Bitshift syntax error

[Syndacate]

Hey,

This is giving me an error, and I'm not sure why:

(stu->attr >> STUDENT_ID_ACCESS_SHIFT) |= (student_number & STUDENT_ID_MASK);

Student is a struct, attr is an int used for bit storage. My hopes was to shift those bits (since I'm storing them in the high order bits) all the way down to the low order, then storing the number in there. The reason I'm masking is because I only want 14 bits of it (5 digit student number max).

Can anybody throw me a bone here on what is wrong?

Thanks.

- Greg

[arisun]

left is an express, you can't assign value to an expression, just break it apart:
a = (stu->attr >> STUDENT_ID_ACCESS_SHIFT);
a |= (student_number & STUDENT_ID_MASK);

[Syndacate]

left is an express, you can't assign value to an expression, just break it apart:
a = (stu->attr >> STUDENT_ID_ACCESS_SHIFT);
a |= (student_number & STUDENT_ID_MASK);

It will copy the ID stored in the attribute into a, then set it in the copy, that doesn't do what I need it to. I need to set it to the original.

There has to be away to shift it to a certain location for bit setting..

[Syndacate]

I came up with this line, it should work how I want it to, no?

stu->attr |= (STUDENT_ID_ACCESS_SHIFT << (sstudent_number & STUDENT_ID_MASK));

Hope that does what I want - think it will..

Thanks for the help, I don't get expressions vs statements or what have you very well, but I generally don't do operations like that, either.

[Jamdog]

This is difficult to follow, and the reason I use #defines for bitvector work. Get the defines right, and it's easier to work with:

#define ST_NUM(x) ((x) & STUDENT_ID_MASK)
#define ST_NUM_ATTR(x) (ST_NUM(x) << STUDENT_ID_ACCESS_SHIFT)
#define ST_ATTR(x) ((x)->attr)

then, you can make you line more readable:

ST_ATTR(stu) |= ST_NUM_ATTR(student_number);

[Syndacate]

This is difficult to follow, and the reason I use #defines for bitvector work. Get the defines right, and it's easier to work with:

#define ST_NUM(x) ((x) & STUDENT_ID_MASK)
#define ST_NUM_ATTR(x) (ST_NUM(x) << STUDENT_ID_ACCESS_SHIFT)
#define ST_ATTR(x) ((x)->attr)

then, you can make you line more readable:

ST_ATTR(stu) |= ST_NUM_ATTR(student_number);

Good call, thanks.