Check if a binary number has a '0' or a '1' at a specific position

Question

Hi

I'd like to check if a binary number has a '0' or a '1' at a specific position.

example:

if the binary number is: 101000100

• checking at position zero (that is at the rightmost '0') should result in '0'.
• checking at position 2 should result in '1'.
• checking at position 3 should result in '0'.

• checking at position 6 should result in '1'.

  etc...

I'm coding in C, so obviously I could use sprintf / scanf and the likes, but I guess there must be something better (read: more time efficient / easier)!

What would be a good mechanism to do this?

Answer 1

This will filter out the bit you're looking for:

number & (1 << position)

If you really need a 1 or 0 response, you can use this to make it a boolean value:

!!(number & (1 << position))

Or even better (thanks Vadim K.):

(number >> position) & 1

Answer 2

Warning: This code is not Standards-compliant. Bjarne Stroustrup says, and he oughta' know, that, "Obviously, it's illegal to write one member and then read another..." Nevertheless I leave this code up for educational purposes...

Here's an alternative that is useful when, say, reading a proprietary binary protocol off of a socket:

#include <cstdlib>


union Value
{
    struct
    {
        unsigned char a_ : 1;
        unsigned char b_ : 1;
        unsigned char c_ : 1;
        unsigned char d_ : 1;
        unsigned char e_ : 1;
        unsigned char f_ : 1;
        unsigned char g_ : 1;
        unsigned char h_ : 1;
    } bits_;
    unsigned char charVal_;
};


int main()
{

    unsigned char someValue = static_cast<unsigned char>(0x42);
    Value val;
    val.charVal_ = someValue;

    bool isBitDSet = val.bits_.d_;

    return 0;
}

Answer 3

This expression evaluates to the value of the bit you're looking for, either 0 or 1:

(number >> position) & 1