Java for-each上的行为不一致

Question

Consider this code:

class Jm44 {

public static void main(String args[]){

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

      for ( int i : arr )
      {
         System.out.println(i);
      }


     }

}

It prints:

0 
0 
3 
0

What's this? For-each is supposed to run over all the elements in the array, why would it run arr[3]=0 but not arr[2]=0 ?

Answer 1

If you look at what happens to arr in the first loop, it becomes obvious.

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

    for (int i : arr) {
        System.out.println(i);
    }

This prints:

i = 1
arr = [1, 0, 3, 4]
i = 0
arr = [0, 0, 3, 4]
i = 3
arr = [0, 0, 3, 0]
i = 0
arr = [0, 0, 3, 0]
0
0
3
0

You are modifying the values in the array, using the values in the array as indexes. The "foreach" loop goes through the values of the array, not the indexes of the array. After removing the syntactic sugar, here is what your foreach loop actually is:

    int[] arr = {1, 2, 3, 4};
    for (int index = 0; index < arr.length; index++) {
        int i = arr[index];
        arr[i] = 0;
    }

    for (int i : arr) {
        System.out.println(i);
    }

To be able to index the array, you need to use the traditional for loop, like this:

    int[] arr = {1, 2, 3, 4};
    for (int i = 0; i < arr.length; i++) {
        arr[i] = 0;
    }

    for (int i : arr) {
        System.out.println(i);
    }

Answer 2

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

Watch arr in the debugger.

1,2,3,4 (initially)
1,0,3,4 (after we change arr[arr[0]], or arr[1])
0,0,3,4 (after we change arr[arr[1]], or arr[0])
0,0,3,0 (after we change arr[arr[2]], or arr[3])
0,0,3,0 (after we change arr[arr[3]], or arr[0] (no change, actually) 

Answer 3

In Java, looping over an array using the

for (int i : arr)

syntax loops over the values in the array, not the indexes . This is different to the JavaScript loop

for (var i in arr)

that loops over indexes. Arrays are also indexed starting from 0 - the first item in the array is arr[0] . @Esko's answer shows what's actually happening.

Answer 4

because

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

doesnt set the array to 0s.

surely you meant to say

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

Answer 5

You cannot use a foreach loop to set values in an array. The for each loop works a little differently than you expect.

(Explanation comming)

Answer 6

you have to be careful not to confuse index and value of an arrayelement

these are the values of the array before and after each iteration of the first loop, including the value of i:

{1,2,3,4}

{1,0,3,4} i=1

{0,0,3,4} i=0

{0,0,3,0} i=3

{0,0,3,0} i=0

Answer 7

不要混淆循环计数器和数组值...我认为这是在这里发生的

Answer 8

Unroll the first loop:

int[] arr = { 1, 2, 3, 4 };
int i;
i = arr[0]; // i = 1
arr[i] = 0; // arr[1] = 0
i = arr[1]; // i = 0
arr[i] = 0; // arr[0] = 0
i = arr[2]; // i = 3
arr[i] = 0; // arr[3] = 0
i = arr[3]; // i = 0
arr[i] = 0; // arr[0] = 0

So, arr[2] (the third element) is never set to zero; it retains its original value of 3.