Question

As an example the following one:

        double b = 1.0; 
        int v[] = {1,1,1,0,1,0,1}; 
        double a[] = {0.001953125, 0.00390625, 0.0078125, 0.015625, 0.03125, 0.0625, -0.125};
        System.out.println(b);
        for(int i = 0; i <7; i++)
        {
            b = b + v[i] == 0 ? a[i] : -a[i];
            System.out.println(b);
        }

gives:

1.0
-0.001953125
-0.00390625
-0.0078125
-0.015625
-0.03125
-0.0625
0.125

and this one:

        double b = 1.0; 
        int v[] = {1,1,1,0,1,0,1}; 
        double a[] = {0.001953125, 0.00390625, 0.0078125, 0.015625, 0.03125, 0.0625, -0.125};
        System.out.println(b);
        for(int i = 0; i <7; i++)
        {
            b = b + (v[i] == 0 ? a[i] : -a[i]);
            System.out.println(b);
        }

gives:

1.0
0.998046875
0.994140625
0.986328125
1.001953125
0.970703125
1.033203125
1.158203125

Does this means that the value of b is changed to 0 first in the first one (without the brackets) or set directly to the value of a[i] ?!

Using ? for if/else in addition means that I have to use brackets around the condition/if/else part ?! Thanks.

Answer 1

This because of precedence of operators, b + v[i] == 0 ? a[i] : -a[i] is parsed as

(b + v[i]) == 0 ? a[i] : -a[i]

Since + has higher priority than ==, the expression has a different semantics from the one you think.

You could avoid the problem by using parenthesis to enforce evaulation order or by using a += operator which makes the problem disappear in this specific case.