CODEWITHSUNDEEP
javaarraysforeach
Devendra Jadhav
Devendra Jadhav

Reputation: 21

Can anyone please explain how this is working?

int[] a = {1, 2, 3,4,1};
for (int n: a) {
    a[n] = 0;
}
for (int n: a) {
    System.out.print(n);
}

This for each loop showing something unexpected result when I make a all array elements 0.Instead of showing all '0' it is giving output something like '00301'.Not able to understand how this is working?

Upvotes: 0

Views: 85

Answers (5)

Andreas
Andreas

Reputation: 159106

If you print out what it does, you'll see for yourself:

int[] a = {1, 2, 3,4,1};
System.out.println(Arrays.toString(a));
for (int n: a) {
    System.out.print("a[" + n + "] = 0;  ->  ");
    a[n] = 0;
    System.out.println(Arrays.toString(a));
}

Output

[1, 2, 3, 4, 1]
a[1] = 0;  ->  [1, 0, 3, 4, 1]
a[0] = 0;  ->  [0, 0, 3, 4, 1]
a[3] = 0;  ->  [0, 0, 3, 0, 1]
a[0] = 0;  ->  [0, 0, 3, 0, 1]
a[1] = 0;  ->  [0, 0, 3, 0, 1]

Upvotes: 0

Akinsola Michael
Akinsola Michael

Reputation: 21

As stated by the Oracle documentation, the java for-each loop hides the iterator, and gives you access to only the values in the iterable (array). Therefore, the for-each loop should not be used for mainupulating the original array, rather should be used for working with the data of the array. https://docs.oracle.com/javase/1.5.0/docs/guide/language/foreach.html If you want to manipulate the original array, your best bet is to use the conventional for loop.

Upvotes: 2

Amit Bera
Amit Bera

Reputation: 7325

In:

for (int n: a)

n is the value of a particular index the array, not the index itself. So, you are basically setting 0 at index 1, 2, 3,4,1. Luckily there is no value greater than 4 (the length of the array) or negative number else you will get ArrayIndexOutOfBoundException.

for (int i=0; i<a.length;i++){
  a[i] = 0;
}

Use the above code and you will get the expected output.

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201447

It's two for-each loops over the contents of an int[].

Loop 1: for each n in 1, 2, 3, 4, 1 set the contents of the array to zero for indices 1, 0, 3, 0 and 1. Which you can see by changing it to

for (int n: a) {
    System.out.println(n);
    a[n] = 0;
}

Leaving the contents of a as 0, 0, 3, 0, 1 then

Loop 2: for each n in 0, 0, 3, 0, 1 print n.

Thus your program prints 00301.

Upvotes: 1

John3136
John3136

Reputation: 29266

a[n] = 0; is setting the a[some_value_from_the_array] to 0, not setting all the array elements to 0.

Upvotes: 2

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