} instead of {1,2,3,4,5}

if you included 5, you will get that error.

Did you try it out, Christian? If you change it to {1, 2, 3, 4} then you will still get an ArrayIndexOutOfBoundsException.

Note that:

for (int i : arr)

is not the same as:

for (int i = 0; i < arr.length; i++)

Instead, it is the same as this:

<blockquote>code:

</blockquote>

Look at it carefully. Do you see now what's happening? i is not the index into the array, but the array element itself.

Originally posted by Christian Polychroniadis:
Hi, Nabila Mohammad

Please change the array initialization, to {1,2,3,4} instead of {1,2,3,4,5}

if you included 5, you will get that error.

After changing it to {1,2,3,4} I am getting 0000 .

I just went through link given by Milind and there the question is same but the value in arr in {1,2,3,4}. If I solve it on paper it gives me the answer 0030 which is the correct answer. But if I try to code it and solve it on my laptop, it gives the answer 0000. Where am I going wrong? Or the JVM is wrong?? Please help.

Hi All,
I changed the arr contents to {1,2,3,4} and tried to solve the problem and here is what i got.
I had the same doubt as phal had.When I solved the problem, I found that 0030 must be the answer.But when I executed the program , i found the answer as 0000.
Then I introducted the following portion in the code ,before the enhanced for loop to print the array elements given at the end.
<blockquote>code:<hr><pre name="code" class="core">
for (int k=0;k<arr.length ;k++ )
{
System.out.println(arr[k]);
}</pre><hr></blockquote>

Then I understood that the for loop given at the end of the program, actually gets the elements of array one by one , assigns it to i and then again gets the content in a[i].

Now as per the program, after the first for loop, the array contents are {0,0,3,0}

Now Consider the execution of
<blockquote>code:<hr><pre name="code" class="core">for(int i : arr)
{
System.out.println(arr[i]);
}</pre><hr></blockquote>
1.First iteration i = a[0], so i becomes 0 and we print a[i] i.e, a[0] = 0
2.First iteration i = a[1], so i becomes 0 and we print a[i] i.e, a[0] = 0
3.First iteration i = a[2], so i becomes 3 and we print a[i] i.e, a[3] = 0
4.First iteration i = a[3], so i becomes 0 and we print a[i] i.e, a[0] = 0

That is how we get {0,0,0,0} printed in our console
If the print statement directly prints 'i', instead of a[i], then we would get 0030
Please let me know if my understanding is correct.
Thanks.
[ July 15, 2008: Message edited by: Meena Subramanian ]

I just went through link given by Milind and there the question is same but the value in arr in {1,2,3,4}. If I solve it on paper it gives me the answer 0030 which is the correct answer. But if I try to code it and solve it on my laptop, it gives the answer 0000. Where am I going wrong? Or the JVM is wrong?? Please help.

you are doing it wrong
when printing loop you might be using this code
<blockquote>code:

<pre name="code" class="core">
System.out.println(arr[j]);
</pre>

</blockquote>
instead you should use this
<blockquote>code:

<pre name="code" class="core">System.out.println(j);</pre>

</blockquote>
I hope this might show you what exactly is your mistake

this code
<blockquote>code:

<pre name="code" class="core">
for(int i : arr)
{
System.out.println(arr[i]);
}</pre>

</blockquote>
is same as
<blockquote>code:

</blockquote>
which is not clearly code for printing our array

1.First iteration i = a[0], so i becomes 0 and we print a[i] i.e, a[0] = 0
2.First iteration i = a[1], so i becomes 0 and we print a[i] i.e, a[0] = 0
3.First iteration i = a[2], so i becomes 3 and we print a[i] i.e, a[3] = 0
4.First iteration i = a[3], so i becomes 0 and we print a[i] i.e, a[0] = 0

this explains

[ July 15, 2008: Message edited by: vaibhav mishra ]
[ July 15, 2008: Message edited by: vaibhav mishra ]

OK, I'm going to explain, first you must change the array declaration to {1,2,3,4}, and that's what happen in the first for,

initial array, value, final array
iteration 1: {1,2,3,4} a[1]=0 {1,0,3,4}
iteration 2: {1,0,3,4} a[0]=0 {0,0,3,4}
iteration 3: {0,0,3,4} a[3]=0 {0,0,3,0}
iteration 4: {0,0,3,0} a[0]=0 {0,0,3,0}

The problem is the second for, if you see it, it do something like this:

print a[0] three times, and a[3] once, that's why you get 0 0 0 0.

If you want to print all the content of the array you need change it:

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

that's how you get 0 0 3 0

Now I think is clear.

Thanks guys for your detailed explanation. It really helped.

Hey, you are right, Christian. I didn't look into it far enough.