realicado realicado - 28 days ago 13
C Question

Bitset Vector int k out of bounds in C

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?

Answer

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.

}
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