Chaining array assignment in Java

Question

Consider the following code snippet in Java. I know that the statement temp[index] = index = 0; in the following code snippet is pretty much unacceptable but it may be necessary for some situations:

package arraypkg;

final public class Main
{
    public static void main(String... args)
    {
        int[]temp=new int[]{4,3,2,1};
        int index = 1;

        temp[index] = index = 0;
        System.out.println("temp[0] = "+temp[0]);
        System.out.println("temp[1] = "+temp[1]);
    }
}

It displays the following output on the console.

temp[0] = 4
temp[1] = 0

I do not understand temp[index] = index = 0;.

How does temp[1] contain 0? How does this assignment occur?

I think it's pretty clear that he expects index to first become 0 making temp[index] equivalent to temp[0] so that only the first element is modified. — Wayne
– Wayne, Commented Dec 15, 2011 at 19:12
The take home message here should be, don't write code this way. Knowing the fiddly bits of Java is a respectable skill, but mostly useless. Code like this simply shouldn't exist, and where it is found should be eradicated immediately. — rfeak
– rfeak, Commented Dec 15, 2011 at 19:18

Bhesh Gurung · Accepted Answer · 2011-12-15 19:54:24Z

9

The assignment is done (temp[index] = (index = 0)), right associative.

But first the expression temp[index] is evaluated for the LHS variable. At that time index is still 1. Then the RHS (index = 0) is done.

edited Dec 15, 2011 at 19:54

Bhesh Gurung

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answered Dec 15, 2011 at 19:13

Joop Eggen

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2 Comments

Wayne Over a year ago

+1 - First person to directly address the question the OP is actually asking (about LHS vs. RHS order of evaluation).

Wayne Over a year ago

Supporting evidence: java.sun.com/docs/books/jls/third_edition/html/…

chubbsondubs · Accepted Answer · 2011-12-15 19:09:02Z

1

Your statement assigned zero to it. The statement temp[index] = index = 0 wrote zero into index AND into temp[index]. That's what that meant. Make all variables to the left of an assignment operator 0.

answered Dec 15, 2011 at 19:09

chubbsondubs

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Comments

Jon Egeland · Accepted Answer · 2011-12-15 19:09:34Z

1

What that line does is say that temp[index] should equal index after index is assigned the value 0.

This is why this syntax is mostly unacceptable. It's hard to read and most people don't understand it.

answered Dec 15, 2011 at 19:09

Jon Egeland

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Comments

user999822 · Accepted Answer · 2011-12-15 19:14:04Z

1

you assigned both temp[1] and index to '0' it running left to right. think ass temp[index/* */]

answered Dec 15, 2011 at 19:14

user999822

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