Problems with Expressions in C - c

Question

I have two functions written that have simple assignment statements with very simple expressions. The expressions are the same for both functions, however, they involve different variable types: One function uses an array of structs, the other just uses a typedef'd struct.
When running the functions, the second function fails to divide by 256, and I get very high values that are not "normalized". I have to uncomment the second line in the second function (valueB = valueB / 256) to get it to work.
The first function, however, works perfectly.
Heres the statement in Function One:
value = ((p[0].value * p2Area)+(p[1].value * p3Area)+(p[2].value * p0Area)+(p[3].value * p1Area) / 256);
Heres the statement in Function Two:
valueB = ((dataPoints.p0B * p2Area)+(dataPoints.p1B * p3Area)+(dataPoints.p2B * p0Area)+(dataPoints.p3B * p1Area) / 256);
//valueB = valueB / 256;
Why would this happen?
Also, I pass the functions the same numbers and it doesn't seem to help.
This is on MacOSX 10.6.8, inside Xcode 3.2.6

Answer 1

Are you absolutely sure the first one works properly? You have
value = ((p[0].value * p2Area)+(p[1].value * p3Area)+(p[2].value * p0Area)+(p[3].value * p1Area) / 256);
I think you want:
value = (((p[0].value * p2Area)+(p[1].value * p3Area)+(p[2].value * p0Area)+(p[3].value * p1Area)) / 256);
Similar thing with the second. I think it should be:
value = (((p[0].value * p2Area)+(p[1].value * p3Area)+(p[2].value * p0Area)+(p[3].value * p1Area)) / 256);
In both cases I think you want to divide the sum of the products by 256. Not just the last one. My change only involves placing an extra set of parentheses around the sum of the product subexpressions and dividing the entire thing by 256
In all languages there is an order by which mathematical (and all other operators are completed). It just so happens that * and / are higher in precedence than + and - in C/C++ You may refer to this link for more details.
To simplify what happened to you, I will create this simple equation:
2 + 4 + 6 + 4 / 2
Since division occurs first (and there are no parentheses to alter the order) it gets computed as:
2 + 4 + 6 + (4 / 2) = 14
Not:
(2 + 4 + 6 + 4) / 2 = 8
So my change to your code was the same as putting parentheses around 2 + 4 + 6 + 4 / 2 giving (2 + 4 + 6 + 4) / 2 and forcing the division to be done last after all the additions are completed.