Search in members of array of data structures in C

Question

I have an array of this data structure:

typedef struct {
unsigned long id; // Node identification
char *name;
double lat, lon; // Node position
} node;

And I want to do a binary search through the id. I tried defining the function like this

unsigned long bi_search(unsigned long *Vector, unsigned long Vlen, unsigned long target) {
    imin = 0; imax = Vlen-1;
    while (imax > imin) {
        imid = (imax + imin)/2;
        if (Vector[imid] == target) {
            return imid;
        }
        else if (Vector[imid] < target) {
            imin = imid + 1;
        }
        else {
            imax = imid - 1;
        }
    }
    return -1;
}

And calling it with

bi_search(&nodes->id, nnodes, 0)

The function works for a vector, but I'm not able to make it work to search inside an array of structures.

Answer 1

You need to adjust the signature of your search function and the way you call it. You have to take a pointer to the first element of an array of the structure type. Inside the function, you'll use vector[i].id to access the id member of the i^th element of the array.

unsigned long bi_search(node *vector, unsigned long vlen, unsigned long target)
{
    unsigned long imin = 0;
    unsigned long imax = vlen - 1;
    while (imax > imin)
    {
        unsigned long imid = (imax + imin) / 2;
        if (vector[imid].id == target)
            return imid;
        else if (vector[imid].id < target)
            imin = imid + 1;
        else
            imax = imid - 1;
    }
    return ULONG_MAX;  // -1 isn't an option
}

You then call it with the start of the array:

unsigned long index = bi_search(nodes, nnodes, 0);