Chap 03 - pag.47

sharpCoder · November 18th, 2010, 01:08 PM

Hi, I have a doubt about this arguments.

In a book is written about postfix ++ operator:

"Placing it after the operand means that the operand is affected after all other computation of the expression is completed"

and in example:

Code:

int var1, var2 = 5, var3 = 6;
var1 = var2++ * --var3;

and then write:
"Before the expression is avaluated, the -- operator preceding var3 takes effect, changing its value from 6 to 5. You can ignore the ++ operator that follows var2, as it won't take effect until after the calculation is completed"

and in the operator precedence chart (prefix++) have more precedence of (postfix++).

Well, all right but....that logic don't function with this code:

Code:

int a = 5;
int b = a++ * a--;

Follow that logic in b the value after calculus would be 25 but.....no is 30.
Why? In this case (postfix++) don't aspect to solve entire expression but...after a++ they change the value of a and take new value in a-- and THEN calculate product.

Can you explain me all this better? Thanks :)

Old Pedant · November 18th, 2010, 03:10 PM

You are absolutely correct!! The book is written very poorly.

The postfix operations take place *IMMEDIATELY* after the value of the expression is fetched. They do *NOT* "take effect until after the calculation is completed". That's just plain wrong!! Good editing should have caught that, but the author should never have written that.

Code:

int a = 5; int b = a++ * a--;
means:
(get the value of a, which is 5)(then increment a to 6)
then prepare to multiply by
(get the value of a, which is 6)(then decrement a to 5)

SO the two "get the value of..." parts there indeed mean you will multiply 6 by 5.

sharpCoder · November 18th, 2010, 03:41 PM

oook

Thank for help me. I wishper that there aren't other logic error some this in the rest of the book

Have a good day!!!

Old Pedant · November 18th, 2010, 04:25 PM

You can perhaps see this more clearly happening if you write slightly messier code:

Code:

int a = 5;
int x;
int y;

int b = ( x = a++ ) * ( y = a-- );

and then when you display all 4 variables you will see:
b : 30
a : 5
x : 5
y : 6

sharpCoder · November 18th, 2010, 06:19 PM

Thanks for help me. :) Now I have understand all. See you next time

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