C ++二进制搜索，如lower_bound

Question

I do have problem with my binary search which should work a little like lower_bound. This gives me a segfault in 5th run. Can anyone see a problem ? Thanks

int BinarySearch ( const char * a, int firstindex , int lastindex){
    if (m_len == 0) return 0; //number of items in searched array
    if ( firstindex == lastindex ) return lastindex;
    int tmp = lastindex - firstindex;
    int pos = tmp/2;
    if ( tmp % 2 != 0 ) ++pos;
    if (strcmp(a,Arr[pos]) < 0) return BinarySearch(a,firstindex,pos-1);
    if (strcmp(a,name) > 0) return BinarySearch(a,pos+1,lastindex);
    return pos;
}

Answer 1

int tmp = lastindex - firstindex;

Should be:

int tmp = lastindex + firstindex;

That is because you are looking for the middle of the indexes x and y which is (x+y)/2.

Your code's behaviour should be unpredictable, possibly looping and causing segmentation fault.

Answer 2

The midpoint between x and y is x + (y - x)/2, so you want

int pos = firstIndex + tmp/2;

Using the slightly more complex expression instead of the obvious (x + y)/2 eliminates an overflow bug that is very common.