Re: Increment and decrement operators

[lwloen@xxxxxxxxxx]

a[i++]++
a[i++]=a[i++] + 1;

As stated, there's no decrement above, so is this exactly right?

For starters, the index variable "i" is incremented twice in the second
statement, once in the first.
So, regardless of which contents of "a" get changed, there's one difference
right off the bat.

Some of this kind of thing can be done with experiments easily enough
nowadays.

Try this out:

public class test {

 public static final void main (String args[]) {

   int a[] = new int[4];
   int i;
   for (i=0; i<4; i++) a[i]=i;
   i=0;
   a[i++]++;
   System.out.println("a[0] is : "+a[0]+
    " a[1] is : "+a[1]+
    " a[2] is : "+a[2]+
    " a[3] is : "+a[3]+
    " and i is :" + i);
   for (i=0; i<4; i++) a[i]=i;
   i=0;
   a[i++]= a[i++]+1;
   System.out.println("a[0] is : "+a[0]+
    " a[1] is : "+a[1]+
    " a[2] is : "+a[2]+
    " a[3] is : "+a[3]+
    " and i is :" + i);
 }

}

For output, I get (IBM Windows 95 1.1.7 JDK):

a[0] is : 1 a[1] is : 1 a[2] is : 2 a[3] is : 3 and i is :1
a[0] is : 2 a[1] is : 1 a[2] is : 2 a[3] is : 3 and i is :2

If you want to figure out more, fire up a decent IDE against this
and watch how the variables change as you let your debugger step through
the program.

I wouldn't be 100% sure about this little example being portable,
by the way.  (It's the a[i++]= a[i++] + 1 that worries me in terms
of which locations in "a" get fetched, all dependent on when the
increments of i take effect, especially the one on the right hand
side of the assignment).

In C, I don't believe that second example would be portable. Two post
increments of the same variable on the same statement
are allowed to optimized any number of ways as I recall the standard.
Perhaps Java restricts this so it is portable, but I'd have to look
it up to be sure; partly this is a "javac" question and partly
a question of what JIT and other Java compilers (e.g. AS/400 Transformer)
are allowed to do.

This is the sort of thing that keeps language gear heads up late
at night, but real programmers quickly learn to avoid that kind of
thing to start with.

But, it's a nice teaching example.



Larry W. Loen  -   Senior Java and AS/400 Performance Analyst
                          Dept HP4, Rochester MN
--
We must do a few things well, rather than everything late.
8-553-3535   (507) 253-3535


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