java operators and operands

Question

consider

    int a =10;
    c = a/a;          //c becomes  1.
    c= a* + a/a ;     //c becomes  1.     
    c= 5+ a* + a/a ;  //c becomes  15.
    c = ++a + a++ - --a - a-- + a * + a/a ;     //c becomes  10.

Can someone please tell me how the above works ? Especially the a* part. The first one is only for reference.

`c= a* + a/a ; //c becomes 1.` This is wrong, c becomes 10 here. — Alexis C., Jan 09 '14 at 17:50
`a* + a` is just a confusing way of writing `a * +a / a`, which is a confusing way of writing `a * a / a` — ikegami, Jan 09 '14 at 17:53

score 1 · Answer 1 · answered Jan 09 '14 at 17:55

In this line:

c= a* + a/a ;

The + is the unary plus operator. So it is equivalent to

c = a * a/a;

The variable a is 10, so 10 * 10/10 is 10, not 1. Then...

c= 5+ a* + a/a;

is equivalent to c = 5 + a * a/a, so c is now 15. Then

c = ++a + a++ - --a - a-- + a * + a/a ;

Pre-increment changes a to 11. Post increment a++ increments to 12 but yields the old value 11, yielding 22 so far. Then Pre decrement decrements to 11 and the subtraction yields 11. Next, post decrement decrements to 10 but yields the old value 11, and the subtraction yields 0. We already know what a * + a/a is, 10.

ikegami · Answer 2 · 2014-01-09T19:02:24.373

Let's look at the third line.

c = a * + a / a;

There is no postfix * operator (EXPR*), but there is an infix * operator (EXPR * EXPR): The multiplication operator. The * must therefore be a multiplication operator.

There is a prefix + operator (+EXPR): The unary plus operator. The + must therefore be a unary plus operator.

The above is equivalent to

c = (a * (+a)) / a;
    (10 * (+a)) / a
    (10 * (+10)) / a
    (10 * 10) / a
    100 / a
    100 / 10
    10

It's exactly the same situation for the EXPR * + EXPR in the next line.

c = 5 + a * + a / a

is equivalent to

c = 5 + ((a * (+a)) / a)
    5 + ((10 * (+a)) / a)
    5 + ((10 * (+10)) / a)
    5 + ((10 * 10) / a)
    5 + (100 / a)
    5 + (100 / 10)
    5 + 10
    15

It's exactly the same situation for the EXPR * + EXPR in the last line.

c = ++a + a++ - --a - a-- + a * + a/a;

is equivalent to

c = ((((++a) + (a++)) - (--a)) - (a--)) + ((a * (+a)) / a);

The last line both uses and modifies a in the same expression. In most languages, the behaviour of such code is undefined because the operand evaluation order is undefined. Even then, understanding when a reference is a placed on the stack as oppose to a copy is crucial, and that makes for poor code.

In Java, the order is defined, but it is not actually the case in my testing. Let's look at ((++a) + (a++)) - (--a) with an initial value of 10 for a.

Using left-to-right evaluation:

((++a) + (a++)) - (--a)    [a=10]
(11 + (a++)) - (--a)       [a=11]
(11 + 11) - (--a)          [a=12]
22 - (--a)                 [a=12]
22 - 11                    [a=11]
11

Using java, you get 13. As such, I won't actually show how your last line is evaluated. That would require inspect the generated byte code, which goes to show it's awful code if it's even valid.

java operators and operands

2 Answers2