Thread: An integer with one bit turned on

Hi!

Here is the quotation from a book called C pitfalls:

Suppose that the defined constant FLAG is an integer with
exactly one bit turned on in its binary representation (in other words, a power of two), and you want to test whether the integer variable flags has that bit turned on. The usual way to write this is:
if (flags & FLAG) ...

I am new to CS and programming. Please, explain what does it mean:
an integer with exactly one bit turned on?
What value does FLAG have in such case? I know that char is 1 bit,
but I assume the FLAG is defined in the following way:

Code:

 #define FLAG somevalue

What value might it be?
Thanks!

Originally Posted by GReaper

You seem to have mixed up "bit" and "byte". A "bit" can have two possible values( 0 or 1, off or on, false or true, whatever ). A "byte" is a collection of 8 consecutive bits. A char is one byte in most systems, but the standard says not to count on that.

Now, about your question. A bit can't be addressed by its own, because it has no distinctive address. A byte, on the other hand, does. When you want to check whether a specific bit is 0 or 1 you simply AND with only that bit set to 1. The powers of two all have just the bit you want set: 1 2 4 8 16 32 64 128
You can do the same with more than just bytes, of course. All the integral types can be used like this.

Thanks a lot for your answer. Just a little question:

to check whether a specific bit is 0 or 1 you simply AND with only that bit set to 1.

I don't understand the part saying "with only that bit set to 1".

Originally Posted by GReaper

Think of an integer as a sequence of bits. For example, 42 would be 101010. When you are ANDing two integers, you do the logical AND operation for each of their bits, separately. For instance, (42 AND 24) equals (101010 AND 011000) which equals 001000(8). See it in vertical format:

Code:

    101010
AND 011000
    ------
    001000

Thank you )