John Meyer's SCJP 5 mock exam doubt

[Rizwan Patel]

First of all thanks for the mock exam provided.

I have gone through this test and have confusion in the below problem.

  question  76:
 
int []arr = {1,2,3,4};
for ( int i : arr )
{
   arr[i] = 0;
}
for ( int i : arr )
{
   System.out.println(i);
}
 
Correct answer is : 0 0 3 0  

Can anyone explain why this happen ?

when we changes the first for loop as

 for( int i=0;i<arr.length;i++)
arr[i] =0;
The output changes to : 0 0 0 0
 

[Christophe Verré]

Take each loop step by step :

i = arr[0] <- 1
arr[1] = 0;
i = arr[1] <- 0 (it's been updated above)
arr[0] = 0;
i = arr[2] <- 3
arr[3] = 0;
i = arr[3] <- 0 (it's been updated above)
arr[0] = 0;

Result:0,0,3,0

Your for-loop is different. You are incrementing i from 0 to the length of the array. The enhanced for loop sets i to the value of each element of the array. It should look like

for( int i=arr[0],j=0;j<arr.length;i=arr[j],j++)
arr[i] = 0;

[Rizwan Patel]

Thanks alot for this nice explaination.
I was making mistake in assuming both the for loops are equals.
Now it is much clear. Once again Thank you