For-each Loop in Java

Question

I have a sample code which creates an "array" of size 10 and tries to initialize it with reverse values in a For loop e.g:(9,8,7,6,5...0):

int[] array = new int[10];
        for (int i = array.length - 1, j = 0; i >= 0; i--, j++) {
            System.out.println("Present value of i=" + i
                    + " Present value of j=" + j);
            array[j] = i;
            System.out.println("Array:" + j + "=" + i);
        }
        for (int k : array) {
            System.out.println(array[k]);
        }

So far so good. This is the output from console which is perfect:

Present value of i=9 Present value of j=0
Array:0=9
Present value of i=8 Present value of j=1
Array:1=8
Present value of i=7 Present value of j=2
Array:2=7
Present value of i=6 Present value of j=3
Array:3=6
Present value of i=5 Present value of j=4
Array:4=5
Present value of i=4 Present value of j=5
Array:5=4
Present value of i=3 Present value of j=6
Array:6=3
Present value of i=2 Present value of j=7
Array:7=2
Present value of i=1 Present value of j=8
Array:8=1
Present value of i=0 Present value of j=9
Array:9=0

The issue is with For-each loop in the end which is just printing the values in array:

for (int k : array) {
            System.out.println(array[k]);
        }

The values printed array are 0,1,2...9 where it should be 9,8,7...0

When I use the regular For loop to print the array, it works normally. Am I missing some funny business here?

What does for (int k : array) mean? What does array[k] mean? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Sep 17, 2014 at 22:47
Throwing this out there; Java != Javascript. The way you're doing things is how javascript works; an advanced for...in loop iterates keys of an object. Java's for-each loop only works with values; it has no concept of keys. — Qix - MONICA WAS MISTREATED
– Qix - MONICA WAS MISTREATED, Commented Sep 17, 2014 at 22:50
Just a rule of thumb; standard for loops are pretty standard across languages. For-each loops, however, are pretty different across the board. Never assume a foreach loop works the same, or even exists, in another language! :) — Qix - MONICA WAS MISTREATED
– Qix - MONICA WAS MISTREATED, Commented Sep 17, 2014 at 22:52

rgettman · Accepted Answer · 2014-09-17 22:48:25Z

11

You are already getting the values out of the array with your foreach loop, which you are using as an index again into the array, yielding the values in order again.

Just print k. Change

for (int k : array) {
    System.out.println(array[k]);
}

to

for (int k : array) {
    System.out.println(k);
}

End of the output:

answered Sep 17, 2014 at 22:48

rgettman

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1 Comment

Qix - MONICA WAS MISTREATED Over a year ago

For anyone confused; for(var key in array){var val = array[key]} is how you can iterate keys in Javascript. Java's foreach loops work with values directly. I think this is the source of the confusion.

ajb · Accepted Answer · 2014-09-17 22:51:48Z

7

Basically, since (int k : array) causes k to go through the values in the array, not the indexes, what you've done is equivalent to

System.out.println(array[array[0]]);
System.out.println(array[array[1]]);
System.out.println(array[array[2]]);
System.out.println(array[array[3]]);
...
System.out.println(array[array[9]]);

which isn't what you want.

answered Sep 17, 2014 at 22:51

ajb

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Comments

Moni · Accepted Answer · 2014-09-17 23:06:55Z

0

This for loop, for (int k : array) is basically gives you the array value in detail this for loop is like -

int k = array[0] = 9 int k = array[1] = 8 .....

and again you are trying to print array[9] , array[8] which gives you result like 0,1,2... replace for( int k : array){ System.out.println(array[k]); }

with for(int k : array){ System.out.println(k); }

answered Sep 17, 2014 at 23:06

Moni

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