About Question enthuware.ocajp.i.v7.2.1040 :

[ETS User]

Could you explain/postLink how are we evaluating these expressions because i thought that "a" becomes 4 and then we add it to itself
so 4 + 4 , but my guess is wrong.
a += (a = 4);
b = b + (b = 5);

[admin]

Could you explain/postLink how are we evaluating these expressions because i thought that "a" becomes 4 and then we add it to itself
so 4 + 4 , but my guess is wrong.
a += (a = 4);

a += (a=4) =>
a = a + (a=4) =>
a = 10 + (value of expression a=4)
The value of a is 10 before a=4 is executed so you put 10 for the first a.
Remember that every assignment expression itself has a value so (a = 4) itself has a value, which is the value that you are assigning to the left hand side i.e. 4.
At this point the value of a is 4, but there one more operation left.
a = 10 + 4
a = 14. So a is set to 14.

HTH,
Paul.

[The_Nick]

I thoroughly understand the point.
However, int a = 10; a = a + (a = 4); By having the parenthesis should not it be prioritized the parenthesis bit of the expression rather than the left part to the "+"?
In few words, why a gets the value of 10 if the mini expression within parenthesis should be valuated first?
Taking into account that () are > than + in terms of priority.
Thanks in advance.

[admin]

I thoroughly understand the point.
However, int a = 10; a = a + (a = 4); By having the parenthesis should not it be prioritized the parenthesis bit of the expression rather than the left part to the "+"?
In few words, why a gets the value of 10 if the mini expression within parenthesis should be valuated first?
Taking into account that () are > than + in terms of priority.
Thanks in advance.

a doesn't get 10 in your example. a gets 14.
YOu can think of it like this: first put the values of the variables and then apply priorty to evaluate. So:
a = 10 + ( a = 4)
a = 10 + 4
a = 14

[The_Nick]

I thoroughly understand the point.
However, int a = 10; a = a + (a = 4); By having the parenthesis should not it be prioritized the parenthesis bit of the expression rather than the left part to the "+"?
In few words, why a gets the value of 10 if the mini expression within parenthesis should be valuated first?
Taking into account that () are > than + in terms of priority.
Thanks in advance.

a doesn't get 10 in your example. a gets 14.
YOu can think of it like this: first put the values of the variables and then apply priorty to evaluate. So:
a = 10 + ( a = 4)
a = 10 + 4
a = 14

I did not mean that the result was 10, sorry if I did not make it quite clear. it was just reporting the code.
For what I understood every left variable gets saved for the right hand expression. isnt' it?

let's say we have:
int a= 1;
a+= (a =4)+(a=4+a);
the result will be 13, basically every variable on the left hand side of a binary operator gets temporary saved and used for the right hand side operand.


The_Nick

[philfrei]

This is kind of interesting:

Code: Select all

		int z = 1;
		z += (z=4) + (z=7) + z;
		System.out.println("z=" + z);

Prints: z=19

Code: Select all

		int z = 1;
		z += (z=4) + (z=7 + z);
		System.out.println("z=" + z);

Prints: z=16

So, the last z is not a 7, but a 4, if it is within the same parens as the assignment to 7. So, I take it that the assignment will not affect a variable in the same immediate parens.

Of course, anyone who actually programs like this is truly asking for trouble.

[flex567]

From your answer:

a += (a =4) is same as a = a + (a=4). First, a's value of 10 is kept aside and (a=4) is evaluated. The statement (a=4) assigns 4 to a and the whole statement returns the value 4. Thus, 10 and 4 are added and assigned back to a.

From the expression a = a + (a=4)
Why isn't 2nd 'a' from left set to 4 instead of 10 ?

[admin]

That is because an expression is evaluated from left to right.
Expression evaluation requires three things to be taken into account - precedence, associativity, and evaluation order. It will a bit too much to explain it in a forum post, so you should google it.
Section 5.1.6 of OCAJP 8 Fundamentals by Hanumant Deshmukh also explains it. You can get it from here.