Bitset Vector int k out of bounds in C

Question

how exactly would I check whether or not a given "item" in a bitset vector is out of bounds

eg:

struct bitset {
    unsigned char*vector;
    int byteSize;
    int bitSize;
};
// create a new, empty bit vector set of 'size' items
struct bitset * bitset_new(int size) {
    struct bitset * theSet;
    theSet = malloc(sizeof(struct bitset));
    theSet->vector = calloc(size, sizeof(char));
    theSet->bitSize = size;
    theSet->byteSize= ((size / 8) + 1);
    return theSet;
}
    int bitset_find(struct bitset * this, int k)
    {
        int arrayIndex = k/8;
        int indexPosition = k%8;
        unsigned int flag = 1;  // flag = 0000.....00001

        flag = flag << indexPosition;     // flag = 0000...010...000   (shifted k positions)

        if()
        {
        }

    }

What exactly should I have in my if statement to see if k is not in my vector?

http://stackoverflow.com/questions/523724/c-c-check-if-one-bit-is-set-in-i-e-int-variable http://stackoverflow.com/questions/127027/how-to-check-my-byte-flag — pm100, Nov 07 '16 at 19:10
struct bitset { unsigned char*vector; int byteSize; int bitSize; }; — realicado, Nov 07 '16 at 19:11
the fields `byteSize` and `bitSize` seem like they should always equal `8` and `1` respectively to me. is that correct? — Mobius, Nov 07 '16 at 19:13
@Mobius yes you're right sorry, had the wrong size down, fixed now, thank you! — realicado, Nov 07 '16 at 19:15

score 0 · Accepted Answer · answered Nov 07 '16 at 19:16

For checking bits from bitfields in general you would use the bitwise and & operator.

You will have to add some logic first to figure out which byte from vector you should be looking at.

This would look something like:

int bitset_find(struct bitset * this, int k)
{
    int arrayIndex = k/8;
    int indexPosition = k%8;
    unsigned int flag = 1;  // flag = 0000.....00001

    flag = flag << indexPosition;     // flag = 0000...010...000   (shifted k positions)


    // check to make sure arrayIndex is in range
    if (arrayIndex > this->byteSize){
       return false; // out of range
    }

    char vector_byte = this->vector[arrayIndex];

    return vector_byte & flag; // return if the field from flag is found in the correct byte from the vector.

}

Bitset Vector int k out of bounds in C

1 Answers1